Falling in love comes at the cost of losing close friends, because romantic partners absorb time that would otherwise be invested in platonic relationships, researchers say.

The research, led by Robin Dunbar, head of the Institute of Cognitive and Evolutionary Anthropology at Oxford University, showed that men and women were equally likely to lose their closest friends when they started a new relationship.

Previous research by Dunbar's group has shown that people typically have five very close relationships – that is, people whom they would turn to if they were in emotional or financial trouble.

"If you go into a romantic relationship, it costs you two friends. Those who have romantic relationships, instead of having the typical five 'core set' of relationships only have four. And of those, one is the new person who's come into their life," said Dunbar.

The study, submitted to the journal Personal Relationships, was designed to investigate how people trade off spending time with one person over another and suggests that links with family and closest friends suffer when people start a romantic relationship.

If you don't see people, your emotional engagement with them drops off and does so quickly.

In a separate study, Dunbar's team looked at how men and women maintained friendships on the social networking website Facebook. They found that women's Facebook friends were more often friends from everyday life that they spent time with, while men tended to collect as many friends as they could, even if they hardly knew them.