New research shows that oldest siblings are more likely to have the highest I.Q.s. From the New York Times:
The eldest children in families tend to develop higher I.Q.’s than their siblings, researchers are reporting today, in a large study that could settle more than a half-century of scientific debate about the relationship between I.Q. and birth order.
I think the most accurate theory is that this phenomenon is not biological, but rather a result of the familial role performed by the firstborn - the eldest are often called upon to play the problem solver. And older children may also benefit from observing the behavior of their younger siblings.