The importance of family relationships to happiness is pretty much viewed as a given. Blood relationships come with a closeness not found elsewhere in social relationships. Geneticists and sociologists tell us through science why this is the case.

Friends, though, ride on the periphery: acknowledged as important anecdotally, but seldom the subject of rigorous introspection and scientific study. This is strange given that many families are geographically distant, as people make interstate and international moves for job opportunities and other reasons. The days of having your uncle around the corner and parents living down the street aren’t the norm even in Midwest rural communities. Moreover, family interactions can be full of politics and stress. These days, friends are, in many cases, people’s surrogate family.

William Chopik, an assistant psychology professor at Michigan State University, published a review this month on friendship that gives insight into how, and perhaps why, friendships are important, especially later in life. But friendship comes with its own stressors, as well.

The most curious find in Chopik’s review is that, for many people, friendship brings greater health and happiness to life than family relationships (excluding one’s spouse and children). This increases in people’s middle and senior years.

“Family relationships are still important, but it looks like friendships become more important over time, and then kind of dwarfs the family relationships,” Chopik says.

Chopik teased this information out of a survey involving 270,000 adults of all age ranges who answered questions about relationships, as well as from a survey that focused on support and strain of social relationships among 7,400 seniors.

But though family relationships can be full of psychological strain, relationships among friends also have a dark side. Studies show that bad habits tend to rub off. One study with a sample of 12,000 people that appeared in The New England Journal of Medicine showed that having friends who are obese increases a person’s risk of obesity by nearly 60 percent.

Chopik discovered that seniors experience more stress from friends than from family. The stress can result in health problems.

“When friends were the source of strain, participants reported more chronic illnesses; when friends were the source of support, participants were happier,” Chopik writes in the review. “This finding is consistent with previous research showing that friendship quality often predicts health more so than the quality of other relationships.”

Why do so many people prefer friends to family despite the challenges? Friends are cherry-picked, family members are not, Chopik says.

“We choose our friends and we do so because we enjoy their company,” he says. “We do a lot of leisure activities with them. By contrast, family relationships can be boring or sometimes really stressful. The selectivity of friendships really matters.”