If you’re looking to reboot your health this year, you might sign up for your first triathlon, kickstart a meditation habit, or cut down on ultra-processed foods. But the latest science suggests the best way to improve long-term health isn’t physical, it’s social: connection. Strengthening relationship ties by exercising what experts call “social fitness” is the most influential brain and body hack. Like weight training staves off bone density loss as you age, social fitness counters the downstream effects of chronic stress. “Not exercising your social fitness is hazardous to your health,” says Robert Waldinger, a psychiatrist and psychoanalyst at Massachusetts General Hospital and a professor of psychiatry at Harvard Medical School.