Just about any massage therapist will claim that word-of-mouth referrals—positive feedback about a massage session shared by a satisfied client with the client’s friend or acquaintance—are one of the most reliable forms of marketing.

New research shines a light on how word-of-mouth referrals work, from a psychological standpoint.

The study, led by Harvard psychology professor Daniel Gilbert, indicates that if people want to know how much they will enjoy an experience, they’re better off knowing how much someone else enjoyed it than knowing about the experience itself.

Gilbert and colleagues studied how people make decisions based on predictions about how much pleasure, satisfaction, utility or reward those decisions will bring them.

“People do not realize what a powerful source of information another person’s experience can be,” says Gilbert. “People believe that the best way to predict how happy they will be in the future is to know what their future holds, but what they should really want to know is how happy those who’ve been to the future actually turned out to be.”