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Americans are getting it on less often.

That’s what my co-authors and I found after combing through data on over 25,000 American adults who were asked how often they had : The average adult now has sex 9 fewer times than the average adult claimed in the late 1990s.

The declines were largest among married people living with their partners, who now have sex 16 fewer times a year. That decline has been in effect on for a while, since at least the late 1990s. Never-married people had sex more often in the 2000s than in the 1990s as the norms around premarital sex changed, but after 2008, that group's sexual frequency began to slide as well, declining by 8 times a year.

The past decade has not been kind to the frequency of sex.

Why did this happen? It's a difficult question to answer, but there are a few intriguing theories:

1. Smartphones have ruined our love lives. It’s tough to have a real conversation with your partner, much less make a move, when he or she is staring at their phone – or when it’s on the bedside table beeping away. Instead of connecting us to each other, perhaps smartphones are driving us apart.

2. Entertainment is more entertaining than it used to be. You can stream TV and movies any time of the day or night, and play video games around the clock.

3. apps are not the boon to sex many have assumed. They might make sex accessible for a certain cohort that seeks easy, quick sex, but they also might make the dating scene less appealing for those who really want relationships – which turns out to be most people.

4. We are less satisfied with our relationships, and life in general, than our parents’ generation was. This may be because we were raised with high expectations. That’s the premise of my book, Generation Me: When we are promised the moon and stars as teens, it's tough to be satisfied with having our feet on the ground as adults. That’s especially true when it comes to relationships, in which many expect that perfect and passionate romance should endure. That wasn’t always the case: Previous generations didn’t expect one person to fulfill their every need, and focused more on the practical realities of . That might explain why they were happier; in fact, we recently found that adults over age 30 are less happy than they once were.

5. Overall, Americans are uneasy. This has been a trend for a few years. Even though the economy has rebounded, there is a widespread dissatisfaction and in the country. Trust in others and in institutions are at all-time lows, and unemployment figures might be artificially low if more people have given up on the idea of working at all. Economist Erik Hurst found that 1 out of 5 young men are not working — he believes this is partially because entertainment options, especially video games, are more attractive (see point 2, above). Young men are among the primary drivers of sexual frequency, and if they are unemployed and playing video games, instead of working and getting married, sexual frequency is inevitably going to decline.

Why do you think Americans have sex less often than they used to?