There is no doubt that the people in our lives have a great influence over us. Our parents instill lasting beliefs in our heads about money. The people we work with can pick us up or put us down.

Most important of all, the friends we choose—and trust me, as grown adults we definitely have a choice when it comes to whom we spend time with—can either lead us to ruin or lift us to greatness.

As a man of science, I’m happy to say that research backs me up. Study after study shows that one of the secrets to life is choosing friends wisely.

This is something mentors have been telling us for thousands of years.

“The key is to keep company only with people who uplift you, whose presence calls forth your best,” said the Stoic philosopher, Epictetus, to his students in Ancient Greece.

More recently, Jim Rohn’s famous quote comes to mind: “You are the average of the five people you spend the most time with.”

The world’s greatest performers know this, wrote Brad Stulberg and Steve Magness, the authors of “Peak Performance.” In their words, “Working to build a better self almost always means working to build a better community.”

Whatever habits and attitudes prevail in your social circle will also occupy your mind. Studies from Harvard show that if one of your friends becomes obese, you are 57% more likely to become obese yourself. If your friends smoke cigarettes, the chance you’ll smoke increases.

Even one weak link in your social chain can be ruinous. In a 2010 study, the United States Air Force Academy examined why some cadets improve their fitness during their time at the academy while others do not. The researchers found that the determining fitness factor within a squadron was the motivation of the least fit person in the group.

If the least fit cadet was gung-ho about getting fit, then his enthusiasm spread and the entire squadron improved. But if the least fit cadet was apathetic, the negativity spread through the squadron like the common cold, and it brought everyone down.

Motivation isn’t the only emotion that can spread like a contagious disease. When we are surrounded by happy people, our brain’s neural network—the one responsible for happiness—becomes active. It’s like yawning. When you see someone yawn, you yawn too. And if you see someone who’s happy, it’s enough to pick up your mood. This also explains why we become melancholy when we spend time with people like Eeyore from “Winnie the Pooh.”

Science provides ample evidence of the effect others have. More importantly, a little introspection delivers plenty of proof from our own lives.