Men's friendships with the opposite sex are driven by sexual attraction, regardless of whether they are single, a new study has revealed.
However, women are more likely to consider their friendships with men as platonic – and only hope for more if their own relationship is in trouble, the Daily Mail reported.
The scientists' findings mirror the plot of 1989 film When Harry Met Sally, in which Harry, played by Billy Crystal, tells Meg Ryan's Sally "Men and women can't be friends because the sex part always gets in the way."
According to the researchers, the study had 'potential negative consequences' for people in long-term relationships.
They said the impact of work, hobbies and university has seen friendships between men and women reach unprecedented levels.
But our mating instincts, which have evolved over hundreds of thousands of years, may get in the way.
In a survey, 88 pairs of young male and female friends were asked to rate their attraction to each other in a confidential questionnaire.
Men – whether attached or single – were more likely to be attracted to their female friends and want to go on a date with them than the other way around.
They also assumed their female friends were more romantically interested in them than they actually were – and women tended to be unaware of this.
Single and attached women showed the same level of attraction to their male friend. But attached women tended only to want something to come of that attraction if their relationship was in trouble.
Women were also less attracted to attached men.
A second questionnaire for 140 middle-aged people, who were almost all married, found levels of attraction between male and female friends fairly equal.
Middle-aged men's attraction to their female friends was much lower than that of the younger men, except among those who were single. For women, levels of attraction had stayed the same.
Participants of both questionnaires said they gained benefits from friendship with the opposite sex including getting good advice and boosting their confidence.
Those who did harbour a secret crush were five times more likely to see it as a potential problem than a benefit. But more men than women saw it as a perk.
The authors of the study, from the University of Wisconsin, said that films and television programmes had helped instill the idea that normal friends can easily become 'friends with benefits' – friends who have sex with each other.
The study has been published in the Journal of Social and Personal Relationships.