Nine years ago my wife had her first sonogram. The technician seemed to be asking routine questions: "How long have you been pregnant?" "Twelve weeks." "Any family history of genetic diseases?" "No." "Any family history of twins?" "No." Then she showed us the screen. "Well, you're having twins." My wife and I were scared. We were first-time parents. How were we supposed to raise two babies at the same time?

Strangely enough, I already knew a lot about twins. I'd been an avid consumer of twin research for years. Identical...