The largest study I know of on birth order is the one carried out by Ernst and Angst themselves. Not content to survey the work of others, they decided to check up on their conclusions by running a massive study of their own: 7,582 college-age residents of Zurich served as subjects. Ernst & Angst used all the proper controls and measured (with a self-report questionnaire) twelve different aspects of personality, including Sulloway's favorite, openness. They found no significant birth order effects at all among subjects from two-child families -- no differences in personality between the firstborn and the secondborn. Among subjects from larger families there was one significant effect: the lastborn tested slightly lower in masculinity. This study was reported in the same 1983 book that produced the data for Sulloway's reanalysis, but he does not mention it either in Born to Rebel or in his interview on Edge.
Perhaps he discounted it because it used the self-report method. Studies that use family members -- parents or siblings -- to assess the subjects' personality are far more likely to produce findings favorable to Sulloway's theory. Several such studies were included in Ernst & Angst's survey and most of them yielded multiple findings. But are the findings valid? Ernst & Angst didn't think so. When you ask people to assess the personality of their children or siblings, what you get is a description of how the subjects behave at home -- how they behave with their parents and siblings. This doesn't tell you much about how they behave at other times and in other places. Parents' descriptions of their kids agree poorly with teachers' judgments. (I imagine that teachers must get tired of hearing parents ask, "Are you sure you're talking about MY kid?") A method Sulloway advocates in his Edge interview is to have subjects compare themselves to their siblings, but what that would give you is a picture of how the siblings behave vis-a-vis each other -- how they behave when they're together, because they don't know how their sibling behaves when they're apart. I have no doubt that such a procedure would generate birth order effects.
In response to John Brockman's question about children without siblings, Sulloway hypothesizes that "only children ought to be intermediate on many personality traits" because "they are not being pushed by a younger sibling into being particularly conscientious or aggressive; and they are not being pushed by an elder sibling into being particularly daring or unconventional." But he also says that only children ought to be more variable because they "are free to occupy any niche." What Sulloway is trying to explain here is the embarrassing fact -- embarrassing not just to him but to all believers in the nurture assumption -- that only children do not differ in any systematic way from children with siblings. These children have missed out on the experiences that play such an important role in Sulloway's theory: they haven't had to compete with their siblings for parental attention, and they haven't had to learn how to get along (or not get along) with a bossy older sister or a pesky younger brother. And yet their personalities are indistinguishable from those of children with siblings.
Occasionally a study does turn up a difference between only children and children with siblings, or between firstborns and laterborns, or between first- and lastborns and middle children. Such results are a testimony to the persistence with which researchers look for them and their refusal to take no for an answer. The fun part comes in thinking up an explanation for each significant effect that is found, because each study that produces a publishable result tends to produce a different one. Sulloway mentions, for example, a study that found that middle children were less likely than first- or lastborns to identify themselves with a family label, presumably because they were more closely identified with their peers. Sulloway's explanation is that "middle children are at a disadvantage -- they don't have the benefit of being first, which leads to greater parental investment because firstborns are closer to the age of reproduction. The lastborn has the benefit of being the last child the parents are going to have, so parents will tend to invest heavily in this child so that it will not die in childhood." Ernst & Angst had something to say about this kind of post-hoc reasoning and I think it's worth quoting here. The italics are theirs.
Birth order research seems very simple, since position in a sibship and sibship size are easily defined. The computer is fed some ordinal numbers, and then it is easy to find a plausible post hoc explanation for any significant difference in the related variables. If, for example, lastborn children report more anxiety than other birth ranks, it is because for many years they were the weakest in the family. If firstborns are found to be the most timid, it is because of incoherent treatment by an inexperienced mother. If, on the other hand, middle children show the greatest anxiety, it is because they have been neglected by their parents, being neither the first- nor the lastborn. With some imagination it is even possible to find explanations for greatest anxiety in a second girl of four, and so on, ad infinitum. *This kind of research is a sheer waste of time and money*.
JUDITH RICH HARRIS is a writer and developmental psychologist; co-author of The Child: A Contemporary View Of Development; winner of the 1997 George A. Miller Award for an outstanding article in general psychology, and author of the forthcoming The Nurture Assumption: Why Children Turn Out The Way They Do.