Men's friendships with women are driven by sexual attraction, regardless of whether they are single, researchers say.
Women, however, are more likely to consider their friendships with men as platonic - and only hope for more if their own relationship is in trouble.
Researchers warn the new study has "potential negative consequences" for people in long-term relationships.
They say 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.