Bill and Melinda Gates are getting divorced. So are increasing numbers of older Americans.

Divorce rates have plummeted among young people even as they have risen among older adults. And it's not just because the population is aging.

Microsoft co-founder Bill Gates and his wife Melinda take part in an interview with The Associated Press in Kirkland, Wash., on Feb. 1, 2018.Ted S. Warren / AP file

After 27 years of marriage, Bill and Melinda Gates are calling it quits. Their high-profile split is emblematic of divorce trends in the United States as a whole. Increasingly, married couples who break up are in the second half of life. Divorce among middle-aged and older adults is so popular now that researchers like me have a term for it: gray divorce.

In the past, many couples would remain in these “empty shell” marriages largely because separations were stigmatized, or couples didn’t believe in divorce.

This phenomenon, which refers to divorce among people 50 and older, doubled between 1990 and 2010, with the rate rising from .5 percent to 1 percent per year, and has since plateaued at this new high. And a generation ago, less than 10 percent of divorces involved a spouse over age 50. Nowadays, though, more than 1 in 4 people getting divorced in the U.S are over age 50.

The growth in gray divorce reflects more than the aging of the population, and is additionally striking given that it’s at odds with the general pattern in the U.S. For the past four decades, the overall divorce rate has been slowly but steadily declining. This modest decrease in divorce reflects diverging trends for younger versus older adults: Divorce rates have plummeted among young people even as they have risen among older adults.

A primary reason why divorce has fallen among people under 39 is that fewer of them get married in the first place, and those who do marry tend to be more advantaged in ways that are protective against divorce. For instance, as marriage has increasingly shifted from a ritual inaugurating adulthood to a capstone experience carried out only after young people complete their education and gain financial independence, the risk of divorce has declined among this age group.

In contrast, older adults face higher risks of divorce than ever before in history. And reaching one’s silver, or 25th, wedding anniversary is not necessarily a marker of marital permanence. Over half of gray divorces occur among couples who have been married more than 20 years.

My Bowling Green State University colleague I-Fen Lin and I have been studying the gray divorce revolution for more than a decade. Our interest in this topic was sparked by another high-profile break up, that of Al and Tipper Gore. We wondered what prompts couples who have stayed together for decades, navigating the highs and lows of married life, to actually call it quits.

We noted that midlife is marked by major life transitions. Children grow up and move out of the house, leaving couples with an empty nest. Careers wind down as individuals transition to retirement. Without the daily grind of juggling children’s schedules and long hours spent at work, spouses can find they have little in common. Gray divorce is often not precipitated by a singular event, but is instead the result of drifting apart.

At the same time, the broader culture has changed when it comes to gender roles and expectations. Marriages work when both spouses feel a sense of personal fulfillment and satisfaction from their relationship. A few generations ago, marital success hinged largely on role performance, meaning being a good wife or husband. Individual happiness was often beside the point.

Now, even the definition of a good spouse is idiosyncratic and must be negotiated among couples. Increasingly, being a good wife is not only about homemaking and raising children, but also about contributing financially to the family. Likewise, being a good husband means more than just being a breadwinner. It also requires doing a fair share of the housework and being an involved father. And marriage as a whole is no longer merely an economic bargain or a haven for child-rearing. Marriage is a true partnership and spouses are to be best friends.

These high expectations for marital success can be challenging to sustain as relationships evolve over time. When people report less satisfying marriages, their risk of gray divorce climbs. And today’s midlife married couples report worse marital quality than their counterparts did a quarter century ago. It’s likely that couples have higher expectations for their marriages these days, particularly around the gendered division of labor.

In the past, many couples would remain in these “empty shell” marriages largely because separations were stigmatized, or couples didn’t believe in divorce. These days, couples are less willing to remain in empty shell marriages. Societal changes also mean that women are often less economically dependent on their husbands, and thus they can afford to get divorced.

In research I conducted with Matthew Wright of the Appalachian State University, we found that adults over age 50 today are more accepting of divorce than younger people. Nearly two-thirds agree that divorce is the best solution when couples can’t work out their marriage problems versus fewer than half among younger adults.

Part of this greater acceptance of divorce is because many older adults have been divorced before. The baby-boom generation is at the forefront of gray divorce. Boomers, those born between 1946 and 1964, as Bill and Melinda Gates both were, came of age during the divorce revolution of the 1970s. Many have married, divorced, and then got remarried.

The gray divorce rate for couples in remarriages is about 2.5 times higher than for couples in first marriages. Some fraction of those in first marriages are in empty shell marriages and simply unwilling to get divorced even though they are unhappy. They may object to divorce on religious grounds, or be concerned about breaking up families or forming stepfamilies, which introduce relationship challenges that can be destabilizing. But people who have been divorced before know that there is life after divorce and they are less reluctant to divorce again if their marriage is not working.

Meanwhile, people are living longer than ever before. If you live to age 65, you can expect to survive roughly another 20 years, which is a long time to spend in an empty marriage with a spouse you no longer like. Once nearly universal, marriage nowadays is just one in an array of options. About one-third of baby boomers are unmarried, and this number is likely only to grow as more boomers experience marital dissolution through either gray divorce or the death of their spouses.

Stating they “no longer believe we can grow together as a couple in this next phase of our lives,” the Gateses join a growing group of baby boomers who are leaving their decades-long marriages. The gray divorce revolution may be viewed by some as evidence of the erosion of marriage as a lifelong commitment. But for many, it means newfound freedom and flexibility as aging adults increasingly reject the confines of empty shell marriages in favor of autonomy and independence.