Researchers have continued to study whether birth order has an effect on intelligence. Past studies have claimed that the first-borns were the smartest, and intelligence dwindled as the number of subsequent children increased.
However: "A study this year by scientists at Ohio State University analyzed data on siblings from 3,000 families collected over 12 years. The researchers found that whether siblings were born first or last in their family generally had no effect on intelligence scores. But children in large families, primarily those in which the mother had her first child at a young age, had lower scores than those in smaller ones. Children in large families generally had high scores only when their mothers were older." (NY Times)
I know this to be true first hand. I'm way smarter than my older siblings.