Birth order theory

The holes in the theories
The birth-order effect, for all its seeming robustness, is not indestructible. There's a lot that can throw it out of balance—particularly family dysfunction. In a 2005study, investigators at the University of Birmingham in Britain examined the case histories of 400 abused children and the 795 siblings of those so-called index kids. In general, they found that whenonly one child in the family was abused, the scapegoat was usually the eldest. When a younger child was abused, some or all of the other kids usually were as well. Mistreatment of any of the childrenusually breaks the bond the parents have with the firstborn, turning that child from parental ally to protector of the brood. At the same time, the eldest may pick up some of the younger kids'agreeableness skills—the better to deal with irrational parents—while the youngest learn some of the firstborn's self-sufficiency. Abusiveness is going to "totally disrupt the birth-order effects we wouldexpect," says Sulloway.
The sheer number of siblings in a family can also trump birth order. The 1% income difference that Black detected from child to child tends to flatten out as you move down the ageline, with a smaller earnings gap between a third and fourth child than between a second and third. The IQ-boosting power of tutoring, meanwhile, may actually have less influence in small families,with parents of just two or three kids doing most of the teaching, than in the six- or eight-child family, in which the eldest sibs have to pitch in more. Since the Norwegian IQ study rests on thetutoring effect, those findings may be open to question. "The good birth-order studies will control for family size," says Bo Cleveland, associate professor of human development and family studies at PennState University. "Sometimes that makes the birth-order effect go away; sometimes it doesn't."
The most vocal detractors of birth-order research question less the findings of the science than the...