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.

A new partner pushes out two close friends on average, leaving lovers with a smaller inner circle of people they can turn to in times of crisis, a study found.

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.

Dunbar's team used an internet-based questionnaire to quiz 428 women and 112 men about their relationships. In total, 363 of the participants had romantic partners. The findings suggest that a new love interest has to compensate for the loss of two close friends.

Speaking at the British Science Festival in Birmingham, Professor Dunbar said: "This was a surprise for us. We hadn't expected it. "

"If you don't see people, your emotional engagement with them drops off and does so quickly. What I suspect is that your attention is so wholly focused on the romantic partner you don't get to see the other folks you had a lot to do with before, and so some of those relationships start to deteriorate."

The questionnaire allowed people to mention whether any of their closest confidants were "extra romantic partners". In all, 32 of those quizzed mentioned having an extra love interest in their life, but these people did not lose four friends as might be expected. Instead, the extra person in their life bumped their original romantic partner out of their innermost circle of friends.

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.

"Boys seem to be in a competition to see who can have the most Faccebook friends and that could be a form of mate advertisting. One of the cues women use for male quality as a mate is the number of other girls chasing them, so signing up lots of girls as Facebook friends seems to be a good idea," said Dunbar.