• Deafness;
  • social cognition;
  • theory of mind;
  • children;
  • longitudinal study


  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References

Central to the interface of social-cognitive and communicative development is the growth of a theory of mind (ToM). ToM is mastered by most hearing children and deaf children of signing deaf parents by the age of 5 or 6 but is often seriously delayed in deaf children of hearing parents. This paper reviews recently published research on deaf children’s ToM development and presents an original study consisting of eight longitudinal case histories that collectively map late-signing deaf children’s ToM performance from 44 to 158 months of age. While five tentative conclusions can be posited from the collective research so far, further investigation of each of these possibilities is clearly needed.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References

Much contemporary research in child social cognition addresses theory of mind (ToM), a topic which, according to Flavell (2004), is “one of the largest and liveliest in developmental psychology” (p. 274). ToM describes children’s early understanding of how behavior is shaped by mental states like thoughts, intentions and beliefs, not only in straightforward situations but also when people’s mental states are at odds with their actions owing to forgetting, fantasizing, being misled and the like. Throughout childhood, an understanding of ToM is fundamental to skilled communication and social interaction, especially in complex situations including reminiscence, sarcasm, deception, humor, interpersonal conflict and persuasive negotiation (e.g., Harris, 2006). Hearing children generally acquire ToM rapidly during the preschool period, as evidenced by their success on “litmus” tests of false belief inference that require predictions about the behavior or thinking of naïve individuals whose beliefs conflict both with reality and with the child’s own accurate knowledge. Most 3-year-olds consistently fail these tests in all their different versions. Yet levels of false belief success by typically-developing hearing children from age 5 years onwards are striking enough to suggest that “understanding of belief, and relatedly, understanding of mind, exhibit genuine conceptual change in the preschool period” (Wellman, Cross & Watson, 2001, p. 655).

However, there are exceptions. Severe delays, specific to ToM, often persist in certain groups of children with disabilities, including high-functioning children with autism (see Happé, 1995; Yirmiya, Erel, Shaked & Solomonica-Levi, 1998; for reviews), blind children (see Siegal & Peterson, 2008, for a recent review) and those severely and profoundly deaf children who grow up in hearing families (see Peterson, 2004; Peterson & Siegal, 2000, for reviews).

ToM development in deaf children of hearing parents

For example, in one of the earliest studies of deaf children, Peterson and Siegal (1995) administered Baron-Cohen, Leslie and Frith’s (1985) two-trial “Sally” false belief test to a group of 26 severely and profoundly deaf Australian children aged 5 to 13 years who were “late signers”. Growing up in an exclusively hearing household, they can be described as “late signing” because they had belatedly acquired their main language, sign, after entering primary school. Despite normal nonverbal intelligence, sufficient language to understand task instructions and no disabilities apart from hearing loss, a majority (65%) of these children failed ToM at a mean age of 10, as compared with near-ceiling performance by hearing 5-year-olds. These findings were soon replicated, initially by Russell, Hosie, Gray, Scott, Hunter, Banks and Macaulay (1998) who gave the same two-trial false belief test to 30 deaf children of hearing parents aged 4 to 16 years at a Total Communication (TC = sign-plus-speech) school in Scotland. Similar to the Australian children, ToM pass rates at 7 and 10 years were only 17% and 10%, respectively. Surprisingly, nearly half (40%) of the high school students (mean age = 15) also failed.

By the end of the 20th century, at least 10 additional studies of late-signing deaf children’s false belief understanding had been conducted in several countries (Australia, France, the UK and the USA). Within and across nations, different approaches to deaf education (e.g., TC versus oral-only instruction) were sampled as were a range of different signed languages -- including ASL (American Sign Language), Auslan (Australian Sign Language), BSL (British Sign Language), LSF (French Sign Language) and Signed English. Thus Peterson and Siegal (2000) were able to review 11 published studies of ToM understanding by more than 200 deaf children worldwide. Serious problems on standard false belief tests were consistently revealed. Throughout primary school, deaf children of hearing parents were outperformed by hearing preschoolers, displaying similar levels of success to high-functioning primary-school children with autism.

Up to 90% of deaf children have hearing parents. But not all of these children grow up to communicate in a signed language. Some severely and profoundly deaf children are fitted with hearing aids or cochlear implants and are trained in a purely oral modality to perceive and express speech as their sole or primary language. This use of the spoken modality does not seem to benefit their ToM development. Peterson (2004) reviewed eight published studies of standard false belief test performance by more than 150 orally-trained severely or profoundly deaf children of hearing parents in several different countries. Irrespective of whether they used cochlear implants or hearing aids, most of these oral deaf children were delayed in ToM development to the same extent as late-signers.

Building on this review, Peterson (2004) conducted an additional study of 26 orally-communicating deaf children of hearing parents aged 4 to 12 years, half with cochlear implants, half with conventional hearing aids. Roughly half of the cochlear-implant users were taught in Signed English or Auslan, while the others were purely orally-communicating, having been mainstreamed in hearing-only classrooms in ordinary primary schools. The same was true of the subgroup with externally fitted hearing aids. No significant effects emerged either for type of schooling or type of prosthesis (implant or hearing aid). Yet all these orally-communicating children, at a mean age of 8 years, scored below hearing 4-year-olds on false belief, supporting previous indications that (a) oral-only deaf children are as delayed in ToM development as late signers and (b) those with cochlear implants score similarly to those using conventional amplification. General language ability on a standardized receptive vocabulary test also correlated with these oral deaf children’s false belief scores, as had been found previously by deVilliers and deVilliers (2000). Interestingly, however, when deVilliers and deVilliers (2000) examined deaf children’s specific language skills with the syntax of embedded sentential complements (e.g., “He says the ball is in the box”), they found that use of complement syntax in spontaneous speech correlated more strongly with ToM understanding than general vocabulary did and, indeed, continued to predict false belief scores once vocabulary and age were partialed out.

The roles of lexical and syntactic language growth in oral and signing deaf children’s ToM development were further elucidated by Schick, deVilliers, deVilliers and Hoffmeister (2007) with a large sample (= 176) of severely and profoundly deaf children aged 4 to 8 years. Irrespective of whether they signed or communicated orally, the deaf children of hearing parents were delayed on general language tests, on specific tests of skill with the syntax of embedded complements and on standard ToM tests, in line with previous evidence. Furthermore, results of multiple regression analyses indicated that, over and above age and nonverbal IQ, scores on language tests significantly predicted false belief understanding for both oral and signing children, with general vocabulary and complement syntax each contributing independently and significantly.

ToM development in deaf children who are native signers

From the perspective of how early linguistic and social experiences influence ToM development, perhaps the most interesting deaf children of all are the 10% minority who are native signers with signing deaf parents. In contrast to hearing parents who, despite extensive efforts, are unlikely ever to achieve the same level of proficiency in sign as a native speaker (Marschark, 1993; Vaccari & Marschark, 1997), signing deaf parents serve as fluent conversational partners for their deaf infants right from the start. As well as enabling the early acquisition of sign language, having a deaf parent is beneficial for ToM development. Beginning with Courtin and Melot’s (1998) seminal discovery of significantly better false belief performance by 13 native signers than by 22 late signers aged 5 to 8 years in France, a number of studies in several countries have consistently shown that native signers in primary school consistently outperform late signers and oral deaf children on ToM tests (e.g., Peterson & Siegal, 1999;Remmel, Bettger & Weinberg, 2001; Schick et al., 2007). Although few native signers under age 5 have been tested, there is some evidence they may even master ToM slightly ahead of hearing children (Courtin & Melot, 1998).

Furthermore, in primary school, native signers score higher than late-signers on ToM tests even after statistically controlling for the effects of general linguistic maturity, knowledge of the syntax of sign language and domain-general cognition (e.g., nonverbal IQ, executive functioning). For example, Woolfe, Want and Siegal (2002) matched a group of young native signers with older signing deaf children of hearing parents on the basis of their levels of skill with BSL vocabulary and syntax. Despite being younger chronologically and no more adept linguistically, the native signers scored higher on false belief understanding of ToM.

Yet, in the context of mainstream oral schooling, deaf children of deaf parents may not match other native signers attending TC or sign-only schools in terms for the early timetable of ToM mastery. Meristo, Falkman, Hjelmquist, Tedoldi, Surian and Siegal (2007) gave false belief tests to samples of deaf children in signing or oral-only schools in Italy, Estonia and Sweden. Native signers who were exposed to sign at school earned significantly higher ToM scores than other deaf children of deaf parents who were in oral-only schools even though, irrespective of schooling, all of these children preferred to communicate in sign rather than speech and had deaf parents who signed at home. Indeed, the Scandinavian native signers in oral-only schools did just as badly on a comprehensive battery of false belief tasks as an age-matched group of late-signers. It seems that the advantage for a native signer of growing up in a fluently signing deaf family is not enough to guarantee optimal ToM development if school days are spent exclusively in the company of oral communicators. Possibly native signers’ opportunities for sharing play and conversation with peers (experiences which are found to be linked with ToM development in hearing children: Harris, 2006) are curtailed in mainstream oral schools, to the detriment of ToM understanding.

In other words, a delayed theory of mind is evidently not a consequence of deafness per se, but rather of deafness in conjunction with upbringing in purely hearing families and/or purely oral schools. Native signers’ early opportunities to share, via sign, in conversations about thoughts and feelings at home and at school may well be crucial to their timely acquisition of false belief understanding.

Family conversation and ToM in deaf and hearing children

As noted above, these findings for deaf children are in line with empirical evidence from hearing preschoolers as to the importance of rich exposure to early conversations about mental and emotional themes for rapid ToM mastery (see Harris, 2006, for a review). Hearing children who pass false belief tests precociously often come from homes where discussions of thoughts and feelings are frequent (e.g., Ruffman, Slade & Crowe, 2002; Slaughter, Peterson, & MacIntosh, 2007) and where child siblings are present to talk about pretending, negative emotions, mistaken beliefs and other mentalistic themes (e.g., McAlister & Peterson, 2006; Peterson, 2000; Ruffman, Perner, Naito, Parkin & Clements, 1998). Indeed, for deaf children, Woolfe, Want and Siegal (2003) found that the closeness of the emotional bond that deaf native signers of BSL had with their deaf or hearing siblings predicted their ToM understanding over and above age and skill with referential communication, suggesting that interaction in the context of positive sibling relationships may afford access to “the inner worlds of beliefs and other mental states” (p. 340).

Late-signing deaf children can also be helped to acquire ToM when their hearing parents know the signs for mental states like “forget” or “guess” and discuss these concepts with their children in sign. Moeller and Schick (2006) gave standard false belief tests to 22 late-singers aged 4 to 10. Only half achieved the false-belief success criterion (75% correct) by age 7, as contrasted with 81% of a matched hearing group by age 5. Although 10 of the deaf children had cochlear implants, this was unrelated to their ToM performance. By contrast, their hearing mothers’ knowledge and use of signs for mental state concepts (e.g., “think”, “guess”) correlated significantly with child ToM, suggesting that: “When mothers do not have the language skills, and have not had the opportunity to learn the sign vocabulary that is needed to talk about mental states, they may not be able to scaffold their children’s understanding of mental events” (p. 763).

Steps and longitudinal continuities in deaf children’s progression towards ToM understanding

All of the studies of deaf children’s ToM that have been reviewed so far have used a cross-sectional methodology and false belief scores as the criterion for ToM. Yet, in the case of hearing children, it is now clear both (a) that false belief is only one of several developmentally sequenced ToM milestones and (b) that the growth of an understanding of minds extends from toddlerhood through middle childhood (Harris, 2006). For example, Wellman and Liu (2004) discovered five orderly steps in hearing 2- to 6- year-olds’ ToM development that conformed statistically to a Guttman scale. (A perfect Guttman scale is one in which passing a later item guarantees all preceding scale items will have been passed). On Wellman and Liu’s scale, children were first aware that different people have diverse desires, then that they have diverse (potentially true) beliefs, then that not seeing leads to not knowing, then of false belief and, finally, of people’s ability to hide their true emotions by controlling their facial expressions. The regularity of this progression across individuals and cultures (e.g., Peterson & Wellman, 2009; Wellman, Fang, Liu, Zhu & Liu, 2006) suggests that sequential ToM understandings may accumulate progressively, with mastery of each earlier step being a prerequisite for the next one. Wellman (2004) concluded, “on many accounts, false belief should be considered only a single milestone within an unfolding conceptual progression” (p. 2).

Given these considerations, it is important to study atypically children’s progression across scaled ToM sequences like these, particularly in the case of deaf children, where both early social-conversational experiences and the progression of ToM development may be atypical. Peterson, Wellman and Liu (2005) explored whether the developmental ordering of the five steps was the same as for hearing children. Of the 45 deaf children in their sample, 24% were native signers with deaf parents and the others were late-signers from hearing families, some with cochlear implants, some with conventional hearing aids and some with no amplification. All had attended a TC school long enough to have good sign language skills, and all performed near ceiling on most of the comprehension control questions that accompanied the ToM measures. Results for false belief understanding were in line with patterns identified in earlier research. The native signers significantly outperformed both the late signers and a hearing preschool control group on false belief tasks and, indeed, were at ceiling on all but the last scale step. The late-signers developed through the five steps sequentially and in the same order as hearing children, despite being chronologically slower. This suggests that even though growing up deaf in a hearing family may delay exposure to crucial ToM-boosting experiences, the orderly progression of accumulating ToM understandings is essentially the same as for a hearing child. Possibly later language mastery (Schick et al., 2007), together with restricted exposure to mentalistic family conversations (Peterson & Siegal, 1999) may delay the late-signer’s progress through each scale step, in line withLohmann and Tomasello’s (2003) observation that “it is difficult for children to construct an understanding of the representational nature of mental states from visual scenes alone” (p. 1139). Social communication in some language (signed or spoken) may well be necessary.

Do all late-signing deaf children catch up in the long run so as to fully master all the steps in ToM understanding eventually? Or is there individual variability, perhaps owing to a critical period for conversational exposure, such that some normally intelligent deaf adolescents and adults never manage to become efficient mindreaders? Data on this point are limited by the relatively young ages of most deaf children tested in previous research and by the cross-sectional nature of the methodologies predominantly employed. There are only three known studies of older deaf individuals. All are cross-sectional and all have rather pessimistic and not fully conclusive results. Specifically, in the UK, Russell et al. (1998) and Edmondson (2006) gave a single two-trial false-belief test to deaf high school students (10 aged 13 to 17 in the former study and 32 aged 12 to 15 in the latter) and found that ToM failure was unexpectedly high despite normal levels of general intelligence and academic performance. In fact, only 60% of the teens in Russell et al.’s and 53% in Edmondson’s studies passed the task.

In Nicaragua, Morgan and Kegl (2006) tested an unusual group of 11 deaf adolescents and adults (mean age = 24 years) who had grown up “in relative isolation from other deaf people and with no access to sign or spoken language” until the age of 11. When tested for false belief understanding in their 20s, they overwhelmingly failed, despite having some knowledge of the creolized sign language to which they had had at least 2 years exposure. This was viewed as evidence for “a crucial period when lack of language exposure can lead to long-lasting deficits in false belief abilities” (p. 813). While intriguing, the (fortunately) highly unique and unusual language deprivations encountered by this cohort of Nicaraguan deaf adults seriously limits generalizability of Morgan and Kegl’s results to other deaf populations. Certainly such an explanation cannot be extended to the UK high school students who failed Edmondson’s (2006) and Russell et al.’s (1998) ToM tests: All of these adolescents had been immersed in a complete and versatile language (BSL and/or Signed English) from at least since their earliest year of primary school at around age 5. However, the question of delays versus lasting deficits in deaf children’s ToM understanding (Baron-Cohen, 1991) cannot be fully answered using cross-sectional data alone.

Longitudinal approaches that follow children through time are advantageous both for directly examining how ToM performance by the same individuals changes as they grow older and for distinguishing developmental patterns from individual idiosyncracies or cohort contrasts. To date, only one known longitudinal study of deaf children’s ToM understanding (Falkman, Roos & Hjelmquist, 2007) has been published. Over a two-year period, Falkman et al. gave a battery of ToM measures of false belief and visual perspective-taking to 10 late-signing deaf 7- to 9-year-olds, repeating the tests four times at 6-monthly intervals. There was little or no evidence of ToM improvement across any of the 6-month gaps. In fact, over the full two years of the study, only one child registered a reliable gain. This was cancelled out by another who displayed a reliable decline in performance (dropping from 75% success at Time 2 to only 25% at Times 3 and 4). Two children had no room to improve, being at ceiling when the study began. None of the remaining six showed either any consistent gains, or any fully persuasive ToM understanding, prompting the conclusion that: “The lack of a clear positive change among the deaf children on the mentalizing tasks over a period of two years is a strong finding” (p. 193), although it is worth noting that their sample was quite small.

Despite the acknowledged strength of Falkman et al.’s use of a longitudinal design, however, the span of the study was relatively brief (2 years), the sample was small (= 8 not counting the ceiling performers with no room to improve) and the 6-month intervals during which changes were tracked were not commensurate with the protracted normative trajectory of deaf children’s ToM development (e.g., birth to age 8 or 9) that has been suggested by the large body of cross-sectional evidence reviewed above. Thus there is a need for further longitudinal investigation. This was the goal of the small empirical study to be reported next.

The present study

In summary, the above review of previous research on deaf children’s development of ToM understanding suggests that, despite impressive consistency in the overall finding of delays for deaf children of hearing parents, a number of more specific questions remain unanswered. Perhaps most pressing among these is the issue of delays versus lasting deficits (Baron-Cohen, 1991). Longitudinal evidence may ultimately help to answer such questions. The following study is therefore reported as a beginning contribution to what it is hoped will become a growing body of longitudinal research into deaf children’s ToM understanding.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References


The present study consisted of eight longitudinal case histories. Participants were late-signing Australian deaf children in a Total Communication (TC = sign-plus-speech) unit whose ToM development was assessed at least three times over an average longitudinal span of approximately 6 years. Children entered the study at varying ages (depending in part on when they entered the unit and achieved sufficient sign language proficiency for valid ToM testing: see below). Thus the longitudinal span of the study as a whole was 9 years 6 months (see Fig. 1) and, collectively, deaf children’s ToM development was tracked over the age range from 44 months to 148 months. While limiting the statistical opportunity for group comparisons at matched ages, this had the corresponding descriptive advantage of enabling a more extended exploration of the developmental trajectory of individual deaf children’s ToM mastery than had proved possible in the one previously published longitudinal study (Falkman et al., 2007) that was described above.


Figure 1.  Developmental changes in ToM scores for individuals in the longitudinal sample at successive ages (in months) when tested.

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The eight deaf children of hearing parents who took part all had profound prelingual hearing losses that had been diagnosed in infancy and no disabilities other than deafness. All were pupils in a government-funded TC unit and used signing (Signed English, Auslan or a combination) as their preferred communication modality. Since their parents and other deaf family members were all hearing, none of these children had access at home to a fluent native signer. (As Vaccari and Marschark, 1997, have shown, even hearing parents who make extensive efforts to learn sign language once their child’s hearing loss is diagnosed almost never achieve native-speaker proficiency especially while the child is young). Thus these children were “late-signers” because, like 90% of other users of Auslan and Signed English in Australia (Power & Carty, 1990) they had acquired their native language “not from parents within a family setting but…in school” (p. 223).

All participants had written parental consent and had been in their signing unit for at least one full academic year, resulting in levels of receptive and expressive language skill that, at minimum, corresponded to “adequate signing skills for everyday communication” according to their teachers (see Peterson & Siegal, 1999, for details of the ratings scales used). As a further precondition to ensure children had adequate command of the vocabulary and syntax needed to understand false belief tasks and test questions, they were required to achieve at least 90% accuracy on a battery of six comprehension control questions given at each testing time. There were no significant gains in mean teacher-rated language skill for the sample as a whole from the first to the second or third longitudinal test, < 1, possibly due to the stringency of the initial language-skill prerequisites. Two of the children had cochlear implants. Table 1 shows this and other participant characteristics.

Table 1. Individual characteristics and outcomes for the deaf children in the longitudinal sample
Child pseudonymCochlear implant?Test 1 age (months)Test 1 ToMTest 2 age (months)Test 2 ToMTest 3 age (months)Test 3 ToMTest 4 age (months)Test 4 ToMLongitudinal span (months)
RolandNo60−18401221  62
DanielYes68−111101371  69
PerryNo82110611292  47
GregNo85011001282  43
LaurenceNo93−113521583  65

All children completed at least three longitudinal assessments over mean span of 4.33 years; 38% of them remained in the study long enough to complete four longitudinal test phases. Graduation from primary school, families moving away, and failure of parents to sign consent forms all contributed to attrition between the third and fourth tests.

Tasks, materials and testing procedure

All children were tested individually by two highly trained and experienced adults. The main experimenter (E) conducted each longitudinal test, assisted by one of three different professionally qualified sign language interpreters (familiar to each of the children as a school employee) who, following a style of interaction that was a familiar part of classroom routine, translated all of E’s spoken narratives and questions into sign (using for each individual child the children’s preferred signing modality -- Auslan, Signed English, Signing-in-English, etc.). Both adults monitored that the child’s attention was appropriately directed (at the materials, interpreter, etc.) before continuing each part of the procedure. Children took three false belief test trials at each longitudinal time, two involving invisible displacements and one involving a deceptive container. Different stimuli (dolls, hiding places, containers, etc.) were used for each test even though the wide intervals between them made recall of these specifics seem unlikely. Tasks were:

Changed location false belief. Baron-Cohen et al.’s (1985) two-trial “Sally” changed location false belief test was borrowed exactly from the original study apart from substituting a boy for one of the girl dolls (overcoming the need to fingerspell and remember proper names). In brief, each of a pair of trials began with the girl putting a marble (or ball or money) in one container (e.g., a basket). In her absence, the boy moved it to a different container (e.g., a box). The girl returned and the test question “The girl looks where for her [marble]?” was followed by two comprehension control questions “Where is the [marble] really?” and “Where did the girl put the [marble] in the beginning?” The second trial was identical except that the boy had moved the item to a new hidden location (e.g., E’s pocket) so that there were three possible hiding places and the same three questions were asked. To pass each trial, the child had to pass both associated control questions (so as to ensure test questions responses were meaningful) and to indicate that the girl would search in the item’s original location on each test question.

Misleading container false belief.  Using Perner, Leekam and Wimmer’s (1987) procedure, a closed, familiar container (e.g., bandaid, egg, jelly bean or crayon box) containing something unexpected (e.g., pencils, rubber balls, a toy car) was initially displayed and the first control question: “What is in this?” was followed by demonstration of the box’s true contents. The box was resealed, followed by the test question: “X [name of classmate] is coming next. He/she hasn’t seen inside this box. I will show it to him/her like this, closed up. What will he/she say is in it?” and the final control question: “What is really in the box?”

Scoring procedure

Comprehension control precautions.  As can be seen above, the control questions all involved similar syntax and vocabulary to the test questions. Thus perfect, or near-perfect, performance on all six control questions indicated that children had the language skills, attention span, and procedural memory needed in order to deem their test question responses to be valid indicators of ToM understanding. Conversely, as a conservative precaution to guarantee that children’s scores were not biased by linguistic or procedural confusion, and accurately reflected their cognition, if a child failed two or more of the six control questions, he or she was excluded from the sample and none of his/her data were reported here. (Interestingly, even though several young deaf children failed to gain entry into the study in the first place for this reason, no child was lost at subsequent tests for control question failure). In other words, all children maintained at least 83% control question success throughout their four or so years of longitudinal testing, reflecting the fact that even the older and more experienced children took the tasks seriously and attended carefully, in contrast to some other studies reporting high ToM control question failure rates by older deaf and hearing children (e.g., Meristo et al., 2007, Experiment 1, where 67% of hearing 9-year-olds failed a true-belief control question).

Given the present stringent control precautions, if a child failed the single control question they were allowed to fail without being excluded from the sample, momentary inattention was deemed to be the most likely reason. The trial was thus failed irrespective of test question accuracy, but the child was retained in the sample. Since failure of a single control question resulted in trial failure even if the associated test question was passed, children could earn total ToM scores of zero in two different ways: either (a) they could have passed all six control questions but failed all three false belief test questions (67% overall question accuracy), or (b) they could have passed one test question but failed its associated control question (67% overall question accuracy). Another possibility was (c) that a child might also fail the test question on the same trial as their single permitted control question failure. Since (c) reflected an even lower overall level of question accuracy (56%) than the zero scores earned for (a) and (b) above, a ToM score of --1 was given for pattern (c).

Total ToM score.  At each of their longitudinal tests, children received a total ToM score reflecting the sum of the false belief trials (out of 3) to which they had responded perfectly with correct responses to all test and control questions. A total ToM score of 3 therefore reflected complete accuracy on these 9 questions (3 test, 6 control), while a score of 2 reflected correct answers to at least 8 of them (89% accuracy). Therefore, as a ToM pass criterion (needed for comparing present results with those of previous studies using pass/fail rather than continuous scoring: e.g., Baron-Cohen et al., 1985; Peterson & Siegal, 1995, Russell et al., 1998), a total score of at least 2 was required for a pass, in keeping with earlier studies.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References

Table 1 shows individual children’s total ToM total scores at each longitudinal test, while Table 2 shows the mean ToM totals and mean ToM gain scores between adjacent tests for the whole group. Figure 1 graphically displays each individual child’s longitudinal trajectory of ToM performance across the period through which his or her development was followed.

Table 2. ToM means and ToM gain scores across successive tests for the longitudinal sample
 Mean initial age in months (SD)Mean ToM total (SD)Percent passing ToM (n)Mean ToM gain since last testGain significance level
Test 1 (= 8)70.62 (16.23)−0.38 (0.92)0 (0)N/AN/A
Test 2 (= 8)99.75 (19.37)0.75 (0.89)25 (2)1.25= 0.038*
Test 3 (= 8)123.12 (20.59)1.75 (0.71)62 (5)1.00= 0.007**
Test 4 (= 3) 143.33 (4.16)2.00 (1.00)67 (2)0.33N/A

As shown in Table 2, on the first test (when the group had mean age of 5 years 10 months), ToM performance was generally poor, in line with much previous research. Indeed, only three of the deaf children passed a single ToM task (38%), and about half failed one of the six comprehension control questions. However, by their second test (at a mean age of 8.33 years), performance had improved significantly. There was an overall mean gain according to a matched-pair t test, t (7) = 2.55, < 0.05, and two children (25%) now met the predetermined pass criterion for full ToM understanding, in contrast to none doing so on the first test. In addition, all children scored perfectly on all control questions at Test 2, and half passed at least one complete task.

From the second to the third longitudinal test, the group’s gain in false belief understanding was also statistically significant, t (7) = 3.74, < 0.01. By this time (at a mean age of 10), a majority of 5 children (62%) now met the ToM pass criterion and all of them passed at least one complete false belief test. The overall gain from Test 1 to Test 3 was also statistically significant, t (7) = 4.82, < 0.005, as was to be expected given the significance of each of the intermediate gains. The number of children in the fourth testing session was too small to permit statistical comparisons. Descriptively, however, performance continued to improve. Upon exit from the study, six of the eight children (75%) had met the ToM pass criterion at some point in their development, even though one child failed his fourth test after having passed consistently on Tests 2 and 3 (see Table 1). (Possibly this latter child’s performance on his final test was undermined by the same motivational and/or performance factors that had led a majority of hearing 9-year-olds to fail a true belief control task in Meristo et al.’s study, as noted above.)

In addition to the group patterns shown in Table 2, the individual developmental trajectories depicted in Fig. 1 reveal a uniformly consistent pattern of developmental gain in ToM understanding. Each of the eight individuals displayed visible improvement over their period in the study, as depicted in Fig. 1. The overall trajectory in this graph is likewise one of marked improvement in false belief performance from early childhood to adolescence. Furthermore, it is notable that a substantial majority of these late-signing deaf children managed to achieve the ToM pass criterion by their third or fourth test (i.e., by age 12), outperforming many of the older high school students in Russell et al.’s (1998) and Edmondson’s (2006) cross-sectional studies.

Whether the differences between these studies’ longitudinal versus cross-sectional methodologies might have contributed to this discrepancy in results requires further investigation. The present longitudinal approach does have certain advantages. For one thing, by comparing each child’s performance against his/her previous baseline, the longitudinal method is more sensitive to individual growth trajectories and to progressive gains that fall short of perfect understanding. For example, as shown in Fig. 1, even though only two of the children in the present sample (25%) managed to progress from failure all the way to the ToM pass criterion between their first and their second test, a further five children (62%) made discernable gains over this period which would have gone unnoticed using cross-sectional methodology. The striking individual variability, even in this small sample, in eventual age when a ToM pass was achieved (age 7 to age 12) would also have been harder to discern with a cross-sectional or a shorter-term longitudinal design.

The more extensive control batteries and language checks that the present longitudinal design allowed for may also have contributed clarity to the present results. By ensuring that all children fully understood procedures, questions and task language, there may have been less noise in the data from spurious accuracy due to guessing. [For example, all children in the present sample passed at least 17 comprehension control questions, as compared to 4 and 2, respectively, in Russell et al.’s (1998) and Edmondson’s (2006) cross-sectional studies and none at all in Morgan and Kegl’s (2006)]. When the overall longitudinal span through which children are followed is lengthy (e.g., the 9 years encompassed by the present study), the use of a longitudinal design can also heighten sensitivity to individual growth trajectories that are unusually protracted or uneven. For example, Fig. 1 reveals considerable variability in shapes of the eight individual growth curves: one child’s performance remained unchanged from age 5 to 9 followed with a sharp rise to criterion by age 12; others displayed performance gains from ages 5 to 9. Yet even these varied sharply in slope. Individual differences like these might not be perceived so clearly using the cross-sectional method.

Counterbalanced against these longitudinal advantages is the signal disadvantage of selective attrition. This is of special concern when sampling from groups of low population prevalence, including the prelingually profoundly deaf. In the present study, children were lost when their families moved out of school catchment areas, opted to place a child in a mainstream school, or simply neglected to sign the one of the many consent forms needed to maintain inclusion in the study. Each of these reasons for dropping out, as well as others, could conceivably be related to ToM performance in non-random ways, especially if residentially-stable parents who sustained an interest both in scientific research and in signing schooling supplied their children with other advantages not so prevalent among the more diverse sample groups likely to be included in cross-sectional studies. The only previous longitudinal study of deaf children’s ToM (Falkman et al., 2007) had a similarly small sample, possibly for similar reasons.

Bearing these methodological contrasts and limitations in mind, the present longitudinal findings hint, at least tentatively, that when ToM problems arise in the context of deafness they may often take the form of temporary delays rather than lasting deficits (Baron-Cohen, 1991), an encouraging possibility that warrants further investigation. Ideally, in order to supply conclusive evidence, such future research should encompass a broad range of ToM measures in addition to false belief (e.g., Peterson & Wellman, 2009; Wellman & Liu, 2004), and should track larger and more diverse samples of deaf children across an age range at least as extended as that encompassed by the full data set in Fig. 1.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References

When the collective results of the numerous cross-sectional studies summarized above are drawn together with the evidence from these two small, but suggestive, longitudinal ones, a number of working hypotheses about deaf children’s socio-cognitive development of ToM understanding suggest themselves. First, for most severe or profoundly deaf children of hearing parents, the development of the skills needed to pass false belief tests is likely to be an unusually protracted process extending well into, and perhaps beyond, middle childhood. Second, ToM delays are likely to arise irrespective of whether deaf children of hearing parents eventually become fluent users of some form of sign language versus skilled speakers and lip-readers equipped with cochlear implants or other prostheses. Nor is there evidence that ToM outcomes differ as a function of mainstream versus specialist, sign-oriented schooling. Third, early and continuing access to signing deaf conversational partners both at home and at school seems to be crucial for deaf children’s timely ToM mastery. Even native signers with unobstructed access to signed conversations at home may encounter ToM delays when educated in oral-only classrooms, in sharp contrast to other native signers in schools where teachers and peers use some form of signing as well as speech (Meristo et al., 2007). Indeed native signers who have easy conversational access to other deaf signers at home as well as at school are found to progress to an understanding of false belief (e.g., Peterson & Siegal, 1999; Woolfe et al., 2002) and through other ToM milestones (e.g., Peterson et al., 2005; Peterson & Wellman, 2009) at least as rapidly as hearing children, or possibly even faster (Courtin & Melot, 1998). Fourth, if hearing parents are both able and willing to converse about mental states in sign with their deaf children, this seems to benefit the latter’s ToM development (Moeller & Schick, 2006), similar to the positive links found between spoken parental mentalistic conversations and rapid ToM mastery in hearing preschoolers (e.g., Harris, 2006; Peterson & Slaughter, 2003; Ruffman et al., 2002) and in ToM-delayed older children with autism (Slaughter et al., 2007). Fifth and finally, there are tentative hints, especially from longitudinal evidence, that a continuing though very gradual upward progression of ToM development may be more possible and more widespread for severely and profoundly deaf children than some previous cross-sectional evidence would have suggested.

This optimistic possibility warrants further investigation both for the sake of a complete and comprehensive understanding of how the social-cognitive ability to “read” others’ minds unfolds itself and also as a practical foundation for intervention efforts on behalf of optimal ToM development for signing and oral deaf children from hearing families.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Method
  5. Results and discussion
  6. Conclusions
  7. References
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