Hikikomori : Multidimensional understanding, assessment, and future international perspectives
Abstract
Hikikomori, a severe form of social withdrawal, has long been observed in Japan mainly among youth and adolescents since around the 1970s, and has been especially highlighted since the late 1990s. Moreover, hikikomori-like cases have recently been reported in many other countries. Hikikomori negatively influences not only the individual's mental health and social participation, but also wider education and workforce stability, and as such is a novel urgent global issue. In this review, we introduce the history, definition, diagnostic evaluation, and interventions for hikikomori and also the international prevalence of hikikomori outside Japan. We propose a hypothesis regarding the globalization of hikikomori based on domestic and international perspectives. In addition, we introduce our latest assessment system for hikikomori (including the latest version of the ‘proposed diagnostic criteria of hikikomori for the future DSM/ICD diagnostic systems’) and propose therapeutic strategies, including family approaches and individualized therapies. Finally, we present future challenges that may lead to solutions for an internationalized hikikomori.
Appearance of hikikomori
The Japanese word hikikomori has long and widely been used in its verb form – hikikomoru – within Japanese society. Hikikomoru is a compound verb made up of the two characters for ‘to pull back’ [hiku] and ‘to seclude oneself’ [komoru]. Collectivism is strongly rooted in Japan and this allows for an easy formation of groups, but when a situation arises where an individual has left the group and is isolated, they are described as ‘that person who has withdrawn into seclusion!’ [hikikomotta; i.e., the past tense of hikikomoru]. In this way, individuals who have withdrawn from the group, in particular school or the workplace, for days, weeks, or months, spending most of the day within their home, are referred to in Japan as hikikomori. Hikikomori became widely used as a noun in the latter half of the 1990s when a Japanese psychiatrist, T. Saito, published ‘Hikikomori – Adolescence Without End.’1 Saito tentatively defined hikikomori as a person who has ceased to go to school or work for more than 6 months and has stayed at home for most of this time.
Initially hikikomori was seen as a phenomenon unique to Japanese society but more recently similar cases have been reported in many other countries and have been widely reported in the international media.2-4 In 2010, the Oxford Dictionary published a new entry for the word hikikomori, signifying its presence and acceptance outside of a purely Japanese context. The Oxford Dictionary's definition is: ‘(in Japan) the abnormal avoidance of social contact, typically by adolescent males.’ Hitherto hikikomori had been discussed as a culture-bound syndrome unique to Japan (as it is defined in the Oxford Dictionary) but we, the authors, now consider the condition to be far more global and perhaps better understood as a ‘contemporary society-bound syndrome.’5 Hikikomori negatively impacts not only the affected individual's mental health, but also population-level education and workforce stability and as such is an urgent issue in the administration of health, welfare, and labor in Japan.6-9
In this review, we introduce the history, definition, diagnostic evaluation, and interventions for hikikomori and also the international prevalence of hikikomori outside Japan. We also propose a hypothesis regarding the globalization of hikikomori based on our domestic and international research, and present our latest assessment system for hikikomori. Finally, we present future challenges that may lead to solutions for an internationalized hikikomori.
A clinical case
Mr A: A 38-year-old man lived with his elderly mother and younger brother. He did not suffer from any particular issue from birth or early development, but as an elementary school student he began to copy the stutter of a popular comedian, which then became a habit. In the second year of middle school, he was separated from a group classmates who he had known from elementary school and came to be ostracized by his new classmates.
An average student, he was accepted to a local high school. He led a typical student lifestyle and regularly socialized with friends. During his first year of high school, his father suddenly passed away due to an acute physical illness. Upon graduating from high school, despite not having any particular motivation or goals, he followed his friends who were all going on to university by applying and being accepted to a relatively easy-to-enter local university.
He hardly attended classes preferring to go out with friends. At the end of his second year, he dropped out of school. From 20 to 30, he worked as a casual shop assistant. According to his own account, he worked quite diligently at this job. The reason he quit this job was that he could not bare the fact that he was over 30 and still doing an unstable part-time job. After quitting he began to look for permanent full-time work (called seishain in Japanese society) but was constantly unsuccessful, finally losing confidence in himself and finding it harder and harder to continue his job search. No longer looking for work, for the past 5 years he has been living in a socially withdrawn way at home, mainly inside his own small room playing online games.
He continues to live with his mother and younger brother, but a few years ago he quarreled with his brother over a trivial matter and has not spoken to him since then. He continues to spend his time idly playing online games until midnight. Avoiding face-to-face human interaction, he continues to be socially withdrawn. He finds it hurtful when his family or relatives say, ‘You should hurry up and get married’ and when he himself thinks of work or marriage he is distressed.
This past year has been particularly hard and he often feels depressed. He eats and sleeps enough but at irregular times. An acquaintance has attempted suicide and he is worried that he himself may also end up the same way. He hopes to be freed from this life as a ‘loser’ soon. After researching online, he himself came to think that he might have avoidant personality disorder. His worried mother came to seek advice from a Hikikomori Support Center and after several consults by her, he too has come to feel that he would like to get help. At the recommendation of the support center, he has now visited the specialty hikikomori outpatient clinic of a university hospital. His main complaints are: (i) the future (when he thinks of his social responsibilities he becomes depressed); (ii) he has no confidence in himself and is reluctant to do anything; and (iii) he cannot connect with his friends as he cannot initiate contact with them. After several assessment interviews by psychiatrists and clinical psychologists, he was assigned to start a weekly psychodynamic group psychotherapy session.
Hikikomori phenomenon and epidemiology in Japan
The above is a real hikikomori case (for reasons of confidentiality some changes have been made). In Japan, the phenomenon of hikikomori can be traced back to what was sometimes referred to as ‘truancy’ or ‘school refusal’ (called futoko in Japanese) in the 1970s and 1980s. In the latter half of the 1990s, many of these cases came to be widely recognized under the term ‘social withdrawal’ or hikikomori by T. Saito.1 A World Health Organization epidemiological survey in Japan between 2002 and 2006 targeting individuals aged between 15 and 49 years found that 1.2% of the population has experienced social withdrawal (hikikomori) for a period greater than 6 months.10 One review of three population-based studies in Japan involving a total of 12 cities and 3951 people showed that between 0.9% and 3.8% had a history of hikikomori.11 Japan's Cabinet Office Survey reported in 2016 revealed that ‘persons socially withdrawn for more than six months,’ even when limited to those aged between 15 and 39 years, numbered 540 000 in Japan.12 In all the above epidemiological data, males outnumber females by a 3:1 margin or more. Furthermore, due to prolonged periods of social withdrawal, sometimes over years and even decades, there are new concerns for an ageing hikikomori population. In fact, including individuals over the age of 40 years would increase the number of hikikomori sufferers even further. Just recently, the Cabinet Office announced that the estimated number of hikikomori aged between 40 and 65 years is 610 000 in Japan.13 It should be noted that these epidemiological surveys are based on simple questionnaires (only one or two questions) and further investigation based on more exact definitions are required in order to grasp a more accurate awareness of the current situation.
Definition of hikikomori
In 1998, T. Saito described hikikomori as ‘those who become recluses in their own home, lasting at least six months, with onset by the latter half of the third decade of life, and for whom other psychiatric disorders do not better explain the primary symptom of withdrawal.’1
In reality, some individuals that may fit into the category of hikikomori have a variety of illnesses and conditions. In other words, it may be said that the argument over whether a particular person has hikikomori is not all that meaningful. Rather what is important to keep in mind realistically is that (i) a variety of people as a reaction to stress may present in a ‘condition of hikikomori,’ (ii) separate from the existence or nonexistence of mental illness in the narrow sense, the fact that the condition is prolonged, and (iii) as a general characteristic of hikikomori, it is often considered the case that even before understanding the detailed nature of an individual and their psychological state it is often the case that it is necessary to initiate some kind of aid.14
As a result of various factors, a withdrawal from social participation (schooling including compulsory education, employment including part-time jobs and other interactions outside of the home), which in principle has continued under the condition of being house-bound for a period of more than 6 months (this may include leaving the home while still avoiding interactions with others). In general, hikikomori is considered to be a non-psychotic phenomenon that is distinguishable from the withdrawal state based on the positive or negative symptoms of schizophrenia, but it should be noted that it is not unlikely that in fact it may include schizophrenia before definitive diagnosis.15
As such, based on the guidelines, hikikomori is a concept that does not generally include schizophrenia, but according to a survey by Kondo et al. conducted before the establishment of guidelines, the DSM-IV-based psychiatric diagnosis of sufferers under the condition of hikikomori attending mental health welfare centers showed a wide coexistence with psychiatric disorders, including schizophrenia, mood disorders, anxiety disorders, personality disorders, and pervasive developmental disorder.16 There is considerable debate as to whether to include psychosis within the definition of hikikomori. As will be described in detail later, for example, we would consider different treatment approaches for hikikomori that is mainly based on psychosis, hikikomori that is mainly based on depression, and hikikomori that is mainly based on autism. Therefore, we believe that it is vital to clarify the coexistence of psychiatric disorders when evaluating hikikomori.
Hikikomori-like condition in psychiatric disorders and other physical and social situations
As described above, there are reports of hikikomori coexisting with a variety of psychiatric disorders.16 At the present time, whether it is such psychiatric disorders that give rise to hikikomori as a symptom or whether it is indeed the condition of hikikomori that is the cause of coexistent psychiatric disorders has not been clearly answered; thus, it could be argued that both possibilities exist. Below, we briefly describe comorbidity issues in each psychiatric disorder that is considered to include hikikomori-like symptoms.
Schizophrenia and psychotic disorders
In patients with schizophrenia, it is not unusual to retreat into a situation of physical withdrawal due to positive or negative symptoms: ‘I'm being chased by Yakuza [Japanese mafia],’ ‘My neighbors are spying on me,’ or ‘Electromagnetic waves are coming in from outside’– such hallucinations and delusions may make venturing outside a frightening experience and lead to withdrawal. ‘Social withdrawal’ is a typical negative symptom of schizophrenia, which is difficult to distinguish from non-psychotic hikikomori. It would be especially difficult to make a distinction in the case of the previously proposed ‘simple schizophrenia’ in which no hallucinations or delusions are observed.17, 18 In the 2010 guideline, schizophrenia was in principle not considered within the definition of hikikomori; however, the definition was somewhat ambiguous, as it did not rule out its possible inclusion. As indicated in this 2010 guideline, physical withdrawal is often observed in prodromal-stage cases of psychosis, and careful attention is required in evaluation. It is necessary to now debate as to whether or not to include schizophrenia in the definition of hikikomori.
Depression
In depression, other than depressed mood, decreased motivation and activity (anhedonia) are major symptoms that may present in the form of withdrawal-like outcomes. Similar hikikomori-like behaviors are observed during the depression phase of bipolar disorder.
Social anxiety disorder and other anxiety-related disorders
Anxiety in social interactions may precipitate hikikomori, and social anxiety disorder is a high comorbid psychiatric disorder among persons with hikikomori.19 In Japan, taijin kyofusho [disorder involving a strong fear of others] has long been identified as a Japanese cultural-bound syndrome in which the sufferer experiences fear of interpersonal relations, especially face-to-face interactions, and is included in the DSM-IV-TR appendix of culture-bound syndromes.20, 21 Interestingly, several commonalities exist between taijin kyofusho and hikikomori.18 Taijin kyofusho is common among the same youthful age group and is more prevalent among males.22 A previous case series study of consecutive taijin kyofusho patients who were treated at an inpatient unit revealed that about 30% of the patients fit what they regarded to be a ‘hikikomori subtype.’23 The core feature of taijin kyofusho is fear of offence or hurting others through awkward social interaction or because of perceived physical defects, such as body odor, blushing, and eye-to-eye contact,24 while such features may not obviously be expressed by typical cases of hikikomori.
Personality disorders
In our previous small-sample-size survey based on the Structured Clinical Interview for DSM-IV Axis II Personality Disorders (SCID-II), avoidant, paranoid, dependent, schizoid, antisocial, borderline, narcissistic, and schizotypal personality disorder may be comorbid with hikikomori.19 From this survey and our recent clinical experience, avoidant personality disorder seems to appear with the highest frequency among personality disorders.
Post-traumatic stress disorder and trauma-related disorders
Hikikomori sufferers in Japan, especially those with the experience of truancy or school refusal [futoko] at the initial stage of their hikikomori, often also have the traumatic experience of bullying [ijime]. Bullying itself may not meet the diagnostic criteria of post-traumatic stress disorder (PTSD); however, in addition to direct physical violence from peers, indirect violence, such as being ignored or ostracized (typical forms of ijime), is in many teen cases, a trigger for hikikomori.
Autism spectrum disorder
Comorbidity of hikikomori with autism spectrum disorder (ASD) has recently been suggested.16, 25 The meaning of the Japanese characters that comprise the word ‘autism’ [jihei] are very similar to the word hikikomori. Due to ASD-like tendencies, including the inability to perceive the feelings of others, which often results in social maladjustment, the above-mentioned susceptibility to bullying and a loss of a ‘place to be,’ there are many cases which result in hikikomori.
Other mental illnesses and neurodevelopmental disorders
In addition, there is a recognition of comorbidity of hikikomori with other mental illnesses and neurodevelopmental disorders, including intellectual developmental disorder.
Adjustment disorder
Some persons with the hikikomori condition may not be diagnosed with any psychiatric disorders, and such persons can be labeled with adjustment disorder based on the DSM-5. Previously, such persons with the hikikomori state were named as ‘ideopathic [ichijisei] hikikomori’ by a Japanese psychoanalyst, T. Kinugasa.26, 27
Suicide
Suicide is perhaps the most drastic behavior seen in a variety of psychiatric disorders. Even though no epidemiological data exist, there have been a number of cases of hikikomori persons who have committed suicide. Although the relationship between hikikomori and suicide is yet to be adequately elucidated, we believe that the act of hikikomori may be considered a precursor symptom of suicide.28 We are proposing that the act of wanting to escape from the real world is common to both suicide and hikikomori. Hikikomori may be an alternative-suicidal behavior. Interestingly, a recent second-analysis study using the data from a survey of young people's attitudes of 5000 residents in Japan (aged 15–39 years) has suggested that the hikikomori condition is one of the risk factors of suicide.29 We believe that further investigations focusing on this perspective should be conducted.
Hikikomori caused by physical illness
In cases of great physical fatigue and pain when it is physically impossible to walk or physically impossible to move, an individual may find themselves in a hikikomori-like state. Furthermore, based on our clinical observation, in certain cases of skin diseases, including dermatitis (especially atopic dermatitis), extreme skin rashes (urticaria), and other strong facial skin conditions, an individual may avoid social relationships and fall into a hikikomori-like situation.30 Furthermore, gastrointestinal diseases, such as irritable bowel syndrome, ulcerative colitis, and Crohn's disease, are suggested to be possible comorbid disorders with hikikomori.31
Hikikomori-like condition caused by social situations
In Japan, the severing of social interactions is sometimes seen as a calling or particular way of life. Examples of this are the mountain recluse and hermit, who have constituted mysterious entities throughout Japanese history. Certain artists have also avoided society during their creative processes but we would not think to label them as pathological hikikomori.
In Japanese society, adult-age daughters who have no external jobs and live with their parents are called Kajitetsudai [domestic helpers]. Some Kajitetsudai and some shufu [housewives] do not have social interactions with people outside their immediate family and are in a hikikomori-like condition. We suspect that such women may have a strong sense of loneliness.
On the other hand, the issue of kodoku-shi [death by loneliness] amongst the elderly has become a major social issue in Japan.28 In such cases, single elderly are left to live alone after the death of a partner with no social interactions and ultimately pass away themselves, remaining undiscovered for days, weeks, or even months. We can suppose that at least several months before death they might have been in a hikikomori-like condition.28
Multidimensional model of hikikomori conditions
In sum, up to this point we have outlined the mental and social bases that give rise to hikikomori and hikikomori-like conditions. In reality, most persons with hikikomori exhibit various psychiatric symptoms/signs and it is important to make a multiaxial evaluation.
Our current conceptualization of hikikomori in the field of psychiatry is proposed in Figure 1. We suppose that even in the absence of a clear diagnosis of psychiatric disorders, many persons with hikikomori find themselves in a ‘gray zone’ and the fact that no formal diagnosis of psychiatric disorders has been made does not equate to the absence of mental suffering (distress); therefore, we strongly believe that above all, due consideration should be made of this suffering.
On the other hand, we believe that hikikomori can be regarded as a reaction to stress that may present in a ‘condition of hikikomori’ and that it might be separate from the existence or nonexistence of mental illness in the narrow sense. Some types of hikikomori might be a particular kind of coping strategy, which is similar to an avoidance strategy in response to stressful situations involving social situations and social judgments. In this sense, these types of hikikomori may not be a disorder in themselves. Yet, at the same time, because of their prolonged condition, they can eventually turn themselves into a disorder. We believe this way of understanding the hikikomori phenomenon helps to clarify the nature of the phenomenon to some extent in contrast to simply treating it as a disorder in itself or as a symptom of another disorder, like autism.
Proposing the novel diagnostic system of hikikomori
In 2015, we initially developed and published to announce the first semi-structural interview system for hikikomori diagnosis32 based on the 2010 diagnosis standards in Japan15 as well as proposals from Teo and Gaw.27 In this diagnostic system, we proposed that the following four criteria should be included and observed for 6 months or more: (A) physical withdrawal (the person stays at home almost all day, almost every day); (B) avoidance of social participation (the person avoids nearly all social situations, such as school and work); (C) avoiding social relationships (the person avoids direct social interaction with family or acquaintances); and (D) distress in social life (the above hinders the individual's social life). Individuals who fulfill all four of these criteria would be defined as hikikomori.
Persons with hikikomori who had been diagnosed with the above semi-structured interviews employing the four criteria also underwent DSM-IV psychiatric evaluation using the SCID-I and SCID-II, which revealed the coexistence of various psychiatric diseases.19 These findings are similar to those reported by Kondo et al.16 In particular, many hikikomori sufferers were comorbid with avoidant personality disorder and major depressive disorder.19 However, some persons with the hikikomori condition did not fall into any psychiatric diagnosis based on the SCID-I/II diagnostic criteria.19
We believe that now is the time to reexamine the definition of hikikomori.
Regarding Criterion A (physical withdrawal),32 hikikomori is in a state of diversification. People with severe hikikomori cannot leave their homes at all, while the majority of people with hikikomori can occasionally go out for shopping and the like. For example, there are cases of people who are physically withdrawn indoors during the day but venture out at night to convenience stores when there are few people about. As we exist within an ‘Internet society,’ it is necessary to reconsider what constitutes social withdrawal. It is now possible to study and work without leaving home as hikikomori-like physical situations, especially owing to information technologies (IT).5
Criterion B (avoidance of social participation)32 should be carefully reconsidered. In the case of housewives, it may be difficult to use the label of ‘work.’ In Japan, though the numbers may not be great, there are some housewives [shufu] who have no connection to people outside of their family and experience a strong sense of loneliness. It may be possible to consider this as a group with a tendency towards hikikomori.
Regarding Criterion C (the avoidance of social relationships),32 it is assumed that there is direct interaction as a condition of the diagnosis. However due to the proliferation of the Internet in modern society, ‘indirect’ communication via web-based or other technologies is increasingly common. In our clinical practice, some individuals with hikikomori describe having friends while the majority of them are revealed to be friends who can meet only during online games. Reexamination of future diagnostic criteria for hikikomori may be warranted in light of the fact that physical social withdrawal will likely still involve social interactions in virtual space. Whether this kind of interaction should change the diagnostic criteria for hikikomori or not may be a theoretical issue that is worth fleshing out. Some might argue that virtual relationships should not count as social interactions, especially if it involves interacting through a fictionalized avatar that is completely different in character from how one would interact in person. Another question is: should the diagnostic criteria distinguish between virtual relationships with people with whom face-to-face interactions generally occur (e.g., Facebook) versus those for whom face-to-face interactions do not generally occur (online gaming)? Further investigation should be conducted to answer such remaining questions.
Regarding Criterion D (distresses in social life),32 especially at the beginning, there are many people who actually feel happy to have achieved a situation of social withdrawal. In other words, a sense of relief at being able to escape from a life of painful reality. However, when the situation of withdrawal continues after a few months or a year or two, there are many cases that result in suffering with feelings of loneliness. Different therapeutic approaches might be provided for persons with hikikomori according to whether or not they have their own distress, especially loneliness.
In hikikomori, more detailed evaluation for each of these criteria is of growing importance. Even without satisfying all four items/categories, some form of hikikomori may exist and it is important to develop a system that can evaluate individuals that fall below these thresholds and constitute ‘pre-hikikomori’ and/or ‘semi-hikikomori’ groups.
To reflect the above-mentioned current issues in assessing hikikomori, just recently we have developed and introduced a novel diagnostic set of criteria in World Psychiatry.33 Table 1 is the latest and most detailed version of our proposed hikikomori diagnostic criteria. The main points of our revised criteria are as follows:
[Definition] Hikikomori is a form of pathological social withdrawal or social isolation whose essential feature is physical isolation in one's home. The person must meet the following criteria:
Individuals who occasionally leave their home (2–3 days/week), rarely leave their home (1 day/week or less), and rarely leave a single room may be characterized as mild, moderate, and severe, respectively. Individuals who leave their home frequently (4 or more days/week), by definition, do not meet criteria for hikikomori. The estimated continuous duration of social withdrawal should be noted (e.g., 8 months). Individuals with a duration of continuous social withdrawal of at least 3 (but not 6) months should be noted as pre-hikikomori. The age at onset is typically during adolescence or early adulthood. However, onset after the third decade is not rare, and homemakers and elderly who meet the above criteria can also be considered. |
The following specifiers are not mandatory criteria; however, they may be useful for additional characterization of hikikomori:
|
Hikikomori is a form of pathological social withdrawal or social isolation whose essential feature is physical isolation in one's home.33 The person must meet the following criteria: (i) marked social isolation in one's home, (ii) duration of continuous social isolation for at least 6 months, and (iii) significant functional impairment or distress associated with the social isolation. We have proposed that individuals with a duration of continuous social withdrawal of at least 3 (but not 6) months should be noted as pre-hikikomori. We have decided to exclude several specifiers (lack of social participation, lack of in-person social interaction, existence of loneliness, and a co-occurring psychiatric condition) from the necessary criteria; however, we believe that these specifiers are useful for additional characterization of hikikomori, especially in the process of assessing the severity and considering the treatment strategy.
Classification of the hikikomori stages based on physical/social situations
In addition, we have herein included the classification of hikikomori based on the physical/social situations following the latest version of our proposed hikikomori diagnostic criteria (Fig. 2). In Japan, many persons with hikikomori live with their families, but a portion of such individuals live alone.34 As shown in Figure 2, we herein propose that individuals with hikikomori can be classified into five stages based on living with others (mainly family members [X category]) or living alone (Y category).34
Here, we briefly focus on hikikomori persons who live alone. Our previous pilot survey has shown that most people with hikikomori live with their families with a much smaller percentage living alone.32 Some hikikomori persons living alone are supported by government welfare programs in addition to the support they receive from their parents. In Japan, the unemployed can obtain full economical support to live alone, if certain conditions are satisfied (such as poverty and mental/physical illnesses) through the governmental social welfare system called seikatsu-hogo and shogai-nenkin. With such support, unemployed single-person households are increasing among younger and middle-aged people in addition to the elderly.35 In many cases, young people start living alone when entering university or finding employment, but even if they drop out of school or quit their jobs and become unemployed, many continue to receive financial assistance from their parents and/or the social welfare system enabling them to remain shut in alone. Human relationships become diluted, contact with family members, friends, and colleagues become scarce and a solitary condition is cemented. When this situation lasts more than 6 months, the individual may be included within the definition of hikikomori. Such individuals sometimes go out to shop at convenience stores and supermarkets, and occasionally go out for fun, but they usually stay at home and spend most of their time watching TV and surfing the Internet. The existence of such single hikikomori persons has been further facilitated by the development of IT mainly based on the Internet. With the development of the Internet, most shopping can be done via the Net (and ‘Net shopping’ coupled with advanced delivery networks is especially advanced in Japan), making it possible to live without going out. Furthermore, withdrawn persons may still easily enjoy various forms of entertainment, such as online games, in complete isolation. Indeed, more and more daily life activities can be achieved without going out and without directly contacting others.
Multidimensional understandings of hikikomori
International relevance of hikikomori
Do hikikomori sufferers exist outside of Japan? Hikikomori-like clinical cases have come to be reported in countries such as Oman and Spain since 2000.36, 37 In 2010, we conducted the first international hikikomori survey targeting psychiatrists in Australia, Bangladesh, Iran, India, Japan, South Korea, Taiwan, Thailand, and the USA using two case vignettes in a self-reporting format and showed the possibility of a hikikomori-like phenomenon in all the countries surveyed.9, 18 Thereafter, the cases of hikikomori sufferers in countries including France, Spain, Italy and Brazil were reported.38-44 Epidemiological surveys of hikikomori outside Japan are very limited. In Hong Kong, a telephone-based epidemiological survey has revealed that hikikomori sufferers (duration 6 months and over) account for 1.9% of the population.45 Just recently, we conducted an epidemiological survey focusing on young people in urban areas of mainland China – Beijing, Shanghai, and Shenzhen – through social-networking services and revealed the existence of hikikomori in mainland China.46, 47
Interestingly, the semi-structured diagnostic interviews employing our previously proposed four criteria conducted in the USA, South Korea, India, and Japan revealed hikikomori sufferers in each country who met the criteria for diagnosis.32 Compared with Japanese hikikomori sufferers, hikikomori sufferers in the USA experienced a stronger sense of loneliness and had a higher level of disability at home. In India, social networks were fairly well maintained but the level of functional impairment was generally great. In South Korea, sufferers experienced a strong sense of loneliness, fewer interactions with friends, and high levels of functional impairment. For hikikomori sufferers in the USA and Japan, we also conducted a SCID-I/II clinical interview. Compared to sufferers in Japan, US hikikomori sufferers were significantly more likely to have been diagnosed with mood disorder, substance use disorder and anxiety disorder.19 Such differences may be reflective of sociocultural influences in each country. Hikikomori may be thought of more as a coping strategy than a disorder and may not be as highlighted in some societies/countries, including the USA, and thus tends not to be as pathologized. In this pilot study, as the numbers of participants were extremely small and case entry methods varied, sampling bias might have influenced this result. It is now important to rectify these points and verify our findings in a large international survey.
Is hikikomori a modern-society-bound syndrome?
Hikikomori, previously considered a culture-bound syndrome, is now seen as having ‘spread’ worldwide,7, 18, 27, 48 and we herein discuss the ailment as a ‘modern society-bound syndrome.’5
Haji [shame]
As an environment that easily gave rise to the phenomenon of hikikomori had long existed in Japan along with sufferers of this ailment, it was easy to consider hikikomori as a culture-bound syndrome. While requiring a leap of imagination, the ultimate roots of hikikomori might be found with the mythical goddess Izanami, who shut herself away in the land of Yomi.49 The Japanese have tended to form social groups and structures that have emphasized indirect communication, and behind this has been the influence of particular values emphasized within Japanese society such as haji [shame].49, 50 Generally, for Japanese, in situations where one is shamed, the idea of ‘making oneself disappear’ has long been considered a kind of virtue. We suppose that this kind of mindset may be linked not only to the high prevalence of suicide but also hikikomori.34 Are these shame-related behaviors limited to Japanese? In Beauty and the Beast, a monster cuts himself off from contact with the outside world due to his perceived ugliness in a story that we can perhaps all sympathize with and understand across the world. In this modern society, we may live in times where shame as a pathology has reached the surface.
Amae [overdependence]
Amae [a culturally accepted ‘overdependence’] as proposed by a psychoanalyst, T. Doi, may be a major influence on the occurrence of hikikomori in Japanese society.51 To a great extent, dependent behaviors related to amae are conducted with the belief that the parent will forgive all. Doi believed that Western societies tend to consider such dependence in children to be something that should be overcome or corrected, whilst in Japan amae remains an acceptable mode of behavior even in adult life.51 Doi discussed ‘sullenness’ or a sullen withdrawal as one transformation of amae; thus, the behaviors of hikikomori may be seen to be a close relation to the classic behavior of amae. Hikikomori persons especially living with families may be affected by amae to the extent that parents accept their child staying at home for prolonged periods of time.18 Interestingly, our latest case–control study using the Rorschach Comprehensive System has revealed some deeper psychological characteristics and society-based unconscious aspects related to amae, indicating that the hikikomori phenomenon may have the aspect of coping behaviors to satisfy one's desire for dependence in at least some persons with the hikikomori condition.52
Emergence of hikikomori from the world historical viewpoint
The phenotype of mental illness and psychiatric symptoms’ manifestation forms have changed throughout eras, cultures, and societies.5, 53, 54 From the end of the 19th century to the beginning of the 20th century, Europe experienced a period of great tension between nations and ethnic groups resulting in forms of social chaos. It is during this period that hysteria seemed to be of epidemic proportions, especially among the ladies of society.55, 56
S. Freud, the founder of psychoanalysis, travelled abroad to study under J. M. Charcot, who himself is considered to be the father of hysteria research predating psychoanalysis.55 Freud engaged in the treatment of hysterical patients and these experiences largely contributed to the development of psychoanalysis with its emphasis on ‘tracing the unconscious source of mental symptoms.’ In the 1950s, as the USA led the way with the era of mass consumption, eating disorders began to emerge and also came to be visible in Japan in the 1970s.57-59 At the same time, borderline personality disorder emerged, becoming a major epidemic in the 1980s and 1990s in Japan.60-62 However, in recent years, at least in Japan, it seems to be rare to see borderline personality disorder patients with severe acting-out behaviors.63, 64
Instead, since around the late 1990s, hikikomori and a novel psychiatric syndrome called ‘modern-type depression’ (MTD) have become a growing issue, especially among adolescents, initially in Japan.9 We believe that the world historical shift of psychiatric disorders is of great importance to understanding the psychopathology of hikikomori and MTD in this era.9, 65 Based on the reports of S. Tarumi66, 67 and our clinical observations, persons with MTD tend to express depressive mood just after stressful events and also have a tendency towards avoidance and social evasion in school and work environments. People with MTD easily complain about their depressive feelings without hesitation, easily escape from social situations, and once escaped their symptoms are quickly relieved, this being the most highlighted feature of MTD. Prolonged social difficulties based on such symptoms can induce social withdrawal condition; thus, we have recently been proposing MTD as a ‘gate-way disorder’ to more serious adjustment problems, especially hikikomori (see Kato et al.68, 69). The commonality between hikikomori and MTD is social evasion tendency and we have proposed that the prolongation of MTD may be an important factor in the occurrence of hikikomori.68, 69 The premorbid characteristics of MTD are self-centeredness, socially evasive and narcissistic tendencies, easy traumatization, and low resilience, which correspond to hikikomori features.70, 71
Phenotypes of mental illness have undergone significant transformations that are far greater than possible changes in human DNA in a mere 100 years. We believe that such changes in phenotypes are greatly influenced by lifestyles characteristic of each era and society, and especially the environments we are raised in as children, as Freud proposed more than 100 years ago. Interestingly, J. Kitanaka has discussed Japan's depression based on her medical anthropological analysis.72 She pointed out that the diagnosis of depression in general became more prevalent in the 2000s in order to address the problem with overwork, death from overwork, and suicide from overwork in Japan. By recognizing depression, organizations had to recognize that workers needed some time off from work. Considering this social background, to some extent, an exploitation of the function that depression has served in this context may be one possible reason why MTD has emerged in this era in Japan.
Impact of IT-based indirect communication on mental development
What were the factors that caused the emergence of hikikomori sufferers outside of Japan and thereby transformed our understanding of hikikomori from a culture-bound syndrome to an international psychiatric disorder or related condition? We have advocated, as important factors, modernization, globalization, the spread of the Internet, and the international diffusion of indirect communication through the IT revolution. In particular, the way children play has shifted from ‘direct’ to ‘indirect,’ and this may strongly affect the behavioral characteristics of modern youth. In 1983, Nintendo ‘Famicom’ was released and since then the way children play has shifted significantly to the indoors. Children who played dodgeball and soccer outdoors while sometimes fighting with each other suddenly began to assemble at the home of a child with a Famicom station and to play indirectly via a TV monitor. Today it is possible through the Internet to play fighting games online even when physically we are totally removed from each other. Thus, worldwide, the youth of today (although not necessarily limited to youth) have far fewer opportunities for direct communication. With social media and texting replacing other activities, such youth spend less time with their friends in person, which is perhaps why they may be experiencing unprecedented levels of anxiety, depression, and loneliness.73 Is it actually the case that there are fewer children who physically fight with each other? On the other hand, the problem of ‘bullying’ via indirect communication has become a major social issue not only in Japan but also in other countries.74, 75 The Japanese saying ‘Rain strengthens the foundation’ means that adversity or conflict can strengthen what came before, and it was not rare that a direct fight between youths could lead to a stronger friendship. The question becomes to what degree can communication skills and trustful relationships be established and built only through ‘indirect’ communication/playing/gaming experiences?
Biological understandings of hikikomori
There is almost universal consensus that the pathogenesis of mental illness cannot be explained solely through the DNA we receive from our parents. Various historical, cultural, and sociological factors (in particular psychosocial stress) cause a variety of changes in the brain, including epigenetic and nerve–glial correlation, which may express a characteristic phenotype that it is one with the time and the society in which it occurs.76 Interestingly, just recently, a meta-analysis has revealed that stressful life events and maltreatment are strongly associated with the occurrence of conversion disorder (known as ‘hysteria’ in the Freudian era) in later life.77 Similar but somewhat different biopsychosocial mechanisms may underlie the occurrence of hikikomori in the 21st century.
Biological bases of hikikomori have not been clarified at all. As an initial step, we are searching for biomarkers of hikikomori using human blood by recruiting actual individuals with hikikomori. We have recently reported that individuals with hikikomori have higher avoidant personality scores in both sexes, and show lower serum uric acid (UA) levels in men and lower high-density lipoprotein cholesterol (HDL-C) levels in women compared with healthy volunteers.78 This is the first report showing possible blood biomarkers for hikikomori. In addition, we have revealed that avoidant personality traits, which are strongly linked to hikikomori, are negatively associated with HDL-C and UA in men, and positively associated with fibrin degeneration products and high-sensitivity C-reactive protein in women among non-hikikomori volunteers, mainly young-adult university students.78 High-sensitivity C-reactive protein and fibrin degeneration products are inflammatory markers,79 and HDL-C and UA are known to have antioxidative effects.80, 81 On the other hand, a non-clinical randomized control trial with healthy volunteers showed that endotoxin treatment by intravenous bolus increased blood-level inflammatory cytokines – interleukin-6 and tumor necrosis factor-α – and also increased self-reported feelings of depression and social disconnection, such as ‘I feel like being alone’ compared to the placebo group,82 suggesting the importance of inflammation in social withdrawal behaviors.
On the basis of such pilot investigations, we have hypothesized that inflammation and oxidative stress may be linked to the underlying pathophysiology of hikikomori, similarly to other psychiatric disorders.76, 83-87 Microglia play crucial roles in brain inflammation via releasing free radicals and inflammatory cytokines, such as interleukin-6 and tumor necrosis factor-α,88, 89 and activation of microglia has recently been proposed to exist in various psychiatric disorders, including schizophrenia, depression, bipolar disorder, and autism.76, 86, 90-97 In addition, microglial overactivation is proposed to exist in the brains of suicide victims and suicidal individuals, especially via the tryptophan–kynurenine pathway.98-103 As noted above, a recent epidemiological data analysis showed that hikikomori sufferers are more likely to have suicide risk,29 thus similar biological mechanisms via microglial activation may exist in hikikomori.104, 105
Figure 3 presents our current hypothesis/understanding of the theoria generationis of hikikomori.
Therapeutic approaches for hikikomori
Providing support to hikikomori sufferers who have withdrawn from society over a long period of time is challenging. According to Kondo et al., the average period from the start of withdrawal to the initiation of first-time support in Japan is 4.4 years.106 Various forms of support, such as telephone consultations, the creation of ‘meeting spaces’ for people with hikikomori, and job-placement support have been undertaken for hikikomori sufferers, mainly through mental health welfare centers and more than 50 MHLW-funded community support centers for hikikomori located throughout all the prefectures of Japan.7 In addition, a variety of private institutions provide support for sufferers with hikikomori and their parents, but there is yet to be a unified evidence-based method for these public/private interventions. A four-step intervention is recommended by the 2010 MHLW Guideline for Hikikomori: Step 1, family support, first contact with the individual and his/her evaluation; Step 2, starting individual support; Step 3, training with intermediate-transient group situation (such as group therapy); and Step 4, social participation trial.15 We have recently established a hikikomori clinical research unit in a university hospital to develop evidence-based therapeutic approaches with the collaboration of public hikikomori support centers, partially by a public research fund. Here, we introduce hikikomori support materials, including our current programs.
Family support
Initially, it is unlikely that hikikomori sufferers themselves will seek treatment, and therefore family interventions are crucial in cases of living with family members. Due to a lack of knowledge (about mental illness in general and hikikomori in particular) and prejudices against such mental conditions, in many cases family members cannot respond directly to individuals with these ailments, are unable to intervene at all, and tend to turn a blind eye for many years without seeking help. Many parents refuse to receive any professional help because of social stigma, such that ‘I don't want to be known to neighbors as having a son/daughter who is a hikikomori and/or who has a mental illness.’ Due to a lack of knowledge about mental illnesses, parents tend to worry that their child might be locked in a strange and fearful psychiatric hospital for life. Moreover, some individuals with hikikomori have violent tendencies toward their parents, and in such situations, parents tend to fear their reprisals after consulting professional help. We suspect that facing the new issue of longer-term hikikomori of ever increasing age is due to these parents' behavioral tendencies. Thus, it is important for family members to acquire the appropriate knowledge and techniques for dealing with individuals with hikikomori for early intervention.
We are now developing an educational program for parents of individuals with hikikomori partially based on Mental Health First Aid (MHFA). The MHFA was originally developed as a 12-h educational course that teaches participants (mainly laypeople) how to identify, understand, and respond to signs of mental illness and crisis, including suicidal and violent behaviors.107, 108 The five-steps of the MHFA (3rd version) are as follows: Step 1, Approach the person, assess and assist with any crisis; Step 2, Listen non-judgmentally; Step 3, Give support and information; Step 4, Encourage the person to get appropriate professional help; and Step 5, Encourage other support.109 We have been promoting usage of the MHFA in Japan,110-114 and are now in the process of developing an evidence-based educational support model that enables families (especially parents) of persons with hikikomori to obtain specific skills and knowledge in dealing with hikikomori based on the MHFA. In this program, we are especially focusing on how to assess one's child and bring them to professional support places smoothly and safely using the MHFA-based materials, including hikikomori-case role-play scenarios.
Also of interest is the Community Reinforcement and Family Training (CRAFT) that was originally developed for family members of individuals with substance use disorders as a cognitive-behavioral therapy (CBT)-based family interventional program.115, 116 The CRAFT has been highlighted as another powerful intervention tool for family members dealing with hikikomori individuals.117
We believe that the combination of MHFA and CRAFT along with other intervention materials may provide a dynamic approach for family intervention in hikikomori cases.
Home visits
Home visits by physicians, nurses, psychologists, and social workers also play an important role as an initial stage of hikikomori support. In some situations, home visits are carried out following a parental consultation, but this is still rare in Japan118 In South Korea, young people exhibiting hikikomori-like behavioral characteristics are referred to as Oiettolie and the existence of a hikikomori-like condition has been pointed out.119 A social-worker home-visit program has been developed for people withdrawn into their homes and a pilot intervention study has demonstrated that this has allowed for appropriate psychological evaluations to be conducted and for such individuals to more efficiently access the next treatment steps, including direct psychotherapy.119 A general hesitancy regarding home visits exists in Japan but given the research in South Korea, the development of an effective home visit approach may be important in the support of hikikomori sufferers in Japan and other countries.
Multidimensional assessment and therapeutic strategies
Before providing therapeutic materials to sufferers of hikikomori themselves, multidimensional assessment is essential. In our hikikomori research clinic, we have been evaluating a variety of aspects based on the biopsychosocial model.
Age of onset, triggering life events, severity of hikikomori conditions (withdrawal duration, frequency of going out from their own room/house, contact with family members and friends, etc.) are evaluated. As shown above, we have just developed the novel hikikomori diagnostic criteria (Table 1). To shorten the evaluation time, we have just recently developed a self-rated questionnaire called the Hikikomori Questionnaire (HQ)-25,120 which takes just 2–3 min to complete and is thus expected to be more widely and easily used at hikikomori supporting faculties even in the absence of hikikomori experts. In addition, we are preparing an online version of the HQ-25, which hopefully can be accessed by pre-stage and/or severe cases of hikikomori.
In addition, psychiatric diagnostic interviews based on the DSM-5/ICD-11 should be conducted due to high comorbidity of psychiatric disorders, and those under the threshold levels should also be evaluated. Psychometrics, especially to grasp personality, attachment, and Internet addiction tendency, are recommended.121-125 To assess behavioral characteristics, economic games, especially trust games, can evaluate unconscious decision-making and consequently estimate interpersonal relationships.126 Biological assessments, including blood tests, and brain functional analysis, such as electroencephalography, may help to understand the biological bases of hikikomori.
According to the results of such assessments, various supports are provided respectively. If hikikomori sufferers have psychiatric disorders, the guidelines of each disorder should be followed, using pharmacotherapy, psychotherapy, and/or psychosocial interventions. To our knowledge, the majority of hikikomori persons require not only pharmacotherapy but also psychosocial supports.
Here we introduce up-to-date individual therapeutic approaches against hikikomori in Japan and other countries.
Animal and robot therapy
Looking outside of Japan, in Hong Kong it is reported that hikikomori sufferers account for 1.9% of the population,45 and that social withdrawal is widely seen as an expanding social issue of concern. As a result, led by social workers and occupational therapists, a variety of hikikomori support measures are being developed. For example, under the hypothesis that hikikomori sufferers may be adverse to direct contact with other people, an animal therapy program has been introduced as a stepping-stone in alleviating this reluctance. Coming in direct contact with animals, such as dogs and cats, can also be a step towards leaving the confines of one's home and this has been demonstrated in a pilot study.127
Furthermore, pet-like robots, such as Sony's Aibo, a dog-like robot, have been developed based on advanced technologies, which might enable humans to communicate with emotional satisfaction.128 These robots have been utilized in a variety of situations to combat social difficulties with psychiatric disorders, especially autism and dementia.129-131 We believe that these robots can resolve loneliness at least to some degree, especially for single-person households with hikikomori.2, 132 The development of pet-like robots capable of conversation on an emotional level is progressing at a fast pace and there is hope that such robots can assist hikikomori cases with the alleviation of loneliness and serve as a first step towards increased social interactions/sociability.
Online-based intervention
The online game Pokémon Go, which was released in the summer of 2016 and utilizes location information and augmented reality, became a global hit. Augmented reality is a technique of superimposing sound and graphics on images reflecting and expanding the real world, and this technique may be useful in the support of hikikomori sufferers. Some people with hikikomori who have not gone out for years are leaving their homes in search of Pokémons.133 Surprisingly, in our clinical practice, a male patient who previously was barely able to go out and who lives alone has begun to venture out daily with the emergence of Pokémon GO.134 Unfortunately, after a few months, he grew tired of the game, and thus its effect was transient, but we believe a hikikomori support approach based on such technology has the possibility of being particularly effective during the early stages of the condition, and much can be expected from the development of such tools through industry–academia collaboration.
Furthermore, the question should be asked: ‘Are hikikomori sufferers themselves seeking treatment?’ Interestingly, our international survey has revealed that many hikikomori sufferers show a preference for psychotherapy over drug therapy that would allow them to have a breakthrough.32 We had previously assumed that they may have a preference for online treatment using web cameras and similar technologies (as telepsychiatry-style) but contrary to our expectations, there were significantly more people who expressed a desire for direct face-to-face treatment.
Psychoanalysis and psychodynamic psychotherapy
Generally, psychotherapy is suggested to be effective for hikikomori.15 The concept of hikikomori was originally developed by psychoanalysis-oriented psychiatrists in Japan, centering on Tamaki Saito, Kazuhiko Saito, and Naoji Kondo. Even now, many hikikomori experts in Japan are psychoanalysis-oriented psychiatrists, psychologists, and psychopathologists. Psychoanalytic (psychodynamic) approaches are not fully empirically validated, but based on the above foundations we herein introduce psychoanalytic understandings and approaches.
We propose that psychodynamic individual/group approaches are especially effective toward individuals with hikikomori in resolving their difficulties with interpersonal relationships between family members and future school/workplace colleagues in the outside world. Until now, we have described hikikomori as a pathological phenomenon and behavior, but is it sufficient to regard social withdrawal only as a negative thing in the first place? From psychoanalytic research into war neurosis (present day PTSD), the British psychoanalyst W. D. Fairbairn imagined only two personality components in all human beings, including healthy individuals: the Exciting Object and the Rejecting Object.135 The hikikomori phenomenon may be an extreme expression of the Rejecting Object. According to Fairbairn's theory, the tendency to withdrawal is intrinsic to what it is to be a human being. With the popularization of the Internet, mobile phones, and other mobile devices, we are connected to someone wherever we are. In such situations, hidden (private) worlds are suddenly in clear (public) view creating a modern society where it is near impossible to ‘be (comfortably) alone.’ Perhaps exactly because we exist in a society where we are constantly connected and without delineation between private and public that ‘withdrawal’ takes on an extreme form. The action of hikikomori (hikikomoru as a verb in Japanese) may be an unconscious but desperate acting out of the primitive desire of a modern person who has lost the space to withdraw.5 We believe that the key to rescuing sufferers with the hikikomori condition is the ‘reacquisition of the mental space where a person can be comfortably/safely alone’ and this reacquisition may be achieved by advocating the usefulness of psychoanalysis. In psychoanalytic psychotherapy, especially psychoanalysis by the non-face-to-face ‘couch’ method, extremes of ambivalent feelings of ‘wanting to engage’ and ‘wanting to leave out’ are shared by both patient and therapist often in silent moments during a therapy session. When two people create a space–time when they are together but in silence, this gives birth to what D. W. Winnicott advocated as the ‘capacity to be alone,’136 and we believe that this approach relieves the person from defensive physical withdrawal. In addition to individual psychotherapy, psychoanalytic group psychotherapy allows sufferers with the hikikomori condition to experience the above ambivalence inside a group while observing the other and is thus highly effective.137, 138
Biological intervention
Until now, no biological interventions have been investigated in the treatment of hikikomori. As inflammation and oxidative stress are possible contributing factors of hikikomori,78 anti-inflammatory and/or anti-oxidative agents may help to improve hikikomori conditions. We have previously reported that minocycline, an antibiotic drug with a suppressing effect on microglia, can change social decision-making in times of strong social stress139-141; thus, such agents targeting microglial activation may also work for solving hikikomori behaviors. Clinical trials are warranted to clarify the effectiveness of such drugs in the future.
Future perspectives
Before concluding, we propose some important aspects that should be considered.
Hikikomori is still a hidden epidemic in many countries, and in order to grasp its worldwide relevance, hikikomori diagnostic criteria should be included in the future ICD/DSM systems. At least, we propose that, similar to ‘catatonia’ in the DSM-5, subtyping and/or a specifier code for ‘hikikomori (social withdrawal)’ should be added and applied for each diagnosis of psychiatric disorders33 in such a way as previously existed for ‘adjustment disorder with withdrawal’ in the DSM-III. Based on such future systems, international and population-level epidemiological surveys should be conducted. We expect that our newly developed self-rated questionnaire, the HQ-25, may help to evaluate risk factors of hikikomori and effectiveness of interventions.120
In addition, as shown in the previous section, hikikomori is now understood to have links to heterogeneous mental illnesses, and we suppose that some common psychopathological mechanisms may exist in the act of ‘shutting-in’ regardless of psychiatric diagnosis. The Research Domain Criteria (RDoC) approach beyond conventional psychiatric classification has attracted much attention142, 143 and we believe that hikikomori may fit well within the RDoC framework because of coexistence of hikikomori within various psychiatric disorders. Beyond psychiatric diagnosis, such as schizophrenia, depression, and ASD, some common mechanisms relating to biopsychosocial factors may exist, and multidimensional research should be conducted. Until now, the majority of hikikomori reports have been empirical; thus, theoretical and scientific studies should be encouraged to understand the multidimensional phenomenon of hikikomori.
Conclusion
In this review article, we have described up-to-date knowledge about hikikomori within and outside Japan and have presented our recent hypothesis of ‘hikikomori as a modern-society syndrome.’ With the IT revolution, the need for direct communication has diminished and it may be that the future will bring a society where it is easier to hikikomoru (becoming hikikomori). For this reason, a continuous international collaboration and multidirectional approach to the hikikomori phenomenon will be required.
Acknowledgments
The authors would like to thank Dr Tae Young Choi, Dr Paul W.C. Wong, Dr Masaru Tateno, and Dr Yatan P.S. Balhara for international collaborative research; Dr Ryo Kawano, Dr Yoko Honda, and Ms Michiko Asami at the Fukuoka City Mental Health Welfare Center/Hikikomori Support Center; and Dr Kohei Hayakawa, Dr Nobuki Kuwano, Mr Hiroaki Kubo, Mr Keita Kurahara, Ms Ryoko Katsuki, Ms Hiromi Urata, Ms Sakumi Kakimoto, and Ms Yoko Zushi at Kyushu University for their research support and/or assistance. This work was partially supported by a Grant-in-Aid for Scientific Research on (i) Innovative Areas ‘Will-Dynamics’ of the Ministry of Education, Culture, Sports, Science, and Technology, Japan (JP16H06403 to T.A.K.), (ii) the Japan Agency for Medical Research and Development (AMED; Syogaisya-Taisaku-Sogo-Kenkyu-Kaihatsu-Jigyo to T.A.K. and S.K.; JP17dk0307073 and JP18dk0307075), and Yugo-No to T.A.K. (JP18dm0107095); (iii) KAKENHI – the Japan Society for the Promotion of Science (JP26713039, JP15K15431, JP16H03741, and JP18H04042 to T.A.K., and JP16H02666 to S.K.); (iv) SENSHIN Medical Research Foundation (to T.A.K. and S.K.); and (v) the JSPS Bilateral Joint Research Project between USA-Japan (to T.A.K. and A.R.T). Dr Teo is supported by a Career Development Award (CDA 14-428) from the Veterans Health Administration Health Service Research and Development (HSR&D) and the HSR&D Center to Improve Veteran Involvement in Care (CIVIC), the Department of Veterans Affairs. The views expressed in this article are those of the authors and do not necessarily reflect the position or policy of the Department of Veterans Affairs or the US government. None of the funders had a role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Disclosure statement
All the authors have declared that no conflicts of interest exist.
Author contributions
T.A.K. drafted the article, and A.R.T. and S.K. revised it critically for important intellectual content. All of the authors provided final approval of the version to be published.
