A new Norwegian study suggests that first-borns average a whole 3 IQ points higher than their later born siblings.
On average, firstborn males had an IQ of roughly 103.2, whereas the second-born child scored about 100.4 and third-borns 99. When the duo accounted for social rank, however, it turned out actual birth order may not be the key to intelligence.
On the other hand, first-borns may be retarded in other areas of life. In his op/ed today (hidden behind the TimeSelect veil of secrecy), David Brooks cites a New Zealand study (which I couldn't scare up) that finds:
"First borns are twice as likely to be virgins at 21 than later-born children."
Proving once again that babes don't necessarily find brains all that attractive.