Countless research and self-help books claim that having more sex will lead to increased happiness, based on the common finding that those having more sex are also happier. However, there are many reasons why one might observe this positive relationship between sex and happiness. Being happy in the first place, for example, might lead someone to have more sex (what researchers call 'reverse causality'), or being healthy might result in being both happier and having more sex.

In the first study to examine the causal connection between sexual frequency and happiness, Carnegie Mellon University researchers experimentally assigned some couples to have more sex than others, and observed both group's happiness over a three month period. In a paper published in the Journal of Economic Behavior & Organization, they report that simply having more sex did not make couples happier, in part because the increased frequency led to a decline in wanting for and enjoyment of sex.

One hundred and twenty eight healthy individuals between the ages of 35-65 who were in married male-female couples participated in the research. The researchers randomly assigned the couples to one of two groups. The first group received no instructions on sexual frequency. The second group was asked to double their weekly sexual intercourse frequency.

Each member of the participating couples completed three different types of surveys. At the beginning of the study, they answered questions to establish baselines. Daily during the experimental period, the participants answered questions online to measure health behaviors, happiness levels and the occurrence, type and enjoyableness of sex. The exit survey analyzed whether baseline levels changed over the three-month period.

The couples instructed to increase sexual frequency did have more sex. However, it did not lead to increased, but instead to a small decrease, in happiness. Looking further, the researchers found that couples instructed to have more sex reported lower sexual desire and a decrease in sexual enjoyment. It wasn't that actually having more sex led to decreased wanting and liking for sex. Instead, it seemed to be just the fact that they were asked to do it, rather than initiating on their own.

"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. If we ran the study again, and could afford to do it, we would try to encourage subjects into initiating more sex in ways that put them in a sexy frame of mind, perhaps with baby-sitting, hotel rooms or Egyptian sheets, rather than directing them to do so," said George Loewenstein, the study's lead investigator and the Herbert A. Simon University Professor of Economics and Psychology in the Dietrich College of Humanities and Social Sciences.

Despite the study's results, Loewenstein continues to believe that most couples have too little sex for their own good, and thinks that increasing sexual frequency in the right ways can be beneficial.

One of the study's designers, Tamar Krishnamurti, suggested that the study's findings may actually help couples to improve their sex lives and their happiness.

"The desire to have sex decreases much more quickly than the enjoyment of sex once it's been initiated. Instead of focusing on increasing sexual frequency to the levels they experienced at the beginning of a relationship, couples may want to work on creating an environment that sparks their desire and makes the sex that they do have even more fun," said Krishnamurti, a research scientist in CMU's Department of Engineering and Public Policy.