By Matthew Warren

The role of birth order in shaping who we are has been a matter of some debate in psychology. Recent research has cast doubt on the idea that an individual’s position in relation to their siblings influences their personality, for instance. But there may be other domains where birth order is still important: in particular, researchers have found that children with a greater number of older siblings seem to have worse verbal skills.

However, a new study published in Psychological Science has found that the situation is a bit more complicated than that. Young children with an older sibling do indeed perform worse on language measures, the authors find — but only if that sibling is a brother.

Parents only have a limited amount of time and attention they can split between their children, so the more siblings a child has, the less input they will personally receive from their parents. As parents play an important role in their child’s language development, this could explain why those with a greater number of older siblings have worse language skills.

Naomi Havron at PSL Université and colleagues were interested in how this effect is influenced by the age and sex of an older sibling. There may be less of a negative impact if there is a bigger age gap between siblings, or if the older sibling is a girl, the researchers reasoned. A much older sibling will have better verbal skills, so could themselves become a useful source for younger children to learn about language. And girls tend to have more advanced language skills than boys, so a sister may also provide better input to their younger sibling.

To test these theories, Havron and colleagues looked at data from an ongoing French cohort study called EDEN, which has followed children, and their mothers, from before birth through to age 11. At ages 2, 3 and 5-6, the children’s language skills were measured: at age 2 this simply involved mothers indicating which words their child could say, but at later ages children completed tests such as repeating words and sentences, naming pictures and listing animals. The team analysed the data from 1,276 children who had completed the language tests, including 547 who had an older brother or sister.

The researchers found that, on average, children who had an older sibling had worse language skills than those who didn’t. But as they predicted, the sex of an older sibling was important: kids with older sisters had better language skills than those with older brothers. In fact, a subsequent analysis showed that children with an older sister didn’t actually differ in their language skills from those with no older sibling. On the other hand, the age gap between the siblings didn’t appear to make any difference to language ability after all.

It’s not yet clear why children perform better when they have older sisters, the researchers write. It could be that sisters have better language abilities or are more nurturing than brothers — but another possible explanation is that sisters are less demanding on their parents, taking less parental attention away from their younger siblings than do brothers. Whatever the reason, the authors say, “it … might be more accurate to think of the well-established negative older-sibling effect as an older-brother effect.”

But having an older brother isn’t all bad. Other studies have found that children with older siblings are actually better at some of the social aspects of language, like joining in conversations. And it’s also important to note that although statistically significant, the size of the effects in the study were rather small — and restricted to just a sample of French speakers. It remains to be seen whether similar results are seen in children in other cultures or who speak other languages.

– The Effect of Older Siblings on Language Development as a Function of Age Difference and Sex

Matthew Warren (@MattbWarren) is Editor of BPS Research Digest