How, then, might demographics favor Republicans? To answer this question, we must expand our definition beyond the race-and-identity obsession we find in American politics to the far simpler demographic variables of birth rates and family formation.

Unlike race and identity, these variables make up the very core of demographic research. Consider a 2017 study by Ron Lesthaeghe and Lisa Neidert. It starts by discussing a phenomenon the authors call the “Second Demographic Transition” (SDT). By this they mean the fall of the stable nuclear family and the rise of more loose-knit forms of family formation. This movement—which started at the end of the 1960s—was accompanied by the “culture war” that has defined American politics ever since.

Lesthaeghe and Neidert try to quantify this shift. Using advanced statistical techniques, they create what statisticians call a “proxy” for the SDT—that is, a single variable that can stand in for a multifaceted phenomenon. The main components of this proxy are, according to the authors, “the postponement of both marriage and parenthood, low total fertility and lower teenage fertility, and in addition, higher levels of unmarried cohabitation, including same-sex couples, and procreation among cohabitors.”

What Lesthaeghe and Neidert find is fascinating. Since the end of the 1960s, their SDT variable predicts presidential elections with increasing power.