Research has shown that when people start a new romantic relationship they are likely to lose two members of their inner circle of friends.
One is sacrificed to make way for the new partner, and the other disappears due to being ignored for too long, scientists believe.
Previous research has shown friendships can wither if insufficient effort is put into maintaining them.
Typically, people have a small number of around five “inner circle” friends who they can confide in.
It is these friendships that can be sacrificed for love, according to new research outlined at the British Science Festival at Aston University in Birmingham.
Study leader Professor Robin Dunbar, from Oxford University, said: “We’ve just shown that if you have a romantic relationship it actually costs you two friends.
“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.
Prof Dunbar is famous for determining “Dunbar’s number” – the maximum number of stable social relationships the human brain is theoretically capable of handling, said to be 150.
Prof Dunbar said: “What I suspect happens is you’re so wholly focused on the romantic partner you just don’t get to see the folks you held onto before, and so some of those relationships start to deteriorate.”
More than 100 men and women aged 18 to 60 took part in the study which involved answering questions about romantic relationships and friendships.
Although women tend to be more socially active than men, both sexes tended to lose the same number of friends when they found a new romantic partner.
“Inner circle” friends were defined as those a person might see once a week or who can be confided in at times of crisis and stress.