Lots of people are excited by the Norwegian study showing 2 or 3 point higher IQs for first-born than latter-born sons in the Norwegian army conscription test.
According to the paper "Explaining the Relation Between Birth Order and Intelligence" by Petter Kristensen and Tor Bjerkedal, the first scientist to notice the higher-level of accomplishment among first-born sons was ... you guessed it, Sir Francis Galton.
The Norwegians have a huge sample size to work with, so their result sounds pretty reliable (although the effect isn't that big, so it might not be generalizable to other countries). They appear to be comparing brothers to brothers within families, so that eliminates hereditary average genetic differences (assuming they are only looking at full, not half, brothers).
They also try to answer the Why question: is it because of social effects (e.g., first-borns get more alone time with Mom and Dad) or because of those obscure gestational effects such as the development of male hormone antibodies. So, they look at conscripts whose older brothers died as infants and find their IQs are almost as high as first-borns, arguing against the gestational wear and tear on mom argument.
I suspect there is a subtle problem with this that reduces the confidence level, besides the much smaller sample size. For the primary How Much question, they should be able to compare living brothers' IQ scores directly to each other. For the secondary Why question, however, they can't compare a younger brother's IQ scores to those of his older brother who died in infancy. They have to estimate what the dead brother's IQ would be based on various demographic factors. And that adds another level of uncertainty to their secondary finding that social factors are more important than biological ones.
Anyway, birth order is an interesting topic. It's kind of odd how it has been out of fashion to talk about it for some years, even though it doesn't seem to be all that politically incorrect. Here is my 1996 review for National Review of the last major book on the subject, Frank Sulloway's Born to Rebel.