Q. What has most surprised you?
A. A few things about fighting were surprising to me: The most physically aggressive of all sibling relationships is not, as you would think, the boy-boy relationship. It's actually the boy-girl relationship — usually the younger brother directed at the older sister. The most damaging of all kinds of fighting between siblings is usually the girl-girl emotional interplay. Girls tend to expose their emotional soft spots to each other more than boys do, and when you do that, you're much more open to the cutting remark.
Q. How important is birth order in shaping personality and life outcomes?
A. There is a not inconsequential minority of researchers who analogize birth order to astrology — [they think] it's too simplistic. The majority think this is one area where the [conventional wisdom] got it right: It is generally true that the firstborn is more aggressive, more successful. The firstborn tend to have a three-point IQ advantage over the second-born. Twenty-one of the first 23 astronauts were firstborn or only children. Last-borns tend to be funnier, more empathic, more intuitive. They're much likelier to be rebels. Middle-borns do tend to be the ones who are stuck in the center seat for life. They tend to take longer to find themselves.
Q. Are older children generally closer to parents?
A. Parents are more invested in older children from the outset. And older children tend to repay that loyalty with greater loyalty to the family. Particularly with the girls, they tend to be the family archivists; they also are the likeliest to be the caretakers of the parents.
Q. Isn't gender the most powerful factor of all?
A. It is, but that's more societal than it is anything else. In terms of temperament and life arc, birth order tends to be a bigger factor than overall age, income, gender, even education.
Q. How did the dysfunction of your childhood home, with a violent father and drug-addicted mother, shape the relationships among you and your brothers?
A. There was almost a hostage situation: We felt like we were people drawn together by crisis and adversity. Those things cause a lot of stress cracks in relationships. Either those stress cracks turn into fractures and relationships break apart, or they have a certain annealing power, and you develop bonds that seem all but unbreakable.