A new study finds millennials are having far less sex than their older counterparts, and that the ‘marriage advantage’ of more frequent sex compared to singles has dramatically declined.

Americans are having sex notably less often than they did 25 years ago, across both sexes, all ages, the country, education levels, and work statuses, finds a new study that analyzes General Social Survey data. It says in 2014, the latest data available, the average American adult had sex nine times fewer per year than the average American adult did in 1989.

Baby boomers, the highly educated, parents of school-age children, and people who do not use pornography saw the biggest declines in sex frequency. The study also finds the “marriage effect,” the pattern of much higher sexual frequency and enjoyment for married folks compared to single counterparts, seems to have diminished over the past 25 years.

“Those with steady partners still have sex more often than those without, but the steady partner advantage in sexual frequency has shrunk,” concludes the study, authored by a team led by prominent researcher Jean Twenge. After running the available data on a variety of hypotheses over reasons for this change, the study also found that “approximately two-thirds of the decline in sexual frequency was due to the decline in the number who were married. The remainder seems to be due to the decline in sexual frequency among married individuals.”

Researchers compared all unmarried folks (never-married plus divorcees) to married folks, which shows essentially no change in sexual frequency for unmarried people but a big decline in sexual frequency for married people. Unmarried Americans continued to average 50 sexual encounters per year, while married Americans averaged 11 fewer sexual encounters per year in 2010-14 than in 1989-94. The steepest declines began in 2000.

A note on study limitations: The GSS asks respondents to estimate how many times they have sex per week, and the study researchers coded the responses on a scale of 0 to 6, with 0 representing none and 6 representing three or more times per week. So these estimates of sex per year should be considered generalities, not as precise as implied by the graph and specific numbers of sex per American per year.

Self-reporting has well-known limits because of people’s notorious inaccuracy at estimating the truth about themselves. It could be that singles are more likely to estimate higher rates of sex and married people to estimate lower rates of sex, and both imprecisely. Sex is more easily available to a married person, who has a dedicated sex partner, than to an unmarried person, who has to actively hunt down partners.

So, since married sex has lower transaction costs, married people may feel more disappointed with their rate of sexual activity even though it is in reality comparatively higher than that of singles’. Meanwhile, singles might feel oppositely, that any sex is a slightly unexpected major positive. Also, for singles more “scores” can be a culturally conditioned positive, but for married folks it could suggest more explicit infidelity, both contexts that could bias estimates in opposite directions.

Marrying Older Seems to Diminish Sex Rates

The study speculates that the older age of first marriage is likely the major contributing factor to millennials’ lack of sex, since sex drives peak for both sexes in their twenties.

“Married individuals in the 1990s had sex more times per year than never married individuals, but by the mid-2000s never married individuals had sex more times than the married,” the study says. “This likely reﬂects fewer married individuals having sex at a very high frequency, perhaps due to the rising age at ﬁrst marriage (which was 23 for women in 1990 and is now 27). Nevertheless, married individuals still had sex with more consistency and thus still exhibited higher sexual frequency…”

When the study authors controlled for age and time period, they found the oldest demographic studied, those born in the 1930s and known as the silent generation, had sex the most often, while those born in the 1990s and later, known as millennials and generation Z, had sex least often. In other words, grandma and grandpa got it on more than their grandkids are at the same age.

Given the social context, this may seem strange. American society has gotten increasingly “sex-positive” in the last 90 years. Pornography and contraception have become ubiquitous. Abortion was forcibly legalized nationwide in 1973. Our current first lady has modeled naked for money. Millennials are perhaps the most open and nonjudgmental generation ever about sexual choices, except perhaps the generation after them, which is even looser. By all the conventional libertine talking points, this should be a new era for exciting free love. But it’s not. Why?

Here’s What the Study Says It’s Not

Before we look at possible reasons, let’s look at some conventional explanations this study did not support. Pornography, which has been widely found to reduce sexual activity and pleasure, was not in this study correlated with the drop in sexual activity. If anything, although the correlation was low, people who didn’t use porn in this study had less sex than those who did.

Partly, the study explains, that’s because the GSS data only contains one question about porn use: “Have you seen an X-rated movie in the past year?” Not only is the response, like all for the GSS, self-reported and therefore susceptible to lying, it also fails to distinguish between rates of porn use. There’s a big difference in effects on sexual performance of succumbing to temptation a few times, or even casually using occasionally, versus being a porn addict.

Another potential explanation not supported by the study was either the increase in dual-income households or work hours. In fact, the study found a small positive correlation between more work and more sex.

Another potential explanation might be young children, because they’re most likely to keep parents up all night and thus more exhausting. But nope again. In this study it was parents of kids who were ages 6-12 who were less likely to have sex than parents of kids in other age ranges.

Yet this could also be explained by the rising age of first marriage, because if people marry at the end of their twenties and hold off on kids until their thirties, they’re in their forties when the little whirlwinds are still very physically active. Let’s face it: a 40-year-old body is not the same as a 30-year-old body at keeping up with kids. So increasingly older parents could play into the sex crash for this demographic.

What Are Other Possible Factors?

Nearly half of women of childbearing age use hormonal contraception, which is linked with a decline in sexual appetite, especially for younger users and particularly with the pill, which 17 percent of fertile women use. Other chemicals Americans increasingly use also depress sex drive, including anti-depression and anxiety medications, which the Twenge study raises as a potential factor.

Maybe it’s because hookup culture treats people like used condoms. If that’s your or your friends’ experience with sex, no wonder kids are opting out.

“In a previous paper, we found that the happiness of adults over age 30 declined between 2000 and 2014. With less sex and less happiness, it’s no wonder that American adults seem deeply dissatisfied these days,” Twenge told ScienceDaily.The overall decline in happiness, which is also affecting millennials, could interact with sexuality to have placed many couples on a spiral downward, since sex increases happiness and relationship satisfaction. Women, in particular, have recorded significant drops in overall happiness since the 1970s. Maybe it’s because hookup culture treats them like used condoms. If that’s your or your friends’ experience with sex, no wonder kids are opting out.

“Women who have better sexual relationships with their partners also have more satisfied relationships in general, and it improves the quality of their relationships,” Elizabeth Lloyd, a professor at Indiana University-Bloomington who has studied female orgasms, told CNN. “So in general, a better sex life leads to a better relationship, which leads to a better sex life. It’s kind of circular.”

Good Sex Requires Commitment and Vulnerability

Or, as my friend D.C. McAllister puts it, “When we are connected, when we explore the person we love, when put forth the effort to spend time with them, we discover the depths of who they are and look for new ways to please and give to that one person. People are not shallow, or they shouldn’t be. We have many layers. When we spend time really diving into those layers, discovering, loving, kissing, caressing, playing, etc., the sex is GOOD!”

It’s hard to explore another person in a safe environment when both of you know the other might leave at any time and for any reason.

That kind of vulnerable play is more probable within commitment. It’s hard to explore another person in a safe environment when both of you know the other might leave at any time and for any reason. Our culture now applies this transience to marriage so that it as an institution no longer offers the stability it once did, which is a key precursor to (among lots of other great things) sex so good it will get partners to turn off the TV or roll over in bed.

Millennials are renowned for fearing risk. It’s one of the main factors in their fear of commitment to marriages, careers, and kids—all the hallmarks of adulthood. Other sociological research has shown they often feel more comfortable interacting with people as avatars through screens than facing even low-level rejection after attempting to initiate conversation in person. Needless to say, initiating sex is several risk levels higher.

This story comes down to one that has been haunting social scientists for several decades now. Everyone has more life choices, women in particular. Broadening choices is often sold as a path to happiness. But research also finds that too many choices make people unhappy. Maybe when marriage is considered just another “reversible” choice that element of relationship stability reduces our ability to open ourselves to another person and create and enjoy a love worthy of and requiring a lifetime to explore.

Maybe that’s why the old folks whose marriages made it that long, and in a culture that expected them to, have the most sex. Certainly the opposite isn’t getting comparably exciting results. Really, what do millennials have to lose by committing themselves to something that has lasted so long and so successfully—other than our apathy, narcissism, and cowardice?