Why, some of my best friends are younger siblings.

Doesn't that pretty much sum up the message of these two paragraphs in this NY Times article

Charles Darwin, author of the revolutionary "Origin of Species," was the fifth of six children. Nicolaus Copernicus, the Polish astronomer who determined that the Sun, not the Earth, was the center of the planetary system, grew up the youngest of four. René Descartes, the youngest of three, was a key figure in the scientific revolution of the 16th century.
First-borns have won more Nobel Prizes in science than younger siblings, but often by advancing current understanding, rather than overturning it, Dr. Sulloway argued. "It's the difference between every-year or every-decade creativity and every-century creativity," he said, "between creativity and radical innovation."

Or, "Sure, also-borns, you gave us Darwin and Descartes, but what have you done for us lately?"

I just wish that articles like this were illustrated along with the relevant distributions, showing not only the difference in means (in this case, 3 points) but also the standard deviations and overlap. That would be much more useful than these wiggle-paragraphs at the end.

Also, I wonder if there is some kind of biasing effect here, where parents are more likely to have successive offspring if their first one is healthier, aka smarter. I'll have to see the study to see if they control for this, but if not then the IQ loss in subsequent siblings may partly be regression to the mean.

UPDATE (6/22/2007): The analysis of children whose older siblings died might test for regression to the mean. Second-born children whose older sibling died tended to have IQs approximately the same as first-born children. So the interpretation is that the social rank of the child is the determining factor.

I say "might" because I'm not convinced this comparison tests for regression to the mean. We'd like to know what actual only children look like in comparison to first-borns of larger families. The Science paper doesn't specifically address the issue, but the comparison of first-born and second-born after death of first-born is very suggestive.

References:

Kristenen P, Bjerkedal T. 2007. Explaining the relation between birth older and intelligence. Science 316:1717. doi:10.1126/science.1141493