“The best environment to grow up in is basically two parents who are chattering away at you with fancy words.”
So says Frank Sulloway, author of Born to Rebel: Birth Order, Family Dynamics, and Creative Lives. Sulloway was quoted in a New York Times story, "Birth Order: Fun to Debate, But How Important?" which I thought mostly missed the point of Sulloway's study.
Sulloway says that there is not a specific character that goes with each birth order position. Rather, the dynamic rule is "first born gets first choice." Normally the first born will gravitate to the things the parents value most, will get the most parental conversation, will seek to work hard, do well in school, and succeed in life, because that is the simplest path to eminence. The later-borns then have to find a different niche, especially when the kids are young and all at home. But if the first-born does not choose that niche, it is available for the second, and so on.
Sulloway cites a recent study of birth order using Norwegian military data. That study found that first borns have a modest but real 3-point IQ advantage over second borns. Sulloway's conclusion from this is the wonderful quote above.