Research Finds Firstborns Gain the Higher I.Q.
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.

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.
http://www.nytimes.com/2007/06/22/science/22sibling.html?_r=1&hp&oref=slogin

Just one more thing I can mention to my kid brother at our next GTG.