Against expectations, no evidence that toddlers with older siblings have superior Theory of Mind

When young kids play together there's often a lot of negotiation involved: "That's my bunny", "No, it's mine", "OK, you have it". There's talk of emotion: "Why are you crying?", "You took my bunny". And role-play: "You be baddie", "No, I'm super-bunny". Perhaps it shouldn't be too surprising then that a recent meta-analysis found that young kids, aged 3 to 7, with more siblings have superior Theory of Mind (understanding other people's mental states and perspectives – an important ability that benefits social and academic performance at school).

A new study in Journal of Cognition and Development asks whether the sibling advantage begins as early as toddlerhood, and whether it matters if a toddler's sibling is older or younger. In fact, against expectations, toddlers with an older sibling showed no Theory of Mind advantage compared with only children, and toddlers with a younger sibling actually performed worse.

Élizabel Leblanc and her colleagues tested 227 two-year-olds (113 boys) on two age-appropriate Theory of Mind tasks. One was focused on perspective-taking and required the toddlers to show their mother a toy and in the process deal with various challenges, such the fact she had her eyes closed or was facing the wrong way. The other task pertained to diverging interests: the toddlers had to choose a book to give the researcher, either a children's book for which she'd previously expressed dislike or an adult book.

On average, the 112 kids with one or more older siblings and the 76 only children performed equally well at the two tasks. Number of older siblings made no difference to these results. Meanwhile, the 29 toddlers with a younger sibling (but no older siblings) performed worse at the perspective taking task than both only children and toddlers with an older sibling; they also performed worse at the divergent interests task than the kids with older sibs.

Perhaps, the researchers speculated, toddlers with a younger sibling are disadvantaged by their parents' attention being diverted to their younger, infant sibling (the toddler also can't engage in sophisticated play with an infant sibling). On the other hand, maybe toddlers without any siblings are able to match the performance of toddlers with older siblings because of the extra attention they receive from their parents.

"Future studies are required to investigate whether a putative negative effect of younger siblings is robust and apparent in the preschool years, whether it is limited to the toddlerhood period, and whether the mechanisms proposed here are indeed at work," the researchers said.