Narr says that when she watched videos, made in the early years of the study, of the teens asking their best friends for advice or support or talking through a disagreement, it was easy to tell which relationships were strong. “These teens tend to be open with one another about difficult topics, and they’re more engaged with one another and helping the other person and connecting with the other person,” she says.

Those strong relationships are paying dividends in adulthood, the study found. When the researchers evaluated the participants at the conclusion of the study, the ones who had close, emotional links showed improvement in their levels of anxiety, depression and self-worth. In other words, they reported less depression and anxiety and more self-worth at 25 than they had at 15 and 16.