We all know about the benefits of friendship in our own lives, but the science behind it is eye-opening. Friends help you experience more life satisfaction, thrive in the midst of challenges, and even live longer.

Researcher Robin Dunbar found that most people have circles of friends—the five people who are the closest to them, then 15 who are the next-closest, 50 who are the next-closest, and 150 who are least close. (Dunbar also found that humans can manage 150 relationships at most).

While today people may have many more social contacts (through Facebook and other means) than ever before, they also have fewer close confidants with whom they can discuss important matters.

However, nearly all the well-being—health, happiness, and longevity—that friendships provide comes from the 5 to 15 people closest to you, not the full 150. Research suggests that those who strive for affinity (the drive to deepen and build close relationships) tend to be happier, healthier, and less depressed than those who value popularity (the drive to have more friends).

Here are nine ways to strengthen and build relationships with your 5 to 15 closest friends:

1. Make time to hang out.

Spending real time with friends can “fire up your nervous system and trigger the release of feel-good neuropeptides called endorphins,” and even boost your immune system. In their analysis of very happy people, Ed Diener and Martin Seligman found that very happy people tended to have strong ties to close friends and family and were committed to spending actual time with them.

Research suggests that spending time with friends (or even anticipating uplifting events with friends) helps people feel less depressed. Face-to-face socializing has been found to be more powerful than phone calls and emails in guarding against depression.

Research also suggests that the more time and effort you invest in friendships, the more you become committed to maintaining the relationship, which drives more closeness and helps the friendship persist through ups and downs.

Whether it’s a mud run, Taco Tuesday, opening night of a new movie, kayaking in the lake, Friday night drumming circle or weeknight baseball game—plan fun and different things to do with friends. Be the organizer, and get things on the calendar. Also, consider setting up a standing date with friends, such as a once-a-month poker night, dinner club, or brunch.

2. Send positive text messages.

Try these:

  • “What are you up to?” texts
  • Congratulations texts
  • Holiday texts
  • Funny memory or old photo text: “I was just thinking about that time we dressed up like cat memes for that Halloween party. Can you believe we did that?!”
  • Checking-in texts like: “How did it go with your dog training lesson?” or “How’d your mom’s surgery go?” or “Is the baby sleeping any better? Anything I can do to help?”

3. Touch base at least once every 15 days.

Don’t let months go by without seeing friends. One study revealed that friends whose relationships persist tend to touch base at least once every 15 days.

4. Call your friends back.

Call friends back within 24 hours of them calling you. Analyzing phone calls between 2 million people, researchers found that one leading cause of persistent relationships is reciprocity, or returning a friend’s call.

5. Send cards (and personalize your holiday cards).

Send thoughtful and personalized (snail-mail) cards, including get well cards, congratulations cards, and thinking of you cards.

Also, research suggests that while non-personalized holiday cards (including those with informative form letters) do not promote relational maintenance, not sending one may have a negative effect on relational maintenance. This research also suggests that people believe holiday cards with a personalized note (especially something meant only for that recipient) maintain relationships better than those without one, likely because the recipient feels special because of the extra effort and disclosure of personal information.

6. Help friends celebrate positive events.

When a friend buys a new house, has another baby, or gets a promotion, help them celebrate. Give them a small gift, take them out to dinner, or just call and ask them to tell you all the details.

7. Support friends through tough times.

One of the top three qualities people look for in friends is supportiveness. Having friends present or available to you during tough times can help you cope better.

One study suggests that having a best friend present for negative experiences buffers their stressful impact. Another study showed that texting with people during their surgery can actually reduce patients’ pain.

Support your friends by:

  • Planning a visit at an appropriate time after a tragedy or hard event.
  • Showing up to a wake or memorial service if a friend’s loved one passes.
  • Providing in-kind support, such as delivering a meal or babysitting.
  • Sending a thoughtful item, such as a prayer rock, bubble bath, or adult coloring book.
  • Listening and providing space for your friend to share their feelings.

8. Comment on—don’t just “like”—their social-media posts.

Make it a point to comment on your closest friends’ posts. Direct interaction on Facebook, including comments, is associated with greater feelings of bonding and social capital, and decreased loneliness.

9. Remember their birthdays.

Call close friends on every birthday—and send a card. And take them out if you can; 85 percent of Americans reported feeling special when someone puts a lot of energy into celebrating their birthday.

Pre-kids it was probably easier to do all of this, but it’s not impossible to do now, either, mama. Your friends will love you for it.