Love hurts, love scars, love wounds and marks any heart not tough enough or strong enough to take a lot of pain.

But what the Everly Brothers, Nazareth, Emmylou Harris, Gram Parsons and Roy Orbison all omitted to mention is that love costs you two close friends.

That’s the conclusion of research at Oxford University that was presented at the British Science Festival this week. It came in a presentation by Professor Robin Dunbar, whose area of expertise includes social networking, whether that be in the stone age or the cyber age.

Dunbar is best known for his previous studies showing that the average person can maintain a basic level of active friendship with a maximum of around 150 people (which appears to be borne out by Facebook user statistics.) That’s not a social constraint, but rather a physical limit of the neocortex, the brain section that covers conscious thought.

He’s also argued that people have two inner circles of friends: those we see in person occasionally and would be particularly upset if they died, and those we see regularly and would turn to in a crisis. The latter group is usually around four to six people.

The new research looked at those groups among people who had begun a new romantic relationship. It found that on average those who had found love had four in the group, while those who didn’t had 5. Bearing in mind that the group of four includes the new partner, that means two people have been dropped.

According to Dunbar, the most likely explanation is that people spend so much time with their new partner that it reduces the frequency with which they meet with other close friends, to the point that that friendship degenerates. The people remain friends, but the relationship falls into the looser secondary category.

Dunbar also reported a possible reason why men are more likely to have a greater number of Facebook friends than women. He said that women tend to use the site to keep in touch with “genuine” friends from real life, while men are more likely to add people to boost their numbers. That isn’t just pure male competitiveness though: men are particularly likely to add women they don’t necessarily know well offline because having a lot of female Facebook friends may make them appear more attractive to women seeking a mate.

(Disclosure: The author’s wife is an employee of the British Science Association, which organizes the British Science Festival.)