Complexity and dynamics: Lifecourse trajectories
Epidemiology is increasingly confronting the problems of complexity and dynamics. The lifecourse approach which centres on life trajectories as dynamic and shaped by many forces moves thinking from the two-dimensional static snapshot approach to one that considers an epidemiological triad – person, place and time. Such a perspective suggests that an individual's position is the end product of a life trajectory and that there are multiple risk exposures along the way [4, 8]. Such a dynamic approach is the first step for social oral epidemiology in considering complex systems wherein disease or illness states arise from dynamic interaction within and between self-adjusting systems (psychological, emotional, cognitive, immune, nervous) not from a failure of specific components . Conventional analytic methods are unable to address situations where risk factors and resources are in flux and a state of interaction. The term dynamic complexity is used to describe such situations . Could dynamic complexity in social oral epidemiology be addressed by using systems modelling methodology in our future research programmes?
The central tenet of a systems approach is that complex behaviours of organic and social systems are the result of ongoing accumulations of people, material assets, biological or psychological states with feedback mechanisms . In systems in which different paths are dependent, actions at certain times called lever points or tipping points can have large effects on outcomes . As these authors outline, tipping points on the macro level are dramatic changes that arise quickly and usually unexpectedly (e.g. a slogan, political idea, a diet). A tipping point is a threshold effect (e.g. tooth whitening in particular subpopulations e.g. the media/advertising) at which individuals or groups adopt an idea or practice. At an individual level, these are likely to be influenced by social norms, whilst at a population level, by taxes or legislation (e.g. decrease in smoking following ban on smoking in public places in the UK). Is it possible by using such thinking to identify patterns both within individuals but also across individuals that predict the increasing likelihood of tipping into healthy behaviour lifestyles (e.g. going to the dentist, brushing twice a day)? This represents quite a different approach – a move away from finding the ‘magic-bullet’ main effect and from a linear framework where unaccounted variance is relegated to ‘error’. In complex systems, error is the thing of interest, as are the interactions. In complex systems, the interaction of factors may be analogous to higher-order interactions terms in regression models (5-, 10-, 15-way interactions), which cannot be examined traditionally because the research will be underpowered (most likely) as well as many of those interactions being nonlinear . As stated by Resnicow and Page, the blessing and curse of complexity is that it is conceptually and analytically complex! Complexity requires tolerance of heterogeneity, unpredictability and uncertainty; the opposite of the epidemiological paradigm and its drive for parsimony.
The conceptual basis of complex systems dynamic models has a long history, and these approaches are increasingly used in other disciplines (e.g. system biology, ecology, economics, organizational science, political science, ). In epidemiology, there has been a growing call for such complex systems models [28, 67] although most have been limited to infectious disease with only a handful applied to noninfectious areas (e.g. smoking). Yet, they are the optimal analytic strategy for lifecourse perspectives in oral epidemiology where we are not concerned with disease as a static product at a given time (caries aged 10) but a result of circumstances over time.
Similarly, agent-based models, which are similar to testing ‘what if’ artificial counterfactual conditions, could be used in future social oral inequalities research to assess if certain public health interventions ‘work’, that is, whether such interventions influence patterning of say the use of dental services in a particular geographical area. In this way, they could be used to examine the distribution of resources relative to the distribution of inequalities. Using such analytic strategies in this way, we can examine a range of system effects an intervention or change in policy might be expected to have if implemented . The modelling of such dynamic processes related to place effects would therefore advance thinking that currently sees ‘place’ or the ‘environment’ as a static entity (see earlier discussion). One example of such an approach is a recent simulation study for designing effective interventions in early childhood caries . The authors used system dynamics to compare the relative effect and cost of six categories of early childhood caries intervention, applying fluorides, limiting cariogenic bacterial transmission from mothers to their children, using xylitol, clinical treatment, motivational interviewing and a combination of these. The resulting model predicted 10-year intervention costs and relative reductions in cavity prevalence with interventions targeting the youngest children having a greater benefit, those targeting high-risk children providing the greatest return on investment, and combined interventions showing the greatest cavity reduction.
By applying systems thinking to social oral epidemiology and then to interventions to improve population oral health, we would begin to target those causes that cannot be manipulated in a randomized-controlled trial. It is possible by using such methods, for example, to model those influencers on health outcomes (tooth decay) but also to evaluate public health policies (e.g. impact of investing in dental service provision on tooth decay under different assumptions about the importance of psychosocial orientations in influencing oral health) (example adapted from ). Again, this approach is not a panacea but one tool that could help push social oral epidemiology forward both conceptually and methodologically.
Embodiment: How bodies register social experiences
In planning future research programmes, we need to move away from decontextualized and disembodied ‘behaviours’ and ‘exposures’ to understanding how the environment or ‘society gets into the body’ . This approach forms part of a long-standing tradition in sociology, exploring how physical bodies are shaped by the body social [71, 72]. In this approach, bodies are seen as an important focus of social regulation. To this end, it has been argued that the principal mechanism of dentistry is disciplinary, that is, it is visited on the body in everyday life through the clinic in the form of the dental examination and through the home in the form of toothbrushing techniques [73, 74]. In this research, the ‘environment’ is defined in terms of disciplinary knowledge and power and how this affects the body, which is in stark contrast to the SES variables commonly discussed as the ‘environment’ in oral epidemiology. In recent years, debates such as these have begun to enter epidemiology more generally. For example, recent work discusses embodiment as a multilevel phenomenon, and how processes become embodied and generate population patterns of health and disease .
The problem of embodiment raises a central issue that might act as a catalyst for new collaborations between epidemiology and sociology. Through this theme, there are ways that both epidemiology and sociology can unpack ‘how’ the body and society are related and perhaps do so more carefully than has been the case to date. So, what then is the problem that underlies embodiment? The problem appears to be that if we start with the question of how society determines patterns of disease, we invariably end up confronting the difficulty of explaining how the acting feeling subject either resists or is less than determined by society. Another way of putting this is that describing how oral disease is socially patterned does not explain ‘how’ or ‘why’ it is patterned the way it is. Some room for the thinking acting subject is required. On the other hand if we start with the internal environment of the thinking acting subject, we end up struggling to explain social patterns of disease and illness .
There have been several attempts to overcome the dualism inherent in sociology's conception of society and the individual or the body. Writers such as Bourdieu  have proposed a way through the impasse with concepts such as ‘habitus’. Habitus refers to the practices that we engage in that reflect regular problems encountered by people in their everyday lives. The practices reflect the regularity of everyday problems, but at the same time, they also reflect the structure of the environments we find ourselves in. So, for example, the practice of tooth brushing reflects the fact that we will all, more or less, experience tooth decay in the so called Western diet since the ‘nutritional transition’ [77, 78]. The practice itself is a reflection of a generalized problem that our food environment presents to us, it is not, however, determined by that generalized problem. The concept of habitus seeks to enable us to reflect on the conditions of the environment through the practices generated in reflection of how we have come to habitually deal with that environment. There are as yet no serious explorations of the everyday habitus and how this relates to the mouth and oral health, and yet, the approach has received widespread attention in social science.
Other approaches may also prove promising. For example, Shilling  takes habitus as one of his points of departure into the pragmatic approach of Mead  in his attempt to unpick the relationship between society and the body. The pragmatism of Mead starts with neither the individual, nor the collective, but from the position that ‘individuals are always already within a social and natural context, yet possessed of emergent capacities and needs that distinguished them from, and also enabled them to shape actively, their wider milieu’ (; p. 4). Our identities are shaped by ongoing interactions and transactions between the internal ‘environment’ of the embodied organism and the external social and physical environment. For Shilling , it is the ability of pragmatism to maintain a view of the internal and external environment that is distinctive. The suggestion then would be that the dental subject is therefore not determined by their social environment, as is often presented in the social determinants approach, rather they can, and often do, intervene creatively in the world to shape and change it.
We can draw on this approach in dental research. One starting point would be to adopt the ‘transactional’ approach of pragmatism between people in their environments. This might lead us to explore, utilizing in-depth qualitative methodologies, the different processes involved in different phases of interaction in oral health and society. For example, by looking at oral health-related habits, crisis and creativity that can combine at different times in the everyday lives of individuals (see  for an example). Such research involves being sensitive to the fact that there is more than one environment for social action and that we need to explore how the different environments constitute the different phases of action. The different environments are effectively the social and physical milieus of Mead .
The social milieu is constituted by people interacting on the basis of three things: their own desires and needs, what they think other people might think of them and from the standpoint of the group as a whole. This latter standpoint, termed the ‘generalized other’, places pressure on members of the group to conform to the standards of the group in terms of how they act. This approach filters the development of a bodily identity. Over time, we learn to evaluate ourselves according to the standards of the social group to which we belong. These organized set of attitudes to others is the way society influences its members. There are already existing data that can direct our attentions to such dimensions of oral health. Take the work of Sussex and colleagues  where it is clear that there was a generalized acceptance of edentulism in New Zealand so that those with less than good teeth can live without stigma and can also support a symptomatic pattern of dental care. As a consequence, in the past, New Zealand society favoured extraction instead of restoration for dental disease.
The social milieu is vital, but it is not the whole picture that there is also a physical milieu that constrains and provides a restricted set of opportunities. For example, the key physical environmental influences on the adoption of these generalized attitudes in New Zealand appears to have been rural isolation . In this respect, the external environment has a social and physical dimension, and both of these dimensions are essential for understanding embodiment and corporeality in older New Zealanders in relation to their oral health.
We can also explore the internal environments of oral health and how this relates to emergent needs. For example, the approach of GH Mead and the pragmatists often begins with the internal environment of embodied action and then explores how our impulses are called out in particular ways by our environments. So, feeling low because of a glucose deficiency calls out a reaction to consume a sugary snack or drink. There is, in other words, a ‘prereflective’ tendency of our bodies towards survival, and we select stimuli that are basically relevant and depend on specific circumstances. Within this approach, the human body reaches out through its senses to manipulate the world around it in a multilayered way, and the senses become the embodied basis of our relationship to our environment . There is almost no detailed exploration of, for example, how sugar occupies a habitual space in the everyday environments of some populations and how its consumption might be related to the internal environment of the self. We are often called to reduce sugar consumption because of its associations with multiple forms of disease, for example, obesity and caries . Yet, this call often neglects or simplifies the complexities of the relationships between our internal and external environments and how these relate to embodiment.
For example, being asked to cut sugar consumption is similar to being asked to express self-control. Yet, recent research has shown that the exercise of self-control has a direct impact on our internal physical environment. In what is called the ‘resource depletion’ account, it is now being demonstrated that increasing self-control is significantly and specifically associated with the depletion of glucose as an energy resource [82, 83]. The paradox, simply stated, is that self-control tasks, controlling ones diet, for example, can have direct impacts on blood glucose. Refraining from the consumption of certain foods can undermine ones will power to continue to control what one eats . This does not mean of course that eating sugary snacks is advisable; there will no doubt be better sources of glucose than sugar.
What this research shows is that the recommendation to cut sugar consumption within dentistry could significantly gain from being able to anticipate in more detail just what is being asked. Research in the social sciences can enable us to better appreciate the social and psychological dynamics behind resistance to such changes. So, for example, it is likely that there will be complex interactions between a person's everyday environments and self-control . As Gailliot and colleagues demonstrated, high degrees of self-control may be required in different occupations, and these in turn can have significant consequences for glucose depletion. Under such conditions, the sources of replacement glucose will no doubt have a significant impact on the health of the individual. Living under the conditions of an occupation where there are high demands in terms of self-control and limited choices in replenishing depleted glucose levels will have negative consequences for the things people can do to avoid the risks to their health and oral health. Reducing glucose intake under such conditions may well result in poorer performance in such roles, but also an added risk of significantly increased glucose consumption at another time.