Researchers talked to 540 people, aged 18 and over, about how their inner core of friends changed when they entered into a relationship with a new partner.
They found the core is usually made up of about five people.
"People who are in romantic relationships - instead of having the typical five [individuals] on average, they only have four in that circle," BBC News
quoted Robin Dunbar, a professor of evolutionary anthropology at Oxford, as saying.
"And bearing in mind that one of those is the new person that's come into your life, it means you've had to give up two others."
Dunbar stated that the intimacy of a relationship correlates with the frequency of the interactions between those involved.
"If you don't see people, the emotional engagement starts to drop off, and quickly,” he said.
"What I suspect happens is that your attention is so wholly focused on your romantic partner that you just don't get to see the other folks you have a lot to do with, and therefore some of those relationships just start to deteriorate and drop down into the layer below."
Previous research he took part in showed that the maximum number of friends it is possible to engage is approximately 150.
The latest research was presented to the British Science Festival at Aston University.