Sibling influences on theory of mind development for children with ASD Karen O’Brien, Virginia Slaughter, and Candida C. Peterson University of Queensland, Australia Background: Research indicates that having child siblings is positively associated with theory of mind (ToM) in typically developing children. As ToM is important to everyday social behaviours it is important to extend this research to examine whether there are similar sibling effects for children with autism spectrum disorders (ASD). Methods: Theory of mind and executive functioning abilities of 60 children clinically diagnosed with ASD were assessed with batteries of standard tasks. Verbal mental age (VMA) and severity of autism symptoms were also measured together with number of child-aged siblings (1 to 12 years) and position in the sibling constellation. Results: Having older siblings was a significant negative predictor of ToM performance for children with ASD, even after controlling for age, VMA, executive function and autism symptom severity. A weaker ToM benefit of younger siblings was not statistically significant independently of control variables. Conclusions: In sharp contrast to findings for typically developing preschoolers, having an older sibling was a disadvantage for ToM development in children with ASD. Conceivably, older siblings may over-compensate for their younger ASD siblings in social interactions, thereby limiting opportunities for social-cognitive growth. Parental attitudes, family resources, cultural norms and access to educational interventions may also conceivably be rel- evant and clearly warrant further research. Keywords: Autism spectrum disorders, siblings, theory of mind. Abbreviations: ASD: autism spectrum disorder; ToM: theory of mind; EF: executive functioning; ADOS: Autism Diagnostic Observation Schedule; VMA: verbal mental ability; PPVT-III: Peabody Picture Vocabulary Test-III. Theory of mind (ToM) is the ability to identify mental states, including intentions, beliefs, desires and emotions, in oneself and other people and to understand how these internal experiences relate to external behaviour. In typically developing children there is a significant advance in ToM understanding between the ages of 3 and 5 years, as revealed via mastery of ToM tests including, notably, inferential false belief (Wellman, Cross, & Watson, 2001; Wim- mer & Hartl, 1991). There is also some evidence that this conceptual mastery of ‘mindreading’ (Baron- Cohen, 2001) may extend to advances in everyday social behaviour (Astington, 2003; Peterson, Slaughter, & Paynter, 2007). By contrast, decades of research (see Happe ´, 1994, for a review) reveal that ToM is severely delayed in children with autism spectrum disorders such that it is rare for even the highest-functioning children with autism to pass false belief tests before 13 years of age (Happe ´, 1994). Everyday ToM skills, including pretend play, are also impaired in children with autism (Baron- Cohen, 2001). Several studies of typically developing children reveal positive correlations between ToM develop- ment and having siblings. Perner, Ruffman, and Leekam’s (1994) initial report of a linear relationship between false belief understanding and family size was shown, in a larger follow-up (Ruffman, Perner, Naito, Parkin, & Clements, 1998), to be age-inde- pendent and attributable to the number of older (but not younger) siblings. Similarly, Lewis, Freeman, Kyriakidou, Maridaki Kassotaki, and Berridge (1996) found that older siblings were more beneficial to Greek preschoolers’ ToM performance than younger siblings whereas, with a Canadian sample, Jenkins and Astington (1996) found that preschool children with at least one sibling – either older or younger – were equally advanced while outperforming only- children. Peterson (2000) suggested that these conflicting results could be resolved by considering the sibling’s age, arguing that preschoolers’ reciprocal social and conversational interactions (pretending, arguing, teasing, etc.) might, in theory, accelerate ToM development most effectively when the sibling was also a child. Infant siblings are often too young to converse or play effectively whereas a preschooler’s interactions with an adult or adolescent sibling might not differ enough from parents’ to prove ben- eficial. Defining ‘child-aged’ siblings as those aged 1 to 12 years, she found support for this hypothesis in the significantly higher false belief scores earned by preschoolers with siblings this age than by those with infant or adolescent siblings, who did as poorly as only-children. Using a broad-ranging battery of standard ToM tasks, McAlister and Peterson (2006) likewise found that preschoolers with child-aged sibling(s) signifi- cantly outperformed both only-children and those with infant or adolescent siblings over and above Conflict of interest statement: No conflicts declared. Journal of Child Psychology and Psychiatry **:* (2011), pp ** - ** doi:10.1111/j.1469-7610.2011.02389.x Ó 2011 The Authors.. Journal of Child Psychology and Psychiatry Ó 2011 Association for Child and Adolescent Mental Health. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main St, Malden, MA 02148, USA