An interesting story on IQ and birth order appeared in the New York Times recently. It makes sense, and jibes with my experience. Money grafs:

The average difference in I.Q. was slight — three points higher in the eldest child than in the closest sibling — but significant, the researchers said. And they said the results made it clear that it was due to family dynamics, not to biological factors like prenatal environment.
…

“Like Darwin’s finches, they are eking out alternative ways of deriving the maximum benefit out of the environment, and not directly competing for the same resources as the eldest,” Dr. Sulloway said. “They are developing diverse interests and expertise that the I.Q. tests do not measure.”

This kind of experimentation might explain evidence that younger siblings often live more adventurous lives than their older brother or sister. They are more likely to participate in dangerous sports than eldest children, and more likely to travel to exotic places, studies find. They tend to be less conventional than firstborns, and some of the most provocative and influential figures in science spent their childhoods in the shadow of an older brother or sister (or two or three or four).

Interesting stuff. The older sibling is the best situated to take advantage of the existing structure, so they take advantage of that, and the younger sibling is shielded from the consequences of risk taking, so they consume more of it.