from Scientific American
Some sniplets from the article:
Past studies of birth order and IQ have produced a mix bag of data; studies of children found that younger siblings fare better on the tests, but research on adults and teens showed the opposite.
Petter Kristensen and Tor Bjerkedal studied 241,300 Norwegians, focusing on men. Not only did they find that first-borns had higher IQs (103.2) than second-borns (100.4) and third-borns (99), but they also say that social rank, rather than birth order per se, is the determining factor.
They reached this conclusion by looking at families whose first-borns died in infancy. In such cases, the second-borns' scores rose, to an average IQ of 102.6. Third-borns whose two older brothers died jumped the most, to 103.5.
Robert Zajonc to explain why, as children at least, younger siblings outscored their top-bunk mates. These youngsters evidently derive wisdom from the older siblings and get an IQ boost; that ends during the teen years because the older kids gain more benefit from tutoring, rather than being tutored. You handle a topic more adeptly when you have to teach it to someone else.
In any case, the study illustrates the power of the environment on IQ. The best example is the Flynn Effect, which shows that IQ scores have been rising over the generations, a rise attributable to the environment more so than to genes.
All emphasis are mine.