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| |    Birth Order and Intelligence by Robert Needlman, M.D., F.A.A.P. reviewed by Laura Jana, M.D., F.A.A.P. What does birth order have to do with intelligence? For years, researchers have noticed that, on average, later-born children seem to score lower on standard IQ (intelligence quotient) tests than do their older siblings. The further down the birth order you are, the lower your IQ is likely to be. The common explanation is that parents only have a limited amount of time and energy to spend teaching their children. Children who are further down in the birth order get less parental stimulation, and as a result wind up with lower IQs. This theory makes sense, particularly if you are a firstborn child. But as it turns out, it's probably dead wrong. A recent article in American Psychologist argues persuasively that birth order has no effect at all on IQ. I bring this up because, as the baby of my family, I have always been a bit irked by the "later equals slower" theory. More important, though, I think that this issue highlights how scientific research can lead us astray--and how, if we're lucky, better research can point us back in the right direction. Getting into the research The idea that later-born children have lower IQs is based on the findings of hundreds of studies, all carried out in much the same way. Basically, the researchers take large numbers of people, or subjects, and measure their IQs by giving them a standard general intelligence test. (IQ itself is a controversial topic, but let's leave it at that for now.) The researchers average the IQ scores across families of various sizes (that is, all the one-, two-, and three-child families, etc.) and also each birth position (all the first-, second-, and third-born children, etc.). When they look at the numbers, they find two trends: the larger the family and the later the birth order, the lower the IQ. Thus, they conclude, being born later in the family lowers your IQ. But these studies suffer from a serious design problem: They mix birth order and family size, which are really two very different factors. For example, all of the "number 5" children come from families with at least 5 or more children, so when you compare the scores of the number 5's with the scores of the number 4's, say, you're actually comparing people from families with five or more children against people from families with four or more. Thus, you're not just comparing birth order, you're also comparing family size. In research jargon, you'd say that the factors of birth order and family size are confounded. In other words, even though you want to study birth order, your research design may be giving you information not about birth order at all, but rather about family size. Getting at the truth Recognizing this potential problem with almost every study on IQ and birth order, how do you get at the truth? The answer is, design a better study! That is exactly what J.L. Rodgers and colleagues did in their article "Resolving the Debate Over Birth Order, Family Size, and Intelligence" (American Psychologist, June, 2000). Instead of taking unrelated people as their subjects, these researchers used a large sample families enrolled in a long-term study called the National Longitudinal Survey of Youth. Because their sample included all the children from each family, they were able to measure how children compared to each other within each family. In this way, they were able to separate out the factors of birth order and family size that had been mixed up (confounded) in the previous studies. Here's what they found: When they looked within families, there was absolutely no connection between birth order and IQ. In this large and carefully chosen sample, fifth-borns were on average no duller (nor any brighter) than fourth-borns, who were in turn no duller or brighter than firstborns. Other research, using similar methods, shows similar results. What about family size? This is a sticky issue: Although birth order wasn't related to IQ, family size was. For many years, social scientists have observed that there is a relationship between larger family size and lower IQ when you look at large-scale population studies. (Much of this research is reviewed in Family Size and Intelligence, a 1989 book by Judith Blake.) Of course, many people from very large families have high IQs, and there are plenty of people from smaller families who don't! It is only when scientists take a few giant steps back and look at whole populations or groups of populations that the association becomes meaningful. When it comes to your colleagues and friends, or your own family, all bets are off! How can you explain the apparent fact that, on average, children from larger families have lower IQs? One theory, which makes some sense to me, has to do with social class. Social class and its extremes--wealth and poverty --are a hard reality in the United States and in most other places in the world. Parents from lower social class backgrounds tend to have larger families (perhaps in response to higher infant mortality; as infant mortality in a culture drops, so does the number of births.) Children growing up in poverty tend to have lower IQs (again, on average; many very brilliant people started life in poverty). So, on the population level, social class might be the link between larger family size and lower IQ. Does this mean that people who choose to have larger families are condemning their children to have lower IQs? Of course not! The moral? As the baby of three, I was relieved to learn that birth order doesn't have any bearing on IQ. Wait 'til I tell my older brother! More to the point, as a parent you don't need to be concerned that each new child is somehow destined to be less intelligent than the one before. Finally, in general, this little exercise points out that scientific studies don't always mean what we think they mean. Often, a small problem in the design leads to a very large error in the results, or in their interpretation. As research results become more and more accessible to parents, it very important that you take a healthy dose of skepticism along with your latest serving of scientific "fact." For more information on children and families, check out our Siblings, Family Relationships, and Parenting programs. Click here to join the discussion on Birth Order
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