Theory of Mind (ToM) can be defined as the ability to work out what other people are thinking and how this might affect their behaviour. In most children ToM shows a significant advance between the ages of three and five. However, the development of ToM is severely delayed in children with autism. Several studies have shown that having brothers and sisters can improve children's ToM and researchers from the University of Queensland in Australia studied 60 children with autism to see if this was the case for them too. The researchers measured the children's ToM, executive functioning, verbal mental age and autism symptoms and compared this to the number of siblings the children had and where they fitted into the sequence. The researchers found that the children with older siblings actually had a weaker ToM, perhaps because their older brothers and sisters helped them to interpret other people's actions and stopped them from developing their own skills. Having younger siblings led to a weak improvement in ToM but this was not significant once mental age and autism symptoms were taken into account.
O'Brien, Karen, Slaughter, Virginia and Peterson, Candida C. - Sibling influences on theory of mind
development for children with ASD Journal of Child Psychology and Psychiatry