Many parents notice big differences in their children. However, they believe that they grew up in the same environment and were not raised differently from each other. Very often, these parents explain this phenomenon by the birth order, which would shape the cognitive development and personality of their offspring. These intuitions have been taken up, amplified and theorized by numerous authors and presented in numerous books.
One of the most widespread intuitions is that children surrounded by numerous siblings evolve in a richer and more stimulating environment than only children and that the youngest necessarily benefit more from a cognitive point of view than the first born. . Equally popular is the idea according to which elders, placed in a dominant position, find themselves defending acquired advantages and developing a rather conservative personality, while younger ones are obliged to challenge the established order and are thus more likely to develop a rebellious, non-conformist and progressive personality. And yet, this is a textbook case of popular beliefs collapsing when confronted with the data of scientific studies.
The differences in IQ depending on the place in the siblings are small
To test such hypotheses, we cannot of course rely on anecdotes and particular cases. It is necessary to analyze large populations for which we know both the place of each individual in his or her siblings and various cognitive and personality measures. It is also necessary to disentangle several confounding factors. Indeed, birth order is partly correlated with the size of the siblings (you cannot be the fourth in a family of two children). In addition, the parents are older at the birth of the last born than the first born. Finally, large families are sociologically different from those with few children. It is therefore necessary to take these factors into account in order to isolate the effect of birth order. Several studies carried out in the United States, Great Britain, Germany, in Norway Thus than in Franceby my team, carried out such analyzes and obtained convergent results.
First, the higher the birth order, the lower the cognitive scores (independent of the factors discussed above). This very general result is valid for language scores, intelligence quotient (IQ), just like academic performance measures. But this effect is weak (1 to 2 IQ points per birth order) and only applies on average. So there are many counterexamples. And this should not dissuade you from having another child if that is your desire. The interpretation most consistent with the data is that of “resource dilution.” Indeed, the first born benefits for a few years from the exclusive attention of his parents, while the next ones must share it with the older brothers and sisters. In fact, our analyzes in the Elf cohort show that the more brothers and sisters there arethe less time parents spend interacting with the newest child.
No effect of birth order on personality
Of course, the elders also interact with the youngest and can make up for this deficit. But it is clear that they only partially fill it, a sign that their way of stimulating it does not have the same quality as that of the parents. However, we also found that the negative effect of birth order on vocabulary was attenuated or even canceled when neither parent speaks French at home. In this particular context, having a French-speaking older brother or sister is likely to contribute positively to the acquisition of French.
Regarding personality, the results are just as clear: no effect of birth order has been demonstrated. This will not fail to surprise parents who notice that the eldest, youngest and youngest often adopt different roles within the family. They are not entirely wrong! But it must be understood that these roles are specific to the family sphere. The same children can behave very differently in other situations, and so can parents. Thus, it seems that these roles played within the family are only induced by the context. They do not have a lasting impact on the stable personality traits that the person will manifest as an adult.