Cristina Souza/Flickr Americans are having less sex than ever. And that change is impacting folks from all walks of life — married and single.

A new study in the Archives of Sexual Behavior draws on data from the General Social Survey, which has asked more then 26,000 people about their sexual habits since 1989. It shows a long term decline in the amount of sex Americans are having, with the steepest decline in the last decade and a half.

In 2002, the average American adult had sex about 64 times a year. By 2014 that had dropped 17% to a mere 53 times per year.

Americans of nearly all age groups, geography, races, and living situations saw some decline in their friskiness.

The biggest reason for the drop seems to be that people are waiting longer to get married.

Stereotypes aside, the best predictor of whether you've had sex in the last month is whether you're married or have a long-term partner. And since more Americans are spending their 20s unmarried and unpartnered, young people seem to be having less sex during the period of their lives typically considered to be their most sexually active.

New dating platforms like Tinder and a more permissive sexual culture may be providing new opportunities for single people to have sex, the researchers write, but not enough to prevent an overall decline.

The decline in marriage doesn't explain the entire drop in sexual activity, however, since married and partnered people are also having less sex than they once were, across age groups.

So what else is going on?

There are several explanations that might seem logical but aren't supported by the study's data. Some people might think, for example, that easily accessible porn is replacing sex, but people who've watched porn recently are actually more likely to have had sex recently as well. Full-time workers also had more sex than those with part-time jobs or no jobs, so heavier work schedules don't seem to be the cause.

The researchers offer a number of plausible explanations: More distractions getting in the way of sex, higher rates of depression, even chemical changes in the environment. But there isn't enough data to conclusively blame the sex slump on any one of them.

Regardless of the reasons, it's clear that younger people just aren't having sex as often as their parents and grandparents did at their age. In fact, older generations are beating younger generations when it comes to sexual activity right now.

In order to compare the habits of different age groups (people born in the '60s versus people born in the '80s, for example) the researchers determined the ratios between how often those various groups have sex, and adjusted the numbers to balance out the known decrease in sexual frequency that comes with aging. The result? Today's older people may be having less sex overall, but they're having a lot more sex for their age than younger generations are.

The cohort having the most sex for their age, according to these adjusted numbers? People born in the 1930s.