Does birth order affect intelligence? Does the number of siblings have a positive or negative effect upon the educational ability of children-or any effect at all? In a recent edition of American Psychologist, a series of seven articles dealing with birth order, family size, and intelligence, address these questions.
As the University of Oklahoma's Joseph Lee Rodgers points out, these are not new questions. In fact, Rodgers says of his own work, "...it follows literally hundreds-even thousands-of research articles dating all the way back to Galton (1874)."
Almost a truism known to all who have an interest in the family, firstborn children seem to have some inherent advantages, and commensurate expectations, over their later-arriving siblings, not the least of which are outlined by R. B. Zajonc, of Stanford University: "Firstborn children, especially boys, are slated to assume responsibility for a family's fortunes, are preferred as leaders, are selected for important positions, are entrusted with power, are accorded primacy in succession to a family's assts, and are expected to assume major responsibility for aging parents."
In Zajonc's understanding, however, firstborn children may have more advantages that we once thought: "When considering the intellectual development of children with in the family, it is clear that each successive child enters into a different environment and begins a particular cycle of growth.... Thus, first children have their parents all to themselves.... On the birth of a second child, experience makes caregiving easier and allows parents to be a lot calmer. Of course, two children are more demanding of parents' time and care than one child is."
Another contention that has been held by many is that the addition of brothers or sisters harms the intellectual development of children-particularly the firstborn. This view is usually called the resource dilution model, and, as espoused by Douglas B. Downey of Ohio State, claims, "...additional siblings reduce the parental resources accrued by any one child, much like a pie being cut into smaller pieces." In fact, Downey states, "The resource dilution model posits that parental resources are finite and that as the number of children in the family increases, the resources accrued by any one child necessarily decline. Siblings are competitors for parents' time, energy, and financial resources, and so the fewer the better. Even one sibling is too many" (emphasis added).
To the contrary, Zajonc contends that "all family members contribute to the quality of the intellectual environment." Furthermore, having many sibling may be beneficial for kids: "When second-born children reach a level of maturity allowing them to ask...questions, then firstborn children's lives change quite dramatically-they become tutors. In this role of tutor, firstborn children gain an intellectual advantage." Not only do the older children benefit from the teaching experience, but also the younger siblings gain the benefit of their older brothers' and sisters' experiences.
This confluence model, posited by Zajonc, asserts a fundamental cooperation between siblings, rather than the resource dilution model's competition. Only children, as firstborn children, do have an initial advantage, claims Zajonc, but that advantage is lost as children with siblings gain experience as tutors, and are exposed to more and more input as tutees: "Only children never become tutors. Thus, although their educational environments include just their parents-a condition favoring higher scores-the absence of siblings denies them the benefits of the tutorial function that children with younger siblings enjoy. However, because the advantage of tutoring accrues slowly, only children show the highest scores until their crossover age (i.e., 11 +/- 2 years), and afterward, their scores drop relative to the top birth ranks."
While disagreeing fundamentally with Zajonc's method, theory, and conclusion, Downey is forced to admit, "that, although sibship size has a negative impact on educational attainment for Ashkenazi Jews and Oriental Jews, it has no effect for Moslem Arabs. ...the Moslems' extended family (the hamula) plays an active role in supporting the nuclear family. ...[W]e found that additional siblings had a less negative effect on Mormon youths' math test scores and Mormon adults' years of educational attained than upon their Protestant counterparts. One explanation for these patterns is that because Mormons have especially strong profamily norms, parents with many children receive substantial support from others outside the nuclear family, buffering the typically negative effects of sibship size."
Zajonc concludes: "Birth order and family size are not the sort of variables that translate into causes. They determine nothing in and of themselves. They are conditions that afford, mediate, or prevent an array of diverse outcomes...." Instead, whether it is the direct influence of their older siblings, or the support of the extended family, we see that a large family is conducive to intellectual development.
