Birth Order Debunked

I recently wrote on my YourParentingSolutions.com blog about the new birth order study that's causing all the fuss.

The short version is: Researchers think they've proven that first borns have higher IQs, and parents are worried about their later-born kids. I say, reading to them makes up the difference and then some. Not to mention that breastfeeding has more impact than birth order on IQ. Click here to read more about the birth order study and increasing your child's IQ.

Although this was a large (that usually means more valid) study, I confess that I wondered about the methodology. The researchers examined 40 years worth of IQ tests from Norwegian men and correlated it to their birth order. The further down the chain you were from first born, the lower your IQ was.

But anyone trained in reading research knows the conclusion can only be drawn that first borns as a group have higher IQs, not that any specific firstborn is smarter than his siblings. I found myself wondering why no one has followed actual families for 40 years and compared the kids within those families. (My uncle-in-law, psychologist Jack Block, was one of the first to conduct longitudinal studies like this, so I am intimately familiar with the challenges they pose, but also with the benefits.)

Today, I read an article by Robert Needleman and Laura Jana over at DrSpock.com that describes exactly the study I've been waiting for, and get this -- it was published seven years ago!

J.L. Rodgers and colleagues reported in the June 2000 American Psychologist their research with 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. There was absolutely no connection between birth order and IQ. The researchers claim that other research, using similar methods, shows similar results.

How did these two different methodologies get such different results? It's a great lesson in research. Obviously, the Rodgers study (presuming they did it using accepted scientific and statistical practices) is more applicable to the question, which is "Does birth order affect intelligence of kids within a family?" Researchers in the Norwegian study -- and hundreds of smaller, similar studies -- didn't have data for specific families. So they looked at groups of first borns and later borns and correlated that to IQ. True, the later the birth order, the lower the IQ, but there's a confound here.

As Needleman and Jana point out: "These studies ... 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."

In other words, the Norwegian researchers were like the inebriated man looking for the keys he had dropped. He dropped them in the dark alley, but since there was no light there, he looked for them on the corner under the lightpost, where there was light to see.

Oh, and family size? The studies indicate that kids from larger families have lower IQs in general. But since women with less education, world-wide, have more children, naturally those women are less likely to offer their kids the intellectual cultivation that hel