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					                        The Effects of Siblings on Twins and Singletons in Theory of Mind Tasks
                       Katie Butera, Ashley L. Melby, Kristin Bell, Sara W. Biebl, Sarah Long, Angela Phillips, and Lisabeth F. DiLalla
                                                       Southern Illinois University School of Medicine

Individual differences related to familial characteristics may be influential factors in theory of mind development. Recently, a few researchers have
attempted to investigate whether or not twins, in spending a significant amount of time with one another, are more likely to develop theory of mind
sooner than other sibling pairs. Some findings have suggested that there are no significant differences between twins and other sibling pairs in the
development of theory of mind, whereas other studies have disagreed. We examined this more thoroughly. The sample consisted of 65 twin pairs (20
MZ, 45 DZ) and 74 singleton children between the ages of 3 and 5 years. Each child was tested on Theory of Mind (ToM) false belief tasks. There was
no significant difference between MZ twins, DZ twins, and singletons on ToM, or between children with and without siblings. However, having more
younger siblings is related to an increased ToM. In addition, a larger age difference between the child and younger sibling was related to increases in
ToM. Increased parental sensitivity was also related to enhanced ToM performance for the twins in this sample. Therefore, having a twin does not
appear to enhance ToM. However, having younger siblings does appear to have an effect, possibly because children need to use ToM skills in order to
communicate effectively with younger siblings.

The development of Theory of Mind (ToM) plays an integral role in helping children to understand that human actions are governed by mental states.
Specifically, it is around the age of 4 that many children are able to understand that another’s actions are influenced by their beliefs, intentions, and
desires. These skills allow children to function socially with others and have been shown to be heritable in 3-year-old children (Hughes & Cutting, 1999;
Ronald et al., 2005). However, several studies have highlighted that there are individual differences in the development of ToM, with some children
developing earlier than others. Researchers have suggested that individual differences related to familial characteristics may be influential factors in
ToM development. In particular, some findings have suggested that children with siblings develop ToM earlier than only children (Cassidy et al., 2005;
Peterson, 2000). Recently, a few researchers have attempted to investigate whether or not twins, in spending a significant amount of time with one
another, are more likely to develop ToM sooner than other sibling pairs. Thus far, some findings have suggested that twin ToM performance is similar
to that of children without siblings (Cassidy, et. al), whereas others have noted that close-in-age siblings may increase ToM performance (Peterson,
2000). In addition, Hughes and Ensor (2005) noted that positive sibling relationships were correlated with increased ToM, but number of siblings was
not. Theory of mind also has been shown to be correlated with language development (Meins et al., 2002). The purpose of the present study was to
expand on the literature examining the development of ToM in twins by comparing them to singletons and by examining twin interactions as well as
controlling for language skills. We examined whether or not having younger and/or older siblings improves ToM and also specifically examined the
age differences of these siblings.
We hypothesized that children who have siblings would have higher ToM scores than those without siblings. However, having a sibling who is a
different age would improve ToM to a higher degree. Thus only children would have the lowest ToM scores, then twins, and then singletons with
close-age siblings. Also, children with close-aged siblings were expected to have higher ToM scores than those who have siblings who are very
different in age. Finally, DZ twins were expected to have higher ToM scores than MZ twins because they are more different from each other.

                                                                                                           Presented to the Behavior Genetics Association
                                                                                                           June, 2008, Louisville, KY
The sample consisted of 65 twin pairs (30 boys, 35 girls) and 74 singleton children (36 boys, 38 girls). The twins (20 MZ, 45 DZ) were part of a larger, longitudinal study, the
Southern Illinois Twins and Siblings Study (SITSS; DiLalla, 2002). Twins were tested at age 3 or 4 years (M = 44.5 months, sd = 5.5 months), and singletons were between the
ages of 3 and 5 years (M = 47.3 months, sd = 6.6 months).
Twin participants were brought to a campus lab within 1 week of their 3rd and/or 4th birthday. The zygosity of each twin pair was determined using a modified version of a
standardized zygosity assessment questionnaire (Nichols & Bilbro, 1966) and 92% also were determined based on buccal cell data. Across the entire SITSS sample,
questionnaire assessments were 96% accurate when compared to buccal cell data. Each twin was then tested individually for cognitive abilities using the Theory of Mind (ToM)
task (see below) and for verbal memory using the Memory for Sentences sub-test of the Stanford-Binet Intelligence Scale, 4th Edition (Thorndike, et al., 1986). The twins also
participated in a ten minute parent-twin triadic interaction. During this time parent sensitivity and twin cooperation with the co-twin was assessed. The singleton children were
tested using the ToM and Memory for Sentences tasks at their daycare facilities in an area separate from their peers. In addition, parents of all children were asked to fill out a
Family Information form, which included an area to record the ages of any siblings.
Theory of Mind Task
Each child was shown a closed Play-Doh container and asked what he or she thought was inside. Following his or her answer, the container was opened to reveal that crayons
were inside. The top was replaced and the child was asked the following probe questions and assigned the appropriate cognitive ability score.
-When I first showed you this Play-Doh container, what did you think was inside it?
  1 – No answer/wrong answer 2 – Play-Doh
- If I showed this to your co-twin/friend, what do you think he(she) would say?
  1 – No answer/wrong answer 2 – Play-Doh
The task was then repeated with a crayon box containing Legos. The four scores were summed to create a total ToM score for each child.

Theory of Mind (ToM) in Twins vs. Singletons
•There was no significant difference between MZ twins, DZ twins, and singletons on ToM (F(2,136) = 1.55, p = ns).
•There was also no significant difference between only children, twins without other siblings, twins with other siblings, and singletons with siblings (F(3,137) = 1.31, p = ns).
Means are presented in Table 1.
ToM Ability in MZ vs. DZ Twins
•Correlations were computed separately for MZ and DZ twins for ToM.
         • At age 3, neither MZ (r(18) = .07, p = ns) nor DZ correlations were significant (r(44) = .27, p = ns).
         • At age 4, both MZ (r(18) = .45, p < .05) and DZ correlations (r(43) = .37, p < .01) were significant.
                   • At age 4, h2 = 0.36 for ToM, but the MZ and DZ correlations were not significantly different from each other.
Impact of Siblings on ToM
• Correlations were calculated to examine relations between ToM and number and age of siblings (see Table 2).
         • After controlling for age and verbal memory, correlations were significant between ToM and number of younger siblings and between ToM and the age difference
           between participants and their next youngest sibling.
                   • There was no relation between ToM and having an older or same-aged sibling.
• Hierarchical multiple regression examined sibling characteristics as predictors of ToM after controlling for age and verbal memory.
         • The full model accounted for approximately 14% of the variance for ToM.
                   • Number of younger siblings accounted for a significant amount of variance in ToM above and beyond that accounted for by verbal memory and age, R2
                     change = .09, F(4,118) = 3.16, p < .05.
The Parent-Child Relationship and ToM in Twins
• A second hierarchical multiple regression examined whether parent-child relationship characteristics predicted ToM for twins.
         • After controlling for verbal memory, the full model accounted for approximately 25% of the variance for ToM.
                   • Parent sensitivity to children’s requests or emotions accounted for a marginally significant amount of variance in ToM above that which was accounted for
                     by verbal memory, R2 change = .07, F(1,44) = 3.81, p < .06.
                   • Responsiveness or cooperativeness toward one’s co-twin was not a significant predictor of ToM.
 Previous research has shown conflicting results concerning the influence of siblings on
theory of mind development. Some suggest having an older sibling is more helpful
(Ruffman et al., 1998), whereas others suggest that number of siblings has no effect                   Table 1. Theory of Mind (ToM) Sample Means
(Hughes & Ensor, 2005). Our study suggests that having younger siblings is actually related                                                Total N                  ToM Mean (SD )
to ToM performance. Also, twins performed similarly to singletons on ToM task.                        Only Children                               25                0.80 (1.26)
 This study contributes to the expansion of current literature on this topic. The sample             MZ Twins                                    20                1.15 (1.42)
size of the study is larger than previous studies such as Cassidy et al. (2005). In addition,         DZ Twins                                    45                0.71 (1.06)
we examined children with same age siblings (twins) as well as children with siblings of
                                                                                                      Singletons with Siblings                    48                1.25 (1.31)
different ages. The children in this study were tested between the ages of 3 and 5 years --
the appropriate age range for examining the development of theory of mind.
 This study demonstrated that having a younger sibling is related to ToM. Additionally, a
larger age difference between the child and younger sibling, as well as a larger number of
younger siblings, were related to increases in ToM.
            o It is possible that having a younger sibling causes children to adjust their own
            understanding of perspective-taking in order to teach or communicate effectively     Table 2. Summary of Regression Analyses Predicting ToM
            with a younger sibling. Having an older sibling may not require children to make     Variable                                                       ToM Total Sum
                                                                                                                                                  B                 SE B               β
            this adjustment because the older sibling is doing the teaching.
                                                                                                 Number of Children                             -0.33                0.20            -0.32
 It was also found for twins that greater parental sensitivity was related to children’s
                                                                                                 Number of Older Siblings                        0.09                0.27             0.05
enhanced ToM. This shows that a strong parent-child relationship may also be an important
                                                                                                 Number of Younger Siblings                      0.77                0.31             .31*
aspect in the development of ToM.                                                                Number of Same-age Siblings                     0.49                0.32             0.21
                                                                                                 R2 Change                                                           0.09

-Cassidy, K. W., Fineberg, D. S., Brown, K., & Perkins, A. (2005). Theory of mind may be              Figure 1.
  contagious, but you don’t catch it from your twin. Child Development, 76(1), 97-106.                              Correlation between ToM and Sibling Characteristics
-Hughes, C., & Cutting, A.L. (1999). Nature, nurture, and individual differences in early
  understanding of mind. Psychological Science, 10, 429-432.                                                0.5
-Hughes, C., & Ensor, R. (2005). Executive function and theory of mind in 2 year olds: A
  family affair? Developmental Neuropsychology, 28(2), 645-668.
-Meins, E., Fernyhough, C., Wainwright, R., Gupta, M.D., Fradley, E., & Tuckey, M. (2002).                  0.3
  Maternal mind-mindedness and attachment security as predictors of theory of mind
  understanding. Child Development, 73, 1715-1726.
-Peterson, C. C. (2000). Kindred spirits: Influences of siblings’ perspectives on theory of                 0.1
                                                                                                                                                                                ToM Score
  mind. Cognitive Development, 15, 435-455.
-Ronald, A., Happé, F., Hughes, C., & Plomin, R. (2005). Nice and nasty theory of mind in
  preschool children: Nature and nurture. Social Development, 14, 664-684.                                  -0.1

                                                                                                                   Older   Younger   Same-              Next       Next
                                                                                                                                      age               Older     Younger

                                                                                                                           Number of Siblings           Age difference
We would like to thank all of the twin families who have participated over the years. We
would also like to thank the daycare directors, who were extremely helpful and supportive,
as well as the daycare families for their participation in this study.

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