Falling in love comes at a cost that does not include flowers or bar bills. In the first experiment of its kind, scientists have discovered that starting a romantic relationship typically costs two close friends from the inner circle of intimate contacts that most people rely on for support and advice.
The study, led by Professor Robin Dunbar of Oxford University, was part of research into the social networks that most people build up over their lifetime. These networks typically consist of a maximum of about 150 friends, family and acquaintances, with only about five, on average, belonging to the "inner circle".
Professor Dunbar told the British Science Festival at Aston University in Birmingham: "When people are in a romantic relationship, instead of having the typical five inner-circle friends on average they have only four.
"Bearing in mind that one of those is the new person who has come into your life, it means you've had to give up two others."
The study was based on on-line questionnaire interviews with 540 people with an average age of 28. It also found that the participants who admitted to a secondary or "extra-marital" affair did not tend to lose any friends as a result.
Professor Dunbar said the most likely explanation for the loss of close friends when someone fell in love was that a romantic relationship took up a person's time. "The intimacy of a relationship, your emotional engagement with it, correlates very tightly with the frequency of your interactions," he said.
The closest friends, or inner circle, are defined as those someone will see at least once a week and who will offer emotional support when needed.
Men typically had four or five close friends, while women had five or six, Professor Dunbar said.
The next layer of friends consisted of people seen at least once a month, or whose death would be upsetting. They numbered 10 to 15.
Beyond this group, there was a much larger number of more distant "friends" in a person's social network who might include work colleagues or fellow members of a sports club.
Professor Dunbar said a survey by the Facebook social networking website showed that the average number of "friends" a member had was around 120 to 130,
This happened to be close to the "Dunbar number'" of 150 friends, family and acquaintances – thought to be the maximum number the human brain can cope with comfortably.
However, men claimed to have far more Facebook "friends" than women.
Professor Dunbar believes this could be because being popular is seen as an attractive trait in men.
"The boys seem to be in a competition to see who can have the most Facebook friends. I would suggest that it's a kind of mate advantage," he said.
The research into human social networks is part of a wider investigation into the related social relationships of apes and other primates, such as chimpanzees, the closest living relatives of man.
Professor Dunbar said that social networks of people today could be related to the small bands of hunter-gatherers who spent tens of thousands of years in social groups numbering no more than about 150 individuals.