Everyone knows that parents are always tougher on the eldest child. Is it because they just get tired of disciplining their kids after the first one? Or is it actually adaptive?

According to this article about a paper published in the April 2008 of the Economic Journal, parents are using an adaptive strategy based on game theory. Essentially, by punishing the older children harshly, parents are gaining themselves a reputation which will potentially deter younger siblings from behaving badly. While parents want to help their children as much as possible, the cost they incur to their eldest children is exceeded by the benefit gained to their younger children.  This makes punishing the older children to be worthwhile overall.

This is because, from an evolutionary perspective, all siblings are equal to the parent.  Each sibling carries 50% of the parent’s genes, so if the oldest sibling has their reproductive success decreased from 1 to 0, but the younger sibling has their success increased from 0 to 2, then this strategy is adaptive for the parents.

However, if this were true, it makes several other predictions. One is that that with each additional child, the parents would become more strict towards the older children. This implies that only children should be treated like younger children, since there is no younger siblings for them to be made an example of. Additionally, one would expect that the eldest children would be on average worse off than younger siblings, since they are incurring cost to benefit the younger children.