Family and friends are two important kinds of relationships in your life, no doubt. But when it comes to making you happier over the long term, you might be surprised which group is the more powerful. While family members are important, when it comes to better health and happiness, it’s friendships that make the biggest difference—especially as you get older, according to new research.

An article published in the journal Personal Relationships, which summarizes the findings of two related studies, revealed that while both family and friends contribute to health and happiness, it was the relationships people have with friends that have the biggest impact later in life. In total, more than 278,000 people of varying ages from nearly 100 countries were surveyed, rating their health and happiness levels. Notably, in the second study (which focused on older adults, specifically), it was found that when friends were the source of tension or stress, people reported more chronic illnesses, while when someone felt supported by their friendship, they reported fewer health issues and increased happiness.

Why? It all comes down to choice, says William Chopik, Ph.D., author of the paper and a professor at Michigan State University. “I think it might have to do with the selective nature of friendships—we can keep around the ones we like and slowly fade out of the ones we don’t,” he explains. “We often spend leisure activities with friends too, whereas family relationships can often be stressful, negative, or monotonous.”

It’s also possible that friends fill in the gaps left by family or provide support in ways family members can’t or won’t, he adds. Friends may also understand you on a different level than family, due to shared experiences and interests. This is why it’s so important to maintain ties with old friends or make the effort to reconnect if you’ve lost touch with your childhood bestie or sorority sister. While life changes and distance can make this difficult at times, the benefits are well worth the effort to pick up the phone or send that email.

“Friendships are among the hardest relationships to maintain across the lifespan,” says Chopik. “Part of that has to do with a lack of obligation. Friends spend time together because they want and choose to, not because they have to.”

Thankfully there are some simple steps to maintain and enhance important friendships. Chopik recommends making sure to be a part of your friends’ day-to-day lives by sharing in their successes and commiserating with their failures—basically be a cheerleader and a shoulder to lean on. In addition, he says sharing and trying new activities together helps, as does expressing gratitude. Telling people that you love them and value their presence in your life is such a small thing to do, but it can make a huge difference in everyone’s lives.