The analysis focuses on the change in marriage patterns over four birth cohorts: the original HRS cohort (born 1931–1941); the War Babies (1942–1947); the Early Boomers (1948–1953); and the Mid Boomers (1954–1959).

It sought to discover what percentage of women’s adult years—ages 20 and older—is spent as part of a married couple.

Allowing for the fact that the youngest group in the study has not lived as long, and thus has not amassed as many married (or single) years as the oldest, researchers calculated the number of years (“total woman years”) for the women in each group and arrived at the percentage of years each group was married.

For the earliest cohort, those born in 1931–1941,” the brief says, “72 percent of women’s years between age 20 and the last interview were spent married. By the Mid Boomer cohort, those born in 1954–1959, the share had dropped to 54 percent. According to this measure, women have gone from spending most of their lives as part of a married couple to spending just 54 percent of their lives married.”

One final calculation, to account for the younger cohorts who have not yet reached the age of the women in the older cohorts, “show that if the Mid Boomers were interviewed at ages 73–83, then women in this cohort would have spent just about half of their life as part of a couple. It may well be that, once the whole lifespan of Mid Boomers has elapsed, women in that cohort will have spent less than half their adult years married.”

Why the drastic change?

The study finds three reasons: an increase in the age of first marriage; a drop in the percentage of women who marry; and, for those who do marry, an increase in divorce.

But the trend is not equal across all demographics.

The report finds that while women in the aggregate are spending less and less time in marriage, “black women have always spent a smaller percentage of years married than white women … [and] the decline in the percentage of years married is greater for black women than white women.”

But while there was substantial variation by race, the same wasn’t true by education.

The report says that regardless of whether women had some college or just a high school degree or less, “the percentage of years spent married declined from about 70 percent to about 50 percent between the HRS and Mid Boomer cohorts. The increase in the percentage of years not married or divorced was consistent across educational group.”

The report concludes that since “women as a group are going to spend less than half of their adult years as part of a couple,” the change “has significant implications for financial planning.”