Birth order theories (e.g., first-borns tend to be more risk-averse) have been around for a long time without making all that much progress. The data is very complicated and how exactly do you specify what you are looking for?
Well, here's an NYT article on a small study that is well-defined enough that they might have actually found something:
In the current issue of Personality and Social Psychology Review, Frank J. Sulloway and Richard L. Zweigenhaft went digging for evidence of siblings behaving differently in the vast database of baseball statistics. Given how younger siblings have been shown to take more risks than their older counterparts — perhaps originally to fight for food, now for parental attention — Drs. Sulloway and Zweigenhaft examined whether the phenomenon might persist to the point that baseball-playing brothers would try to steal bases at significantly different rates.UPDATED: A reader tries to reproduce this just for brothers where both were batters (i.e., no pitchers) and finds younger brothers more likely to steal in only 56% of the pairs.
In fact they did: For more than 90 percent of sibling pairs who had played in the major leagues throughout baseball’s long recorded history, including Joe and Dom DiMaggio and Cal and Billy Ripken, the younger brother (regardless of overall talent) tried to steal more often than his older brother....
“We tend not to exhibit birth-order differences all the time in adulthood — we employ them in situations with siblings, because that’s where the behavior comes from,” Dr. Sulloway said. “But we found that here, and that’s significant.” ...
In other words, Sulloway is making a fairly limited claim for the effects of birth order -- birth order has more effect within the family than in the outer world. The oldest son is much more likely to become CEO than the youngest son of the family firm, but not all that much more likely to become CEO of a publicly traded company.
And there remains the plausible issue of whether younger brothers learned baseball strategy more fully simply by watching their older brothers growing up, which Dr. Zweigenhaft, a professor of psychology at Guilford College in Greensboro, N.C., said could very well be a contributing factor.
Another explanation about why this study's finding might be restricted to baseball might be that when brothers play on the same team growing up, the older brother will usually be stronger (because he's more mature), and thus be put lower in the batting order in a slugger's RBI slot. In contrast, the younger brother will bat higher in the order where players are more expected to steal.
For example, say you are lucky enough as a high school baseball coach to have two future major leaguers on your team, a pair of brothers two years apart, Al and Ben. Both are much more coordinated at putting the bat on the ball than all your other players. Where do you put them in the batting lineup? When the older one Al was a 150-pound sophomore, you had him batting third or fourth (clean-up) to maximize the number of runs he could drive in because even though he was still thin, he was so much better at hitting the ball than anybody else. But when Al's a 180 pound senior and and his younger 150-pound brother Ben is a sophomore, you still have Al in the RBI slot, but you put Ben in the leadoff slot so he can get on base and steal.
So, by virtue of being older, Al is never given training as a leadoff batter, but Ben, by virtue of being stuck on Al's team, is trained to get on base and steal so his big brother Al can drive him in.
Here's my 1996 book review from National Review of Sulloway's book Born to Rebel.