The order of birth of an individual would appear to determine in many ways the family environment to which he is exposed during his developmental years. A person born early in the birth order will be in constant contact with siblings who are younger and therefore weaker and more helpless than he is. A late birth order, on the other hand, means years of close interaction with people who are also labeled children and whom the parents may strive to treat equally but who are bigger, smarter, and more powerful than the newcomer. In addition, the characteristics of the parents may change with birth order. The early-born children have parents who tend to be inexperienced in raising a family. They are likely to be eager to have children but unsure and nervous in handling them, and they may have unrealistic expectations regarding the child's immediate and distant future.
FARINA A, BARRY H, GARMEZY N. Birth Order of Recovered and Nonrecovered Schizophrenics. Arch Gen Psychiatry. 1963;9(3):224-228. doi:10.1001/archpsyc.1963.01720150034005