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Research has long documented a positive correlation between and . Generally, people who are having more sex also report being happier. This link holds both in the U.S. and around the world, for young and old couples alike.

Up until now, the data on this link has been correlational, so the question of causality has remained unanswered. Still, the notion that more sex can cause more happiness seems quite plausible: In our personal experience, sex tends to make us feel good, so we intuit that the same effects may work as a general rule. Second, the notion that sex may increase happiness aligns with our understanding of the reciprocal link between attitude and behavior. Many people can see clearly how attitude gives rise to behavior (I feel aroused, therefore I kiss you), yet the causal arrow works in the other direction as well (As we start kissing, I become increasingly aroused).

In fact, research has shown that one of the most surefire ways to affect a swift attitudinal and emotional change is by changing behavior. Given this, it would make sense to assume that a change in sexual behavior (such as having more sex) will result in a change in attitude and emotion (becoming happier).

A recent study conducted by George Loewenstein of Carnegie Mellon University and his colleagues sought to find out whether more sex in fact causes more happiness.

The researchers reasoned that if more sex causes more happiness, then having more sex should improve couples' happiness. They recruited 64 adult, married, heterosexual couples and established (through online questionnaires) their baseline sense of well-being, mood, frequency of intercourse, and enjoyment in sex.

Half the couples were then randomly assigned to continue with their normal sexual patterns. The other half was asked to double their frequency of sex for the next three months. Participants completed daily online questionnaires about the frequency and quality of their sex, and their subsequent mood.

Results, however, showed that while couples in the experimental group did increase their frequency of sex markedly (40% on average), this increase in the frequency of sex led to small declines in well-being—and in the reported quality of the sex. Participants, regardless of , reported that additional sexual activity just wasn’t that much fun. "They were less happy with the sex and they were less happy overall,” according to Loewenstein.

So, more sex appears to have led to less happiness. What’s going on?

For starters, as the researchers themselves noted, the study had actually answered a different question than it claimed to have asked. What they ostensibly asked was, "Does more sex cause more happiness?" But what they answered was the question, "How does requesting couples to have more sex affect their happiness?" Requesting couples to have more sex led to less enjoyment and happiness for them. Why would that be? The researchers considered two plausible hypotheses:

Perhaps people’s sex frequency is already attuned to their needs, and yields optimal happiness. If that’s the case, more sex will upend the delicate balance, such that too much of a good thing becomes a bad thing. The data, however, failed to support this hypothesis. There was no correlation between the amount of participants' increase in sex and their mood and well-being ratings.

Most participants did not, in fact, manage to meet the researchers’ directive to double the frequency of intercourse. Perhaps they became disappointed by this, leading to declines in mood and satisfaction. The data, however, failed to support this explanation: The size of the gap between the researchers’ request and the couples’ actual increase did not correlate with participants' mood ratings.

So the declines in mood and well-being were not the result of too much sex. Neither were they due to a failure to adhere to researchers’ instructions to double the frequency of sex.

Perhaps the reason that requesting people to have more sex did not lead to more happiness had to do with the effects of the request itself. As Loewenstein noted: “Perhaps couples changed the story they told themselves about why they were having sex, from an activity voluntarily engaged in to one that was part of a research study.”

Indeed, a large body of research shows how intrinsically motivated behavior can, under some conditions, become less appealing once it is externally rewarded. This is known as "the over-justification effect."

It is possible that switching sexual behavior from intrinsically motivated (I feel like having sex) to externally controlled (We are supposed to have more sex for this study) had the effect of reducing happiness. Now participants could interpret their sexual encounters not as expressions of their authentic , but as external demands independent of how they felt. "Whether you do something because you want to or because you are instructed to can have a huge impact on how much you enjoy it," Loewenstein said.

This hypothesis was in fact supported by the researchers’ analysis. “They wanted [sex] less and liked it less. That seemed to explain most of the change in their overall mood," Loewenstein said.

This finding that externally-motivated sex leads to reduced enjoyment is consistent with research from other areas. For example, studies have shown that couples in fertility treatment report less pleasurable and satisfying sex.

The researchers speculated that more sex could, in fact, cause more happiness under certain conditions; for example, if sex frequency can be made to increase organically or unobtrusively, without being imposed as a request or demand.

"If we ran the study again…we would try to encourage subjects into initiating more sex in ways that put them in a sexy frame of mind, perhaps with babysitting, hotel rooms, or Egyptian sheets, rather than directing them to do so," said Loewenstein.

Seeking to have couples increase their sex frequency by improving their desire to have sex is a sensible approach. After all, sex, in one sense, is a bit like exercise: You know you’ll feel better having done it, but it’s difficult to get off the couch to start doing it. The desire to have sex fades more quickly—and is more subject to habituation and outside influences—than the enjoyment of the sexual act itself. If we can get couples to have more sex because they feel like having more sex, then we may indeed see the resulting increase in sex causing an increase in happiness.

Or this could not be the case. Perhaps it will turn out that the causal agent in the sex-happiness link is actually happiness and not sex. After all, scientific data often contradicts our wishes and basic intuitions.

(One notorious precedent for this kind of surprise was set in the 1980s with regard to the link between and school performance. At the time, research had begun showing that kids with higher self-esteem were more successful in school. It felt intuitive to conclude that self-esteem causes school success. Self-esteem became a buzzword, and much money and energy were spent on programs aimed to increase children’s self-esteem, with the hope that their school performance would increase as a result. Alas, it did not. With time, it became apparent that the direction of causality was actually the other way around, and that school success is more likely to boost self-esteem than vice versa.)

Finally, it is certainly possible that sex and happiness are not linked causally to each other at all. Correlations are often caused by an external variable. The fact that I have a headache every time I wake up with my shoes still on doesn’t mean that my headache is caused by my shoes, or vice versa—it’s probably the causing both. Perhaps there is a third variable, like health or similarity of values and , that causes both sex and happiness to rise and fall together.