"The study of 240,000 Norwegian men in the journal Science found the IQs of firstborns were two to three points higher than those of younger siblings.
While that may not sound like much, experts said even a few IQ points can make a big difference over a lifetime and set firstborns on a trajectory for success. University of California, Berkeley, researcher Frank Sulloway, who wrote a commentary accompanying the study, said two to three IQ points could translate to an added 20 to 30 points on an SAT college-entrance exam."and this,
"The findings suggested the mechanism behind the birth-order effect is not biological but related to social interactions within families. He surmised older children are showered with attention early in life and treated as leaders within the family. They are handed more responsibility after siblings are born and live with higher expectations from their parents.
Spacing between births also was a factor, Kristensen said. Children born less than a year apart had the greatest IQ gaps. Differences in IQs diminished when there were more than five years between the first and second child, he said."My kids are nearly 4 years apart, both very bright but very different in their approaches to learning. I think the leadership point and attention point are probably true, though. Of course, this completely leaves out other gifts like arts ability, social skills, etc.