Moms, you don’t need to send your second sons to remedial school just yet.
Once again, Science Magazine is a tool of publicity hounds who trumpet their incredible breakthroughs in press releases, knowing that almost nobody will read – let alone understand – the fine print and discover that their claims have all the foundation of a sand castle.
Petter Kristensen and Tor Bjerkedal (2007. Explaining the relation between birth order and intelligence. Science 316:1717) managed to squeak a one-pager in this week that gives even those of us trained to read scientific papers a run for their money. However, if one delves into the online supplements to the article, and an accompanying paper also newly available(1) it becomes clear that the results are biologically meaningless, despite the media’s ever present desire to milk them for controversy. This is why:
First and foremost, this is yet another paper that uses an enormous sample size (over 250,000) to support its conclusions. Those who do not fully grasp the nature of statistics often believe that this makes results more credible, when in fact, it makes them less so. This is because in huge sample sizes, any tiny difference between groups is amplified to the point where it is likely to be statistically significant. However, statistical significance does not necessarily imply biological significance, especially when large sample sizes are required to identify a statistical effect. The authors of these papers themselves give the best demonstration of the fallacy of this technique when they cite a previous Science paper(2) that reached exactly the opposite conclusion using 400,000 Dutch men. If they repeat the study tomorrow using a different huge population, they could easily find the results reversed again.
If birth order has no correlation with IQ at all, then in randomly selected Norwegian family, there is a 50% chance that the first son has a higher score than the second son. My statistician colleague did some rough calculations based on the data in the Science paper, and concluded that it shows that in a randomly selected family, there is a 52% chance the first son scores higher than the second son. The statistical standard for biological significance in most contexts is 95%. For a first son to have a 95% chance of scoring higher than a second son, the average difference in IQ points would have to be 80. (As far as I know, there is no data showing that second sons are more likely to be retarded.)
In addition, the Norwegian soldiers did not take a standard IQ test, which has a population mean of 100. They took a test given by the Norwegian military which gives single-digit scores of 1-9, with a mean of 5. The scores are therefore much less precise than those from an IQ test, and a difference of 3 IQ points after the data transformation corresponds to a difference of 0.25 point on the Norwegian test. If two brothers both have a score of 6, which one is a quarter point smarter?
The media love these stories and will continue to broadcast them, analyze them, and give people complexes about them forever. But it is disheartening that Science, an internationally respected journal that scientists throw elbows to get published in because of the instant fame it brings, is in it purely for the publicity as well. People should no longer assume that a paper in Science must be actual science.
(1)Bjerkedal T., Kristensen P., Skjeret G.A., and Brevik J.I., 2007. Intelligence test scores and birth order among young Norwegian men (conscripts) analyzed within and between families. Intelligence doi:10.1016/j.intell.2007.01.004 (in press)
(2)Belmont, L., and Marolla, F.A., 1973. Birth order, family size and intelligence. Science 182:1096-1101.