From Nature:
Eldest sibblings are, on average, 2.3 IQ points more intelligent than their younger brothers and sisters, says a study of Norweigan kids. And it’s not necessarily being born first that makes the difference — it’s being raised as the eldest child.
It has been proposed for some time that, on average across a population, first-borns are more intelligent than their younger brethren. There are more first-born sons in prominent positions than might be expected, for example. And some studies have shown a link between birth order and intelligence: the later born, the less smart the child. But the reasons behind this trend, and even whether it’s real, have been hotly debated. Families with low-intelligence children tend to be large (perhaps a big brood leaves little time for helping with homework), so the observation that sixth-born children aren’t very smart, for example, could just be a side effect of this, critics have said.
More here.