"When the firstborn comes along, there are no younger siblings and they've got 100 percent of parental attention," he said. "It seems to be reflected not only in IQ differences ... but firstborns essentially do things that the parents value."
So it makes sense that firstborns score 3 points higher than their younger siblings on IQ tests. Twenty-one of the first 23 American astronauts to fly in space were firstborn, along with a disproportionate number of our nation's presidents and CEOs.
"The good story about younger siblings is they diversify in other ways," Sulloway said. "Younger siblings are always looking for alternative niches to explore and to exploit as a way of getting back some of that parental investment and attention they may have otherwise lost."
While more Nobel Prize winners for science are firstborns, more winners of the Nobel Prize for literature are younger siblings, Sulloway said. Younger siblings are also 50 percent more likely to play dangerous sports.
Sulloway said it's important to keep in mind that most birth order effects are "fairly modest in size," and that there are other contributing factors that influence human behavior and personality development.
"You just have to keep in mind that there are a lot of different things that are influencing behavior, and birth order is just one player in that whole complex process of how we become who we become," Sulloway said."