BBC News  Photo Mark Velasquez

Many women are left unhappy in the aftermath of casual sexual encounters, a survey has revealed.

Just under half of women who answered the internet poll, published in the journal "Human Nature", said they felt it had been a bad idea.

Four out of five men, in contrast, said they were happy with a brief fling.

The academic leading the research said it showed that there was no evolutionary advantage for women in one night stands.   The survey was answered by 1,700 people who had all experienced this kind of relationship.   While men reported feeling more content, sexually satisfied and confident after meaningless sex, women were more likely to worry about feeling used and "letting themselves down".

Some of the their reasons for the encounters were because they felt there was the possibility of a longer term relationship.

Professor Anne Campbell, from Durham University, who carried out the research, said this was evolution at work.   "In evolutionary terms women bear the brunt of parental care and it has been generally thought that it was to their advantage to choose their mate carefully and remain faithful to make sure that their mate had no reason to believe he was raising another man's child.

"Recently, biologists have suggested that females could benefit from mating with many men - it would increase the genetic diversity of their children, and, if a high quality man would not stay with them forever, they might at least get his excellent genes for their child."

However, she said that if women were designed by evolution for short-term relationships, they would enjoy them more, and the survey suggested this was not the case.   "Basic emotions guide us down pathways that have been advantageous for our ancestors," said Professor Campbell.