I've seen this study before and while its certainly measuring something, what its measuring doesn't seem to fit the definition of IQ. Considering it says that younger children score higher when under 12 then vice versa, IQ isn't really suppose to be changeable especially at that kind of age.
The social expectations on the first born may cause them to concentrate harder perhaps, who knows. And while its a large sample its from a particular source that may include systematic biases. If smarter younger kids managed to get out of military service in disproportionate numbers that would skew the results.
I'm intrigued that the stated IQs seem above-average (100):
"In the study, Norwegian epidemiologists analyzed ... military records. ... the researchers found that eldest children scored an average of 103.2, about 3 percent higher than second children (100.3) and 4 percent higher than thirdborns (99.0)."
Especially considering that there will always be more first-borns than second- and third-borns. I'm not sure if the bell-curving is done internationally, but I would bet it is - otherwise it would be useless as a measure of intelligence between different nations. So the difference would be explained if Norway has a slightly higher IQ than average.
So the study concludes that eldest children turn out smart, as do those who are the youngest by a large margin. One more example of middle children never catching a break.
There's something to it, e.g. MIT students are overwhelmingly eldest or only children, but I can't see how individual observations like you and I can make would be able to distinguish a 3 to 4 point IQ difference.
Finer measurements and larger sample spaces are required, I'm sure.
I certainly can't tell apart people with a 3 point IQ difference but as the article mentions it is possible that this kind of differences would accumulate over time. For example the difference between an A and a B is pretty small on a case by case basis, but it's a big difference between a straight A student and B average student.
The social expectations on the first born may cause them to concentrate harder perhaps, who knows. And while its a large sample its from a particular source that may include systematic biases. If smarter younger kids managed to get out of military service in disproportionate numbers that would skew the results.