The Development of Theory of Mind in Deaf Children: A Systematic Review Pim A. C. Looze 10581529 Bachelorthesis University of Amsterdam 25-01-2016 Word count: 6026
Table of Contents
Abstract p. 3
The Development of Theory of Mind in Deaf Children p. 4
Method p. 10
Article Selection: Data Sources and Keywords p. 10
Inclusion Criteria p. 10
Results p. 11
ToM in Native and Non-Native Signers p. 11
ToM in Non-Native Signers and Oral Deaf Children p. 16
Conclusion and Discussion p. 19
References p. 23
Abstract
This systematic review summarizes the results of 23 studies of deaf children and their Theory of Mind (ToM) development, published between 2000 and 2015. Studies were included if they discussed native signers, non-native signers and/or oral deaf children. Moreover, they had to include a control group and the different groups had to be compared on ToM
performance. Results show that native signing deaf children do not lag behind their hearing peers. In contrast, non-native signers are often found to be substantially delayed in ToM. Differences found in ToM could not be attributed to whether a child communicates in sign language or speech. A consensus on the cause(s) of the observed difficulties and delays deaf children experience is yet to be reached. Intervention work surrounding this topic has recently begun and is already showing promising results. Therefore, it should be continued to prevent the ToM delay often observed in deaf children.
Keywords: theory of mind, false-belief, deaf, children Abbreviations: ToM: Theory of Mind
The Development of Theory of Mind in Deaf Children: A Systematic Review Not having access to the world of sound puts deaf children at risk of leading segregated lives, unable to participate in social dialogue with the hearing population (Marschark & Clark, 1993). One of the most severe consequences of profound deafness during early childhood is the impaired development of verbal language (Macaulay & Ford, 2006). Furthermore, deaf children are found to frequently experience difficulties when it comes to understanding other people’s emotions (Rieffe & Terwogt, 2000). It has been suggested that an impaired ‘Theory of Mind’ (ToM) can account for these difficulties. Having Theory of Mind is a person’s ability to understand that other people have minds, and those minds contain beliefs, knowledge, desires, and emotions that may be different from those of the person himself (de Villiers & de Villiers, 2014). When someone can figure out the content of other people’s minds, their behavior will begin to make sense to this person. Consequently, other people’s behavior will become predictable and explicable (de Villiers & de Villiers, 2014). Therefore, it is not surprising that this skill has shown to be an essential element for successful communication (Premack & Woodruff, 1978). Typically developing hearing children acquire ToM between 4 and 5 years of age (Wellman, Cross, & Watson, 2001). Most often, ToM is measured with false-belief tasks. According to Dennett (1978), these false-belief tasks represent ToM because the understanding that people’s actions are not determined by the real state of the world, but by their own mental representations of the world is needed in order to succeed on such tasks.
Baron-Cohen, Leslie and Frith (1985) have created one of the most renowned false-belief tasks worldwide: the Sally-Anne task. A brief explanation of this task will be given to clarify what such a false-belief task entails. The Sally-Anne task has two dolls as main
characters that are, not surprisingly, named Sally and Anne. Firstly, Sally places a marble into a basket, after which she leaves the scene. The marble is then transferred by Anne and hidden
in her box. After Sally returns, the experimenter asks the participant, who has witnessed these series of events, the following question: “Where will Sally look for her marble?”. If the participant points to basket, thus understanding that Sally did not witness Anne transferring the marble, they pass the question. However, if they point to the current location of the marble, the box, they fail the question by not taking Sally’s belief into account. Conclusions from this test can only be drawn if the two control questions are answered correctly: “Where is the marble really?” and “Where was the marble in the beginning?”.
Wellman and Liu (2004) have demonstrated that there is a certain sequence of five steps when it comes to developing ToM: (a) diverse desires: knowing that different people may want different things, (b) diverse belief: knowing different people may have different beliefs about the same situation, (c) perceptual access to knowledge: knowing that not seeing something leads to ignorance of it, (d) false belief: predicting a naïve observer’s false belief about misleading contents, and (e) hidden emotion: awareness that expressed emotion may not match true, subjective emotion. This study suggests that false belief might represent only a certain fragment of the acquisition of ToM. Therefore, false-belief tasks might not be as representative as often and long presumed.
Language development is thought to be critically connected to the development of ToM (de Villiers & de Villiers, 2014). De Villiers and de Villiers (2014) give three main arguments to support this claim. First of all, it seems necessary to be able to hear language used about feelings, desires and thoughts in order to learn how to understand and express these concepts in our culture. This approach to ToM development focuses on the importance of learning words as labels for mental states that may not be directly observable in behavior. Secondly, the information that conversations contain seems important. Information about minds, such as feelings, desires and thoughts, seems to be richer when it is conveyed through conversation rather than through behavior, eye gaze or gestural expression. Finally, the role
played by the child’s own language mastery is emphasized. Mastering the appropriate linguistic structures gives the child a new ability to reason about the contents of others’ minds.
The crucial role of language in the development of ToM has also been demonstrated by empirical research. For example, Astington and Jenkins (1999) have shown that earlier language abilities predicted later ToM test performance. However, the exact mechanism by which language influences ToM is still disputed (Peters, Remmel, & Richards, 2009). For instance, Schick, de Villiers, de Villiers, and Hoffmeister (2007) found an understanding of syntactic complements specifically to be important in acquiring a mature ToM, rather than general language. Syntax can be described as the arrangement of words and phrases to create well-formed sentences in a certain language (Oxford English Dictionary, 1989). Syntactic complements are linguistic structures where one sentence is embedded within another
(Hauser, Chomsky, & Fitch, 2002). In contradiction with the findings of Schick et al. (2007), Cheung et al. (2004) found general language skills to correlate more highly with ToM
performance than complementation. In addition, the use of mental state language, which is the use of words that refer to the mind, has been found to correlate with performance on false-belief tasks (Brown, Donelan-McCall, & Dunn, 1996). Although it is clear that language plays a key role, different areas of language are implicated as determinants for the ToM
development.
Rather than language as crucial ingredient for the ToM development, it is also possible that more general features of early communication play a role in the first stages of social-cognitive development (Morgan et al., 2014). For instance, studies of spontaneous ToM abilities, such as visual preferences and pointing gestures, have shown that social-cognitive abilities can already be observed several months before children use language. For typically developing hearing children, social-cognitive abilities and social emotional understanding
have been linked to the use of mental states in family conversations (Brown et al., 1996). Furthermore, through interactions between mothers and their deaf children, access to mental state language has been found to be an important predictor of the ToM development (Moeller & Schick, 2006). Therefore, besides language development, it is also possible that general early communication features play a role in the development of ToM.
The study of deaf children offers a unique opportunity to investigate the ToM development and its connection to language (Stanzione & Schick, 2014). Because of their different ranges in language and communication experiences, and due to the apparent crucial role of language in the development of ToM, it is likely that deaf children experience
difficulties with the development of this skill. Research has actually shown that deaf children consistently show problems in ToM (e.g. Jones, Gutierrez, & Ludlow, 2015). This is thought to be due to difficulties in language acquisition and opportunities to talk about mental states (Jones et al., 2015). At least two factors seem to be of influence on the ToM development of deaf children: whether they are born to deaf- or hearing parents and whether they use sign- or spoken language.
Firstly, differences are found between deaf children born to deaf parents and deaf children born to hearing parents. When deaf children have deaf parents, they generally learn and converse in sign language only, from the moment they are born. Therefore, they will be referred to as native signers. When deaf children have hearing parents, they are usually not exposed to sign language from birth, since most hearing parents do not master sign language yet. Therefore, they tend to learn sign language at a later age than deaf children with deaf parents, and hence are referred to as non-native signers. However, when deaf children have hearing parents, they do not always end up using sign language (Peterson, 2004). Some of these parents choose a purely oral approach to family communication and mainstream oral-only schooling. These deaf children are referred to as oral deaf children. Oral deaf children
can be defined as children who converse in speech rather than sign language (Woolfe, Want, & Siegal, 2002). This review will first look at the differences between native and non-native signers. Oral deaf children will be left out of this comparison, to make sure that possible differences due to the use of sign language or spoken language will be minimized. Furthermore, considering non-native signers lack early language development and native signers do not, this comparison will clarify the role of early language development for ToM.
When it comes to language development, native signers and hearing children appear to be quite similar to each other. Children learning sign language from early childhood show remarkable resemblance in the onset, rate and patterns of development compared to children learning spoken languages (Woll & Morgan, 2012). This is not surprising, since sign language is considered to be a natural language (Mayberry & Fischer, 1989). Therefore, anything that can be said through spoken language can be communicated through sign language as well. For instance, all signed languages have manual signs to express mental states (Morgan & Kegl, 2006). Moreover, it was found that sign languages and spoken languages are processed by the brain in the same way (Petitto et al., 2000). Therefore, it can be expected that native signers perform similar to hearing children on ToM tasks.
In contrast to native signers and hearing children, non-native signers are likely to be delayed in their ToM development. Since language is thought to be critically connected to ToM (de Villiers & de Villiers, 2014), non-native signers’ lack of early language development is likely to negatively affect their ToM development. Research by Vaccari and Marschark (1997) has shown that the families of deaf children with hearing parents have limited skills in sign language. Consequently, the topics they discuss with their deaf children have a limited complexity (Moeller, 2002). This makes it plausible that the children’s reduced opportunity to converse about mental states leads to mind-reading difficulties. Furthermore, the parents’ lack of sign language skills will reduce their ability to teach their children words, or signs, as
labels for mental states. Consequently, the child’s own language mastery will likely lag behind typically developing hearing children. Since most severely or profoundly deaf children are born to hearing parents (90%) (Marschark, 1993; as cited in Peterson, 2004), which means the majority of deaf children are non-native signers, this is a substantial problem.
Secondly, when it comes to the role of language in ToM development, there is another interesting factor to look at: deaf children communicating through sign language or spoken language. Having hearing parents is likely to come with difficulties for the language
development of the deaf child. On one hand, in the case of non-native signers, parental sign language skills are often lacking, which causes several difficulties for their child. On the other hand, an oral deaf child will mostly depend on lip-reading, which is also likely to cause delays in language development (Peterson, 2004). It is not clear whether one of these approaches to language has a more promotional or harmful effect on the ToM development than the other. Much research has shown non-native signers to be delayed in their ToM development (e.g. Peterson & Siegal, 1995). Similarly, oral deaf children have been found to lag behind their hearing peers (Schick et al., 2007). Along with a delayed language development, restrictions upon the oral deaf child’s opportunities to exchange information about thoughts, feelings, or intentions may constrain the ToM development (Peterson, 2004). According to de Villiers and de Villiers (1999; as cited in Peterson, 2004), the limited speech, vocabulary and syntax typically presented in oral deaf children not exposed to sign language at the age of 4, is insufficient to support elaborate mind-talk, especially reference to others’ beliefs. This
corresponds with the findings that oral deaf children are 3 to 4 years delayed in their ToM development compared to their hearing peers (de Villiers & Pyers, 2000; as cited in Jackson, 2001).
The purpose of this systematic review is to create an overview of recent literature about different groups of deaf children and their ToM development. This will be investigated
by further looking into the existing literature on the two factors named above. Although most research points towards the same population of deaf children that is experiencing difficulties with ToM, the magnitude of their delay is still disputed. Furthermore, many different factors are indicated as crucial factors for this development. This systematic review will shed light on what is known about deaf children and their ToM development, and which areas are in need of more research. For example, by knowing which areas to target, interventions can be
developed to support their ToM development. This will lead to a higher chance at success and ensuring deaf children are not unnecessary lagging behind their hearing peers. Since ToM is essential for successful communication, and because solely being deaf already comes with restrictions for communicating with the world of sound, this is specifically important for deaf children. To sum up, a clear conclusion on the magnitude and causes of the ToM
developmental delays of certain groups of deaf children is still lacking. This review can clarify what is known about this subject and provide some useful information to give deaf children more equal opportunities to develop this skill that is crucial for successful
communication.
Method Article Selection: Data Sources and Keywords
A systematic review was conducted by using the following combination of terms in the literature search: (theory of mind OR false-belief) AND (deaf*) AND (child*). The databases Web of Science, PsycINFO, ERIC and Medline were used to identify relevant records on the basis of these search criteria.
Inclusion Criteria
All articles published between 2000 and 2015 were considered for inclusion in this review. The search language was set at English. Furthermore, articles were included in this systematic review if they satisfied the following criteria: inclusion of at least one of the three
factors discussed in this review: native signers, non-native signers or oral deaf children; inclusion of at least one control group; and the different groups were compared on the basis of their ToM performance.
At first, 35 articles were identified through database searching. After further assessing the eligibility of these articles based on the inclusion criteria, a total of 23 studies were retrieved to be discussed in this review.
Results
This systematic review has used 23 articles published between 2000 and 2015 to look at the development of ToM in deaf children. Firstly, the studies found about the ToM
development of native and non-native signers will be discussed. Secondly, the results from the studies about deaf children communicating in sign language or speech will be examined. The appendix contains a table that provides information about the sample sizes, mean ages of the participants, used measures of ToM and research designs of the 23 studies used in this review. ToM in Native and Non-Native Signers
Native signers usually have at least one deaf parent, while non-native signers are generally born to hearing parents. In contrast with deaf parents of deaf children, hearing parents with deaf children are challenged to fluently engage in conversation about mental states using sign language, which limits the conversational experience with their children (Lederberg & Everhart, 2000). As mentioned earlier, besides deaf children using sign language, there are also deaf children who use spoken language to communicate. However, this section will only compare the ToM of native signers and non-native signers, so
differences that might be due to the use of sign language or spoken language will be minimized and a fairer comparison can be made. Since native signers are exposed to sign language from birth and non-native signers only from a later age, comparing native signers
with non-native signers will shed light on the importance of early language development for the development of ToM.
Sign languages differ from all spoken languages. However, if this is one’s native language, it does not seem to have a major effect on the development of ToM. Many studies have shown that the development of ToM is alike for typically developing hearing children and native signers (Falkman & Hjelmquist, 2006; Meristo et al., 2007; Peterson, Wellman, & Liu, 2005; Schick et al., 2007). Courtin and Melot (2005) have found native signers to surpass hearing children on false-belief tasks. Furthermore, Woolfe et al. (2002) showed native
signers to excel in their ToM performance compared to non-native signers, even though the native signers were younger in age. However, a study by O’Reilly, Peterson, and Wellman (2014) showed contradicting results. In childhood, they found both native and non-native signers to perform worse at ToM tasks compared to hearing children. Still, the native signers had caught up in adulthood, while the non-native signers did not (O’Reilly et al., 2014). From all studies reviewed, this is the only to find native signers to perform worse on ToM tasks than hearing children in childhood. Since only ten native signers participated in this study, these contradictory results might be coincidental. Generally, being deaf and having deaf parents does not seem to significantly affect the usual trajectory of ToM development.
Where most research found no difference in ToM when it comes to native signers and typically developing hearing children, several studies on non-native signers showed a
different outcome. Non-native signers are consistently found to perform worse on ToM tasks than hearing children and native signers (Courtin & Melot, 2005; Falkman & Hjelmquist, 2006; Lecciso, Petrocchi, & Marchetti, 2013; Levrez, Bourdin, Le Driant, Forgeot d’Arc, & Vandromme, 2012; Meristo et al., 2007; Meristo et al., 2012; Moeller & Schick, 2006; O’Reilly et al., 2014; Peterson, 2002; Peterson & Slaughter, 2006; Peterson & Wellman, 2009; Peterson et al., 2005; Peterson, Wellman, & Slaughter, 2012; Schick et al., 2007;
Terwogt & Rieffe, 2004; Wellman, Fang, & Peterson, 2011; Woolfe et al., 2002). Even at the age of 8 to 10 years old, non-native signers were found to fail a standard false-belief task (Peterson et al., 2005). Similarly, Peterson et al. (2012) showed non-native signers to have a delay of more than five years compared to hearing children. Since typically developing hearing children are generally able to pass ToM tasks between 4 and 5 years of age (Wellman et al., 2001), it can be said that non-native signers are considerably delayed in their ToM development.
The importance of early exposure to language, early talk about mental states and conversational interaction are most commonly identified as the underlying mechanisms associated with the differences in ToM growth between native and non-native signing
children (Courtin & Melot, 2005; Falkman & Hjelmquist, 2006; Lecciso et al., 2013; Meristo et al., 2007; Meristo et al., 2012; Moeller & Schick, 2006; O’Reilly et al., 2014; Peterson, 2002; Peterson & Slaughter, 2006; Peterson & Wellman, 2009; Peterson et al., 2012; Schick et al., 2007; Woolfe et al., 2002). Since native signers are generally found to perform like their hearing peers, it seems that being deaf by itself is not a cause for a delayed ToM. However, when there is interference in the language development, like with non-native signers, the ToM development is often found to be delayed. Therefore, it seems likely that these commonly found causes and language development in general are indeed crucial for developing ToM.
The 5-item ToM scale of Wellman and Liu (2004) was identified through research with typically developing hearing children. Peterson et al. (2005) have shown that these steps apply to deaf children as well, and in the same order. Only the non-native signers were found to be delayed in ages of attainment. Similarly, Wellman et al. (2011) found that at each step, non-native signers acquire these understandings several years later than typically developing children and native signers. Peterson and Wellman (2009) have extended this five step
sequence, for both deaf and hearing children, to six sequential developmental steps by adding social pretence understanding. However, the placement of this sixth step in the ToM sequence differed for deaf and hearing children. Deaf children were found to understand social
pretending earlier in the ToM sequence than hearing children, although at a later chronological age (Peterson & Wellman, 2009).
Even though intervention work on ToM and deaf children is still in its first stages, it is already showing promising results. For instance, dramatic improvements on several ToM tasks were observed after an intensive six-week program of learning to use thought bubbles to talk about and represent beliefs. Results showed that the deaf children in primary school gained understanding of ToM concepts focal to their training and generalized their gains to a new type of false-belief task. Moreover, they progressed significantly on a broad
developmental scale of ToM concepts. In this six-week program, on average, the ToM-trained children gained a whole step of the five-item ToM scale developed by Wellman and Liu (2004). Longitudinal research by Wellman et al. (2011) suggests that acquiring a whole step without this training would require at least two years for a deaf child. However, it should be noted that the sample size of this study was modest, participants were not randomly divided between the conditions and this study has not been replicated yet. Fortunately, the delays that non-native signers are often found to experience are not intractable (Wellman & Peterson, 2013).
Even though most studies found non-native signers to be delayed compared to native signers and their hearing peers, one study showed contradicting, more positive results.
Marschark, Green, Hindmarsh, and Walker (2000) found non-native signers, between the ages of nine and fifteen, to be fully capable of attributing mental states to others, as well as to themselves. Interestingly, they did not use a standard false-belief task to measure ToM. They explored ToM by examining stories told by children for statements ascribing
behavior-relevant states of mind to themselves and others. Marschark et al. (2000) found 87% of the deaf children and only 80% of the hearing children to demonstrate ToM. However, from this study it cannot be concluded that non-native signers do not show a delay in ToM compared to hearing children, because the participants were older than the four to five years of age when ToM typically emerges in hearing children.
Since a different methodology showed contradicting results, Marschark et al. (2000) state that assertions that deaf children of this age lack ToM, based solely on their performance on a false-belief task, are wrong. Most researchers now consider false-belief understanding as only one of several aspects of ToM (Peterson et al., 2005). It is possible that the participants of the study by Marschark et al. (2000) possess some of the steps required for a mature ToM, but still would have failed a standard false-belief task. Since the use of a different method, which requires advanced language skills, shows such different results (Marschark et al., 2000), and because research (Wellman & Liu, 2004) has shown that there is more to ToM than solely false belief, it can be said that a false-belief task alone does not answer the question whether a child masters ToM or not.
To sum up, native signers do not seem to have a delay in ToM compared to typically developing hearing children. It is highly likely that this is due to the fact that they are exposed to and learn sign language from the moment they are born, which puts them on track in their language development, similar to hearing children. Unlike native signers, most research on non-native signers shows they are several years behind in both their language and ToM development. These studies often assume a causal relation between their lack of early language development and their ToM development. It is important to note that the use of a standard false-belief task alone to measure ToM can be disputed. This can be argued, since the use of a narrative methodology has shown different outcomes and false belief forms only
one step out of five in total when it comes to acquiring ToM (Marschark et al., 2000; Wellman & Liu, 2004).
ToM in Non-Native Signers and Oral Deaf Children
Besides non-native signers, who are generally known to perform worse at ToM than native signers and typically developing hearing children, there are also hearing parents who choose for an oral approach to family communication and/or schooling (Peterson, 2004). Oral deaf children communicate by verbal language and depend mostly on lip-reading (Peterson, 2004). Possible differences in ToM between oral deaf children and their hearing peers might be due to difficulties or delays in language development. Comparing oral deaf children with hearing children can shed light on whether their difficulties in language acquisition has an effect on their ToM development, and if so, the magnitude of this effect. Besides that, it is interesting to compare the different approaches to language of hearing parents who have deaf children. Since oral deaf children and non-native signers both show delays in language acquisition, they will also be compared on their ToM development. This way, it might become clearer whether delays in sign- and spoken language have a different effect on the development of ToM or if it is delay in language development itself that causes a delay in ToM.
Several studies have shown a delay in the ToM development of oral deaf children when they are compared to typically developing hearing children (Courtin & Melot, 2005; de Villiers & de Villiers, 2012; Lecciso et al., 2013; Peterson, 2004; Schick et al., 2007). For example, a study by de Villiers and de Villiers (2012) compared oral deaf children with their hearing peers, and found oral deaf children to perform significantly worse. They found language, in particular complement syntax, to be the best predictor of false-belief reasoning. Furthermore, all language measures used in this study showed oral deaf children to be significantly delayed. However, even when language demands were made minimal, their
results proved oral deaf children to have a significant delay in ToM compared to hearing children.
Lecciso et al. (2013) have also compared oral deaf children with hearing children. Again, oral deaf children performed worse than the hearing children, on two different false-belief tasks. In this study, the hearing mothers of both the oral deaf and hearing children have taken the same two false-belief tasks. One task assessed their ability to read complex
emotional mental states from gazes in photographs of people’s eyes, and the other task investigated the understanding of inappropriate actions. Interestingly, it was found that the mothers of the oral deaf children performed worse than those of the hearing children on both ToM tasks. No explanation was found for this remarkable finding. However, it is possible that an explanation can be found in the area where these mothers differ most from each other. Due to different communication opportunities, the interactions of these mothers with their deaf- and hearing children differ from each other. The finding that the mothers of the deaf children referred more often to concrete objects in conversations instead of to mental states
corroborates this possible explanation (Lecciso et al., 2013).
In addition, research has been done on deaf children with a cochlear implant. This is an electronic implant that transfer sounds to electric impulses. Through a cochlear implant, sounds and speech can be made perceptible to a limited extent. Therefore, children with a cochlear implant are able to communicate orally more easily. Remmel and Peters (2009) researched ToM in children with a cochlear implant. The results showed no delay on either ToM or spoken language, relative to the hearing children. Their results suggest that having a cochlear implant can benefit spoken language ability, which may then benefit ToM, perhaps by increasing access to mental state language. Furthermore, Ziv, Most, and Cohen (2013) have shown oral deaf children with cochlear implants to perform similar to hearing children on false-belief tasks as well.
Whether deaf children born to hearing parents use sign language or communicate orally, there are many indications that both groups are generally delayed in their ToM development compared to typically developing hearing children. Just like spoken language, sign language is considered to be a natural language (Mayberry & Fischer, 1989). Therefore, no differences were expected between non-native signers and oral deaf children on ToM performance. Findings of a study by Courtin and Melot (2005) corroborate this expectation. Their results showed these two groups to perform no different from each other on two different ToM tasks. Furthermore, Peterson (2004) found that the rate of language development and verbal maturity are significant predictors in the variability in ToM.
Moreover, peer interaction and early fluent communication with peers and family, whether in sign or speech, is likely important to optimally facilitate the growth of social cognition and language. So it is not about sign or speech, but it is only the rate of language development and verbal maturity that matters for ToM (Peterson, 2004).
In conclusion, oral deaf children are generally found to perform worse on ToM tasks than typically developing hearing children. This is probably due to the fact that their language development is delayed. Overall, it is found that non-native signers and oral deaf children perform alike on ToM tasks. These similarities are not surprising, since language
development delays are often observed in both groups. However, it is interesting to see that the usage of sign language or speech is not of influence on the ToM development. This corresponds to the fact that native signers are mostly found to perform similar to typically developing hearing children. Both these groups are native speakers in respectively sign language and spoken language, just like non-native signers and oral deaf children are both non-native speakers in their own language. It has become clear that the use of sign language or speech is likely not of influence on the ToM development. Furthermore, these findings highlight the importance of early language development for a timely development of ToM.
Conclusion and Discussion
The purpose of this systematic review is to create an overview of recent literature on the development of ToM in deaf children. Based on the results of this systematic review, several conclusions can be drawn. First, when deaf children have deaf parents, and are
therefore native speakers of sign language, there is no discrepancy between their performance on ToM tasks and that of typically developing hearing children. Second, deaf children who have hearing parents and are taught to communicate in sign language, non-native signers, are generally found to do worse than their hearing peers and their deaf peers born to deaf parents. Third, oral deaf children have shown to be delayed in acquiring ToM as well. Fourth, no differences are found between oral deaf children and non-native signers. This brings us to the final conclusion that can be drawn from this review: the differences observed in the ToM development of deaf children could not be attributed to whether a child communicates in sign language or speech.
However, there are some alternative explanations for the results found in this review. For instance, different methodologies have been used to measure ToM. Mostly this concerns the use of different false-belief tasks. Furthermore, different adjustments have been made to make these false-belief tasks understandable for deaf children, such as adapting a verbal task to a low- or non-verbal task. Besides alternative explanations, another factor was identified that might be of influence on the development of ToM: the different school environment that deaf children can attend. There are four different types of school environments that deaf children can attend: regular schools, where they are usually assigned a teaching assistant to translate; deaf schools, where sign language is the only form of communication; oral schools for deaf children, where there is only oral communication and the children depend mostly on lip-reading; and bilingual schools, where a combination of sign- and spoken language is used.
This has not been taken into consideration in this review, since only little research has been done on this specific factor.
However, Tomasuolo, Valeri, Di Renzo, Pasqualetti, and Volterra (2012) have done research on two different school environments deaf children can attend. The results showed that deaf children attending a regular school performed significantly worse than their hearing classmates and deaf children attending a bilingual school. Furthermore, deaf children
attending the bilingual school outperformed their hearing classmates. This corresponds to the findings of studies by Goetz (2003) and Kovács (2009), which have shown that bilingual children outperform their monolingual peers when it comes to false-belief tasks. Furthermore, a study by Meristo et al. (2007) found that native signers attending a bilingual school
outperformed the native and non-native signers attending an oral-only school. From this it can be concluded that, particularly for native signers, access to sign language in a bilingual school environment may facilitate conversational exchanges. In turn, these exchanges promote the expression of ToM by enabling children to monitor other’s mental state effectively. Based on these two studies on school environments for deaf children, it seems that the ToM
development suffers when signing deaf children are educated in a regular or oral-only classroom.
There are three important limitations of this systematic review. Firstly, research on the different school environments of deaf children is too limited to include this factor in this review. Therefore, it has been excluded, even though this might be of influence on the ToM development of deaf children. Secondly, even though most studies carried out tasks
measuring language skills, besides ToM tasks, these results have not been taken into
consideration. Potential differences on ToM performance caused by differences in language skills might have been overlooked. Thirdly, the sample sizes of all studies reviewed were relatively small. This means that conclusions should be drawn with caution.
Based on these limitations, some suggestions for further research can be made. It seems very important take a closer look at what kind of school environment fits best for which groups of deaf children. This way, deaf children do not have to be disadvantaged by their school environment and maybe their school environment can even be promotional for their ToM development. More research is also needed to determine the exact underlying mechanisms that are essential for developing a mature ToM. At this moment, literature is suggesting many different factors as crucial elements for developing ToM. When it comes to language for instance, the understanding of syntactic complements, general language skills and the use of mental state language have been implicated as determinants or the development of ToM. Besides language, also more general features of early communication are thought to play an important role. For now, the underlying mechanisms for the development of ToM remain unclear.
Another area that requires more research is interventions for the development of ToM. Only recently, interventions have begun to improve false belief of deaf children by using thought bubbles to represent mental state understanding (Wellman & Peterson, 2013). ToM training may create new opportunities for everyday conversational exchanges, which will then further stimulate their ToM development (Wellman & Peterson, 2013). Furthermore, a study by Lecce, Bianco, Devine, Hughes, and Banerjee (2014) has suggested that for typically developing populations, targeted conversations about mental states significantly improve ToM performance. Clearly, we are still in the early stages of intervention work when it comes to ToM, especially for the ToM development of deaf children. Deaf children, especially with hearing parents, could benefit from this greatly, since it could lessen the restrictions put on their opportunities for communication.
For years, research has been studying the difficulties and delays that deaf children experience in their development of ToM. It has become very clear that non-native signers,
which form the majority of deaf children, experience difficulties when it comes to ToM. However, there is yet to be reached a consensus on the exact reasons for these difficulties. It is important for deaf children to be assisted in their ToM development, so their social skills do not suffer because of their deafness. This is especially important since their deafness already puts restrictions upon their communication opportunities with the world of sound. More interventions should be developed to prevent this delay and to create more equal opportunities for deaf children to develop this skill that is crucial for successful communication.
References
Astington, J. W., & Jenkins, J. M. (1999). A longitudinal study of the relation between language and theory-of-mind development. Developmental Psychology, 35, 1311– 1320. doi:10.1037/0012-1649.35.5.1311
Baron-Cohen, S., Leslie, A. M., & Frith, U. (1985). Does the autistic child have a “theory of mind”?. Cognition, 21, 37–46. doi:10.1016/0010-0277(85)90022-8
Brown, J. R., Donelan-McCall, N., & Dunn, J. (1996). Why talk about mental states? The significance of children’s conversations with friends, sibling, and mothers. Child Development, 67, 836–849. doi:10.1111/j.1467-8624.1996.tb01767.x
Cheung, H., Hsuan-Chih, C., Creed, N., Ng, L., Wang, S. P., & Mo, L. (2004). Relative roles of general and complementation language in theory-of-mind development: Evidence from Cantonese and English. Child Development, 75, 1155–1170. doi:10.1111/j.1467- 8624.2004.00731.x
Courtin, C., & Melot, A. M. (2005). Metacognitive development of deaf children: Lessons from the appearance–reality and false belief tasks. Developmental Science, 8, 16–25. doi:10.1111/j.1467-7687.2005.00389.x
de Villiers, P. A., & de Villiers, J. G. (2012). Deception dissociates from false belief
reasoning in deaf children: Implications for the implicit versus explicit theory of mind distinction. British Journal of Developmental Psychology, 30, 188–209. doi:10.1111/j. 2044-835X.2011.02072.x
de Villiers, J. G., & de Villiers, P. A. (2014). The role of language in theory of mind
development. Topics in Language Disorders, 34, 313–328. doi:10.1097/TLD. 0000000000000037
Dennett, D. C. (1978). Beliefs about beliefs. Behavioral and Brain Sciences, 1, 568–570. doi:10.1017/S0140525X00076664
Falkman, K. W., & Hjelmquist, E. (2006). Do you see what I mean? Shared reference in non- native, early signing deaf children. Journal of Deaf Studies and Deaf Education, 11, 410–420. doi:10.1093/deafed/enl002
Goetz, P. (2003). The effects of bilingualism on theory of mind development. Bilingualism: Language and Cognition, 6, 1–15. doi:10.1017/S1366728903001007
Hauser, M. D., Chomsky, N., & Fitch, W. T. (2002). The faculty of language: What is it, who has it, and how did it evolve?. Science, 298, 1569–1579. doi:10.1126/science.298.
5598.1569
Jackson, A. L. (2001). Language facility and theory of mind development in deaf children. Journal of Deaf Studies and Deaf Education, 6, 161–176. doi:10.1093/deafed/6.3.161 Jones, A. C., Gutierrez, R., & Ludlow, A. K. (2015). Confronting the language barrier:
Theory of mind in deaf children. Journal of Communication Disorders, 56, 47–58. doi:10.1016/j.jcomdis.2015.06.005
Kovács, A. M. (2009). Early bilingualism enhances mechanisms of false-belief reasoning. Developmental Science, 12, 48–54. doi:10.1111/j.1467-7687.2008.00742.x
Lecce, S., Bianco, F., Devine, R. T., Hughes, C., & Banerjee, R. (2014). Promoting theory of mind during middle childhood: A training program. Journal of Experimental Child Psychology, 126, 52–67. doi:10.1016/j.jecp.2014.03.002
Lecciso, F., Petrocchi, S., & Marchetti, A. (2013). Hearing mothers and oral deaf children: An atypical relational context for theory of mind. European Journal of Psychology of Education, 28, 903–922. doi:10.1007/s10212-012-0146-1
Lederberg, A. R., & Everhart, V. S. (2000). Conversations between deaf children and their hearing mothers: Pragmatic and dialogic characteristics. Journal of Deaf Studies and Deaf Education, 5, 303–322. doi:10.1093/deafed/5.4.303
impact of verbal capacity on theory of mind in deaf and hard of hearing children. American Annals of the Deaf, 157, 66–77. doi:10.1353/aad.2012.1610
Macaulay, C. E., & Ford, R. M. (2006). Language and theory‐of‐mind development in prelingually deafened children with cochlear implants: A preliminary investigation. Cochlear Implants International, 7, 1–14. doi:10.1002/cii.22
Marschark, M., & Clark, M. D. (Eds.). (1993). Psychological perspectives on deafness. Hillsdale, NJ: Lawrence Erlbaum Associates.
Marschark, M., Green, V., Hindmarsh, G., & Walker, S. (2000). Understanding theory of mind in children who are deaf. Journal of Child Psychology and Psychiatry, 41, 1067–1073. doi:10.1017/S0021963099006496
Mayberry, R. I., & Fischer, S. D. (1989). Looking through phonological shape to lexical meaning: The bottleneck of non-native sign language processing. Memory & Cognition, 17, 740–754. doi:10.3758/BF03202635
Meristo, M., Falkman, K. W., Hjelmquist, E., Tedoldi, M., Surian, L., & Siegal, M. (2007). Language access and theory of mind reasoning: Evidence from deaf children in bilingual and oralist environments. Developmental Psychology, 43, 1156–1169. doi: 10.1037/0012-1649.43.5.1156
Meristo, M., Morgan, G., Geraci, A., Iozzi, L., Hjelmquist, E., Surian, L., & Siegal, M. (2012). Belief attribution in deaf and hearing infants. Developmental Science, 15, 633–640. doi:10.1111/j.1467-7687.2012.01155.x
Moeller, P.M. (2002). Mothers’ input and theory of mind understanding in deaf and hearing children (Unpublished PhD thesis). University of Nebraska-Lincoln, Nebraska. Moeller, M. P., & Schick, B. (2006). Relations between maternal input and theory of mind
understanding in deaf children. Child Development, 77, 751–766. doi:10.1111/j.1467- 8624.2006.00901.x
Morgan, G., & Kegl, J. (2006). Nicaraguan sign language and theory of mind: The issue of critical periods and abilities. Journal of Child Psychology and Psychiatry, 47, 811– 819. doi:10.1111/j.1469-7610.2006.01621.x
Morgan, G., Meristo, M., Mann, W., Hjelmquist, E., Surian, L., & Siegal, M. (2014). Mental state language and quality of conversational experience in deaf and hearing children. Cognitive Development, 29, 41–49. doi:10.1016/j.cogdev.2013.10.002
O’Reilly, K., Peterson, C. C., & Wellman, H. M. (2014). Sarcasm and advanced theory of mind understanding in children and adults with prelingual deafness. Developmental Psychology, 50, 1862–1877. doi:10.1037/a0036654
Oxford English Dictionary. (1989). Oxford: Oxford University Press.
Peters, K., Remmel, E., & Richards, D. (2009). Language, mental state vocabulary, and false belief understanding in children with cochlear implants. Language, Speech, and Hearing Services in Schools, 40, 245–255. doi:10.1044/0161-1461(2009/07-0079) Peterson, C. C. (2002). Drawing insight from pictures: The development of concepts of false
drawing and false belief in children with deafness, normal hearing, and autism. Child Development, 73, 1442–1459. doi:10.1111/1467-8624.00482
Peterson, C. C. (2004). Theory‐of‐mind development in oral deaf children with cochlear implants or conventional hearing aids. Journal of Child Psychology and Psychiatry, 45, 1096–1106. doi:10.1111/j.1469-7610.2004.t01-1-00302.x
Peterson, C. C., & Siegal, M. (1995). Deafness, conversation and theory of mind. Journal of Child Psychology and Psychiatry, 36, 459–474. doi:10.1111/j.1469-7610.1995. tb01303.x
Peterson, C. C., & Slaughter, V. P. (2006). Telling the story of theory of mind: Deaf and hearing children's narratives and mental state understanding. British Journal of Developmental Psychology, 24, 151–179. doi:10.1348/026151005X60022
Peterson, C. C., & Wellman, H. M. (2009). From fancy to reason: Scaling deaf and hearing children's understanding of theory of mind and pretence. British Journal of
Developmental Psychology, 27, 297–310. doi:10.1348/026151008X299728
Peterson, C. C., Wellman, H. M., & Liu, D. (2005). Steps in theory‐of‐mind development for children with deafness or autism. Child Development, 76, 502–517. doi:10. 1111/j.1467-8624.2005.00859.x
Peterson, C. C., Wellman, H. M., & Slaughter, V. (2012). The mind behind the message: Advancing theory‐of‐mind scales for typically developing children, and those with deafness, autism, or Asperger syndrome. Child Development, 83, 469–485. doi:10. 1111/j.1467-8624.2011.01728.x
Petitto, L. A., Zatorre, R. J., Gauna, K., Nikelski, E. J., Dostie, D., & Evans, A. C. (2000). Speech-like cerebral activity in profoundly deaf people processing signed languages: Implications for the neural basis of human language. Proceedings of the National Academy of Sciences, 97, 13961–13966. doi:10.1073/pnas.97.25.13961
Premack, D., & Woodruff, G. (1978). Does the chimpanzee have a theory of mind?. Behavioral and Brain Sciences, 1, 515–526. doi:10.1017/S0140525X00076512 Remmel, E., & Peters, K. (2009). Theory of mind and language in children with cochlear
implants. Journal of Deaf Studies and Deaf Education, 14, 218–236. doi:10.1093/ deafed/enn036
Rieffe, C., & Terwogt, M. M. (2000). Deaf children's understanding of emotions: Desires take precedence. Journal of Child Psychology and Psychiatry, 41, 601–608. doi:10.1017/ S0021963099005843
Schick, B., de Villiers, P., de Villiers, J., & Hoffmeister, R. (2007). Language and theory of mind: A study of deaf children. Child Development, 78, 376–396. doi:10.1111/j.1467- 8624.2007.01004.x
Stanzione, C., & Schick, B. (2014). Environmental language factors in theory of mind development: Evidence from children who are deaf/hard-of-hearing or who have specific language impairment. Topics in Language Disorders, 34, 296–312. doi:10. 1097/TLD.0000000000000038
Terwogt, M. M., & Rieffe, C. (2004). Deaf children's use of beliefs and desires in negotiation. Journal of Deaf Studies and Deaf Education, 9, 27–38. doi:10.1093/deafed/enh013 Tomasuolo, E., Valeri, G., Di Renzo, A., Pasqualetti, P., & Volterra, V. (2012). Deaf children
attending different school environments: Sign language abilities and theory of mind. Journal of Deaf Studies and Deaf Education, 18, 12–29. doi:10.1093/deafed/ens035 Vaccari, C., & Marschark, M. (1997). Communication between parents and deaf children:
Implications for social‐emotional development. Journal of Child Psychology and Psychiatry, 38, 793–801. doi:10.1111/j.1469-7610.1997.tb01597.x
Wellman, H. M., Cross, D., & Watson, J. (2001). Meta-analysis of theory-of-mind
development: The truth about false belief. Child Development, 72, 655–684. doi:10. 1111/1467-8624.00304
Wellman, H. M., Fang, F., & Peterson, C. C. (2011). Sequential progressions in a
theory‐of‐mind scale: Longitudinal perspectives. Child Development, 82, 780–792. doi:10.1111/j.1467-8624.2011.01583.x
Wellman, H. M., & Peterson, C. C. (2013). Deafness, thought bubbles, and theory-of-mind development. Developmental Psychology, 49, 2357–2367. doi:10.1037/a0032419 Wellman, H. M., & Liu, D. (2004). Scaling of theory‐of‐mind tasks. Child Development,
75, 523–541. doi:10.1111/j.1467-8624.2004.00691.x
Woll, B., & Morgan, G. (2012). Language impairments in the development of sign: Do they reside in a specific modality or are they modality-independent deficits?. Bilingualism: Language and Cognition, 15, 75–87. doi:10.1017/S1366728911000459
Woolfe, T., Want, S. C., & Siegal, M. (2002). Signposts to development: Theory of mind in deaf children. Child Development, 73, 768–778. doi:10.1111/1467-8624.00437 Ziv, M., Most, T., & Cohen, S. (2013). Understanding of emotions and false beliefs among
hearing children versus deaf children. Journal of Deaf Studies and Deaf Education, 18, 161–174. doi:10.1093/deafed/ens073
Appendix
Table
Characteristics of Included Studies
Authors (year) n M age (y;m) [range] Measures of ToM Research design
FB task, 5-item ToM scale and/or other task
Courtin & Melot (2005) NS: 28 NNS: 26 ODC: 34 CG: 36 NS: 6;1 [4;10–7;6] NNS: 6;6 [4;11–7;6] ODC: 6;6 [5;0–7;5] CG: 5;11 [4;11–7;3] • False-belief task • Other task Cross-sectional design
De Villiers & De Villiers (2012)
ODC: 45 CG: 45
ODC: 5;10 [4;6–7;11] CG: 4;4 [3;1–5;11]
• False-belief task Correlational design
Falkman & Hjelmquist (2006) NNS: 10 CG: 10
NNS: 9;4 [7;4–11;3] CG: 9;5 [7;7–11;3]
• False-belief task Correlational design
Lecciso, Petrocchi, & Marchetti (2013)
ODC: 17 CG: 17
ODC: 9;1 [5;6–14;6] CG: 8;11
• False-belief task Cross-sectional design
Levrez, Bourdin, Le Driant, Forgeot d’Arc, & Vandromme (2012)
NNS: 12 CG: 12
NNS: 10;8 [9;3–12;1] CG: 7;2 [6;7–8;3]
• False-belief task Correlational design
Marschark, Green,
Hindmarsch, & Walker (2000)
NNS: 15 CG:15
NNS: 13;1 [9;7–15;1] CG: 13;2 [10;6–15;5]
Meristo et al. (2007) NS: 56 NNS: 41 CG: 105 NS: 8;7 [4;6–12;8] NNS: 10;1 [6;5–12;9] CG1: 4;3 [3;5–4;10] CG2: 6;7 [5;1–6;11] CG3: 7;9 [7;0–8;8] CG4: 9;10 [9;0–10;8]
• False-belief task Correlational design
Meristo et al. (2012) ODC: 10 CG: 10
ODC: 1;11 [1;5–2;2] CG: 1;11 [1;7–2;4]
• False-belief task Correlational design
Moeller & Schick (2006) NNS: 22 CG: 26
NNS: 6;11 [4;3–9;11] CG: 5;0 [4;3–5;11]
• False-belief task Correlational design
O’Reilly, Peterson, & Wellman (2014) NS: 10 NNS: 32 CG: 39 NS: 9;0 [5;4–12;1] NNS: 9;3 [5;7–12;8] CG: age-matched
• False-belief task Correlational design
Peterson (2002) NNS: 21
CG: 25
NNS: 9;3 [6;8–12;6] CG: 4;9 [4;1–5;8]
• False-belief task Cross-sectional design
Peterson (2004) ODC: 26
CG: 17
ODC: 7;9 [4;2–12;1] CG: 4;10 [4;1–5;8]
• False-belief task Correlational design
Peterson & Slaughter (2006) Study 1: NNS: 21 Study 1: NNS: 8;8 [6;4–11;4] Study 1: • False-belief task Study 1: Correlational design
CG: 13 Study 2: NNS: 17 CG: 17 Control: 8;0 [6;11–10;1] Study 2: NNS: 9;8 [6–12] CG: 4;9 [4–5] Study 2: • False-belief task Study 2: Correlational design
Peterson & Wellman (2009) NNS: 33 CG: 60
NNS: 9;8 [5;10–13;6] CG: 4;5 [2;8–5;9]
• 5-items ToM scale • Other task
Correlational design
Peterson, Wellman, & Liu (2005) NS: 11 NNS: 36 CG: 62 NS: 10:8 [6;4–12;6] NNS: 10;0 [5;5–13;2] CG: 4;6 [3;10–5;9]
• 5-item ToM scale Cross-sectional design
Peterson, Wellman, & Slaughter (2012)
NNS: 31 CG: 29
NNS: 9;7 [6–12] CG: 8;9 [7;6–11;6]
• 5-item ToM scale Correlational design
Remmel & Peters (2009) ODC: 30 CG: 30
ODC: 7;6 [3;1–12;0] CG: 5;2 [4;6–6;5]
• False-belief task • 5-item ToM scale
Correlational design
Schick, De Villiers, De
Villiers, & Hoffmeister (2007)
NS: 49 NNS: 41 ODC: 86 CG: 42 NS: 6;1 NNS: 6;1 ODC: 6;1 CG: 5;4 [4;0–6;8] • False-belief task • Other task Correlational design
Terwogt & Rieffe (2004) NNS: 21 CG: 36
NNS: 11;8 [10;5–12;14] CG: 11;1 [9;8–12;11]
Wellman, Fang, & Peterson
(2011) NNS: 31 CG1: 31 CG2: 30 NNS: 8;3 [4;2–12;8] CG1: 3;6 [3;1–3;11] CG2: 3;11 [3;1–5;0]
• 5-item ToM scale Longitudinal design
Wellman & Peterson (2013) NNS ToM training: 13 NNS non-ToM training: 14 CG: 16 NNS ToM training: 9;10 [7;8–13;0] NNS non-ToM training: 8;8 [5;8–12;2] CG: 9;8 [6;8–13;2]
• 5-item ToM scale • False-belief task
Quasi-experimental design
Woolfe, Want, & Siegal (2002) NS: 20 NNS: 40 CG: 40 NS: 5;10 NNS: 6;8 CG: 3;11
• False-belief task Correlational design
Ziv, Most, & Cohen (2013) NS: 10 ODC: 20 CG: 23
NS: 6;2 [SD=0.75] ODC: 6;6 [SD=0.71] CG: 5;10 [SD=0.61]
• False-belief task Correlational design
Note. n = number of participants; M = mean age (years; months); ToM = Theory of Mind; NS = native signers; NNS = non-native signers; ODC = oral deaf children; CG = control group; SD = standard deviation; FB = false belief; 5-item ToM scale = (a) diverse desires, (b) diverse beliefs, (c) knowledge access, (d) false belief, and (e) hidden emotion.
