These results are in line with the notion that, presumably due to a differential treatment by their parents during early childhood, firstborns prefer self-referenced standards to evaluate their competence. That is, they approach tasks with the desire to develop knowledge, skills, and task mastery. On the other hand, secondborns tend to evaluate their competence in terms of other-referenced standards. They are more strongly inclined to approach tasks with the desire to demonstrate competence relative to others.
Firstborns consistently rank higher on intelligence tests. The going theory is that they get more attention and resources from parents. Indeed, Robert Zajonc says that
Studies have also shown that sexual orientation correlates with a man’s number of older brothers. And in fact,The going theory is that mothers become increasingly immune to certain antibodies with each subsequent pregnancy. Accordingly, the anti H-Y antibodies produced by the mother during a pregnancy pass through the placental barrier to the fetus, which in turn affects various aspects of sexual orientation in the fetal brain.
Also, children with older siblings are more likely to experience respiratory symptoms at four years of age. One possible explanation is that