The men and women, who were aged between 35 and 65, filled in psychological surveys at the start and end of the study, as well as daily assessments of their mood.
Those prescribed to have more sex didn't quite manage to double their efforts but they did increase the number of sessions by more than 40 per cent. This meant they averaged nearly nine sessions a month, rather than six.
However, the effects were unexpected. Lead researcher George Loewenstein, said: "This is the first study to assess whether increased frequency really does lead to greater happiness.
"Our main finding is surprising and shows that people who increased frequency had reduced happiness and sexual enjoyment."
Libido also fell, the Journal of Economic Behaviour & Organization reports. Professor Loewenstein, of Carnegie Mellon University in Pennsylvania, said the extra activity may have proved exhausting.
However, he thinks a more likely explanation is that being ordered to have more sex turned it from a pleasure into a duty. In other words, spontaneity may be key.
The professor, a psychologist and economist, said: "Luring couples into having more sex - through travelling to new places or staying in hotels, or perhaps even counselling - rather than directives, might be a better way of improving relationship quality and happiness."
Professor Mike Wyllie of the team of scientists that invented Viagra, described the results as an example of interesting statistics that do not reflect real life. He added: "A survey of any physician's office would undoubtedly show many more patients there to discuss too little sex and few, if any, too much."
- Daily Mail
