Friends do your health so many favors. They protect your health as much as quitting smoking and a great deal more than exercising, according to a large 2010 review in the journal PLOS One. More research has shown that socially isolated people are more than twice as likely to die from heart disease as those with a solid social circle.
“Strong social relationships support mental health, and that ties into better immune function, reduced stress and less cardiovascular activation,” says Dr. Debra Umberson, a sociologist at the University of Texas, Austin. Umberson says emotional support is just one of a dozen ways friends may safeguard your health and extend your life.
Unfortunately, though, many of us don't have enough of them. According to data from the General Social Survey (GSS), the number of Americans who say they have no close friends has roughly tripled in recent decades. “Zero” is also the most common response when people are asked how many confidants they have, the GSS data show. And adult men seem to be especially bad at keeping and cultivating friendships.
That may seem strange in the era of Facebook, Twitter and boundless digital connectivity. But the “friends” orbiting at the farthest reaches of your digital galaxy aren’t the ones that matter when it comes to your health and happiness.
The vital friendships—the pals you hug and laugh and lament with—are the ones who have the greatest impact on your health and happiness. You need between three and five of them for optimal wellbeing, suggests research from Dr. Robin Dunbar, an evolutionary psychologist at the University of Oxford.
Dunbar’s name comes up a lot when you start digging into the subject of friendship. From his early work studying the brains and social circles of primates, he recognized that the size of a human’s social network might be limited by the size of a certain part of the human brain called the neocortex, a critical site for higher brain functions. After some complicated study, he came up with a figure now known as “Dunbar’s number.”
That number—usually cited as 150, but actually a range between 100 and 200—is the approximate size of a person’s social circle, or the perpetually changing group of friends and family members that you would invite to a large party. While you may have far fewer than 150 of these people in your life, your brain really can’t hold a close connection with more than 150, Dunbar’s research shows. Within that group, he says your closest 15 relationships—including family members or “kin”—seem to be most crucial when it comes to your mental and physical health.
But that’s not to say a brother or sister offers you the same benefits as a close friend, Dunbar says. While your kin are more likely to be there for you when you need help, your good friends tend to fire up your nervous system and trigger the release of feel-good neuropeptides called endorphins. Whether you’re laughing with your pal or feeling him or her touch your shoulder in sympathy, the resulting rush of endorphins seems to “tune” up your immune system, protecting you from disease, Dunbar explains.
So yes, for the sake of your health, you need friends—ideally the really close kind you see face-to-face on a regular basis. But even one very good friend can improve your life in profound ways, says Dr. Mark Vernon, a philosopher, psychotherapist and author of The Meaning of Friendship.
Despite their value in terms of your health and wellbeing, don't think of them as your personal social doctors. Vernon warns against turning your friends into what he calls “service providers”—that’s not what friendship should be about, he says, even if your pals are good for you.
In the end, Vernon says Ralph Waldo Emerson may have offered the best advice when it comes to making and keeping close pals: “The only way to have a friend is to be one.”
Read next: 5 Types of Friends That Everyone Has
Illustration by Peter Oumanski for TIME