For example firstborns are bigger than their younger siblings. They also are older and tend to have more status. In competition with their siblings, there are certain strategies that eldest children can employ that younger children cannot. A younger child can decide to hit an elder sibling, but this is usually not a smart idea because the elder sibling can hit back harder. This birth-order difference reflects difference in the niches that firstborns and younger children typically occupy.
Firstborns tend to occupy the niche of a surrogate parent.
For this reason, firstborns tend to identify more closely with their parents, and they also tend to identify with whatever their parents value. Parents value a child's doing well in school, so firstborns are conscientious, do their homework, generally do better at school, and tend to be over-represented as academics and in Who's Who . The niche of the responsible achiever is particularly likely to be open for an eldest child. Once this niche is taken, it is difficult for a younger sibling to compete effectively for the same niche, although they often try. The typical strategy of younger siblings is to see whether they can compete successfully in a niche already occupied by an elder sibling.