If what we watch is a touchy subject, what we buy is even more intimate and revealing. Programs like Facebook Beacon, designed to advertise users’ recent purchases to their friends, have been widely reviled. But the study, published today in Science, isn’t about personal sharing. It’s based on testing what the researchers call unicity: the odds that if you know fragments of a person’s shopping history, you can match them against a much larger amount of data, uncovering everything else they’ve bought. As it turns out, those odds are very high.