(CNN) Are you younger than 25 years old? You may want to appreciate this moment in life when your social circle is at its greatest. If you're older, you may relate to what you read next.
Soon after your mid-20s, your social circle shrinks, according to a recent study by scientists from Aalto University in Finland and the University of Oxford in England.
The teams analyzed data from 3 million mobile phone users to identify the frequency and patterns of whom they contacted and when, as well as overall activity within their networks.
Men and women were found to be socially promiscuous -- making more and more friends and social contacts -- until the age of 25, after which point they started losing them rapidly, with women losing them at an initially faster rate than men. The average 25-year-old woman contacts about 17.5 people per month, while a man contacts 19 people.
No, Facebook friends don't count.
This decline continues for the rest of your life, or at least until retirement, where it plateaus, probably due to reduced data among this age group.
Why does it happen? It comes down to investments.
The theory is that around this age, people begin to decide who is most important -- and valuable -- in their life and make a greater effort to hold on to those friends.
"People become more focused on certain relationships and maintain those relationships," said Kunal Bhattacharya, a postdoctoral researcher at Aalto University who co-authored the study. "You have new family contacts developing, but your casual circle shrinks."
This applies to both partners and friends, and it stems largely from people wanting to settle down and raise a family.
"At the beginning of this age range, women are more focused," Bhattacharya said, meaning women are more intent on finding the correct partner. Once they believe they have, they invest more time in nurturing that relationship and lose others of less value.
"Once you've made decisions and found the appropriate people, you can be much less socially promiscuous and invest your time in these people," added Robin Dunbar, a professor of evolutionary psychology at the University of Oxford who co-authored the paper.
"But they can't be just anybody," he added.
Narrowing down the people you're close to includes friendships as well as life partners, particularly for women, due to the support and help they can provide in times of need.
"Women have this idea of a best friend, who is similar to a romantic partner ... and women work hard at these relationships," Dunbar said. "Particularly with friendships, if you don't invest in them or see those friends, they will decay and quite rapidly drop."
Trends were seen to change slightly in people's late 30s: Men begin losing buddies at a faster rate. "You get a secondary switchover later in life," said Dunbar.
By the age of 39, the average man was contacting 12 people, while women were calling 15 people each month.
Initial numbers of contacts during younger years are higher for men, but by these later years, they soon drop contacts faster than women, and their totals become lower.
"You see this [reduction] in them about seven years later," Dunbar said. "It's the women who make up their mind very early on."
Though the team emphasized that the rapid loss of friends happens in both men and women, experts generally consider this formation of an "inner circle" to be more important to women, mainly due to them having children.
"You make the effort in return for some benefits," said Dunbar, who believes that at this point, people will prioritize those who are more useful to them.
"That investment will help certain aspects of your life," Bhattacharya added.
At this point, contacts such as mothers, mothers-in-law, close friends and family come into play as they help people raise their children, known as the grandmother effect.
"It's the 'tend and befriend' idea, meaning relationships become more important when you have children," said Michael Price, director of the Center for Culture and Evolution at Brunel University London who was not involved in the study. "You're now investing in offspring for the rest of your lives."
In an evolutionary aspect, such such networks are believed to help women raise children.
Price believes that men instead value more individualistic criteria, such as their achievements or status, once they have a family. "It's well established that close, personal relationships are more highly valued by women in general, while men value status more," he said.
Though the study was an opportunity to analyze a large data set across many age groups within a population, Price noted that it did not reveal much about the quality of the relationships being counted or what people were actually talking about. "The quantity doesn't reveal the quality of the relationships," he said.
For many years, social networks have been raising people's "friend" count, making millennials believe they have hundreds or even thousands of friends. But even with these added means of communication, experts believe the time taken to invest in a true close-knit friendship will continue to keep the trend going. Although it may become more international, the value of face-to-face friendship may never change.
"Our natural psychology is small, very small, like a village," Dunbar said. "The internet may allow you to keep relationships going over a much wider geographical area, but [for now], a shoulder 2,000 miles away isn't as good to cry on."