Almost any way you slice it, marriage is increasingly a luxury. Nearly 30 percent of those who make less than $40,000 a year are married, compared to 59 percent of those who make more. Nearly 60 percent of those with a college degree are married, compared to 43 percent of those with a high school degree or less. More than half of whites are married, outpacing 31 percent of Blacks and 45 percent of Hispanics.

Disentangling these measures is hard. Still, the cumulative effect is remarkable, which is why marriage rates are attracting scrutiny from social scientists and demographers.

Those looking to explain the gap have explored everything from incarceration rates to the tax code to find an answer. Recent research on the dearth of “marriageable men” points to factors as diverse as declining labor force outlook for low-status men, mass incarceration, and even women’s entry into the university. Demographer Lyman Stone noted in congressional testimony on Tuesday that much of the tax structure is biased against marriages—a dynamic he said is partially responsible for America’s plummeting fertility rate.