I “clipped” this off of CNN for a couple reasons. One is to comment on the fact that I first saw this research months ago – CNN is pretty slow sometimes – okay, slow most of the time – at reporting new research. Second, even though there was a statistically significant difference between IQ scores of 1st, 2nd, and 3rd born children, there is only a difference of 3 points between 1st and 3rd. That doesn’t really mean anything in real life. Also, this is significant in part as a result of the huge same size – 200,000+. Additionally, the subjects all were male. There are other limitations pointed out in the CNN article.
We shouldn’t be so set on statistical significance (I worry about it in my research, of course). There is internal and experimental validity and then there is ecological validity. Intelligence research means very little without good ecological validity.