If you're looking for a boost in happiness, having more sex may not be your best bet. New research suggests that upping the frequency of sex can make individuals less happy under some circumstances.

Researchers divided couples into two groups, asking one group to double how many times they had sex every week, while asking the control group to stay the course and have as much sex as they normally would. By the end of the three-month study, the people who increased their sexual forays were actually less happy than they were at the beginning of the study, the researchers found.

"The findings were a surprise and a disappointment," said the study's lead researcher, George Loewenstein, a professor of economics and psychology at Carnegie Mellon University in Pittsburgh. "We were expecting that the people who had more sex would enjoy it a lot and would be happier, and it would be good for the relationship.

"Instead, what we found was that the group who had more sex enjoyed it less, they wanted it less and they reported lower levels of happiness," Loewenstein told Live Science.

However, the finding doesn't necessarily mean that more sex makes people unhappy, he said. It could be that being ordered to have sex puts a damper on it.

"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.

Countless studies show a link between sex and happiness. For instance, one analysis of 16,000 American adults found that people who had more sex had high levels of self-reported happiness. Another study found that people who have more active sex lives also reported having happier relationships.

But both of these studies are correlational; it's not clear whether sex directly causes happiness, or whether other factors, such as health or personality, are at play.

The researchers of the new study attempted to determine a more direct connection between sexual frequency and happiness, they said.

They recruited 128 healthy people between the ages of 35 and 65 who were in married heterosexual relationships. The participants completed a number of surveys on their personalities, emotions and libidos at the beginning and end of the study, as well as a daily survey to help the researchers monitor changes throughout the 90-day study.

Contrary to Loewenstein's expectations, the couples who were directed to have more sex reported lower levels of happiness by the study's end. Still, sex is crucial to any healthy marriage, he said.

"A lot of people rate sex as the number one most pleasurable activity," Loewenstein said. "It's good for your health and it burns some calories."