When you’re online dating, why do you swipe left on one person and swipe right on another? Are you carefully weighing every factor that makes someone a good romantic match? Not according to a study of more than 1 million interactions on a dating website published this week in the Proceedings of the National Academy of Sciences . Instead, the results indicate that you are probably looking for "deal breakers," harshly eliminating those who do not live up to your standards.

Not long ago, dating produced no data at all. People met their romantic partners through the recommendations of friends, family, or even at real-world locations known as "bars." Whatever signals and decisions led people to couple up were lost to science. But that's changing. According to the Pew Research Center, 5% of Americans in a committed romantic relationship say they met their partner through an online dating site. Those 30 million people have generated billions of pieces of data. And because most dating sites ask users to give consent for their data to be used for research purposes, this online courting has played out like an enormous social science experiment, recording people's moment-by-moment interactions and judgments.

A team led by Elizabeth Bruch, a sociologist at the University of Michigan, Ann Arbor, tapped into this torrent of dating data. Because of a nondisclosure agreement, the researchers can't reveal the exact source of their subjects, describing it only as an "established, marriage-oriented, subscription-based dating site" from which they randomly selected 1855 people, all based in New York City. Besides photographs, each user's profile could include any number of personal details including age, height, weight, education, marital status, number of children, and smoking and drinking habits. The data set includes some 1.1 million interactions between users. But beyond someone's looks, how much do any of these factors matter for mate selection?

One complication is that online daters are not making just one decision, but several in a series: First, people are swiping their way through profiles and deciding which to dismiss immediately or browse more closely. Then comes the choice to send a person a message, or to reply to one. And of course, the final, crucial decision, which isn't captured by these data: whether to meet the person in the real world. Bruch's team devised a statistical model that maps the "decision rules" people follow during the first two steps.

Bruch and her team divided the rules into two broad categories, "deal breakers" and "deal makers," used to exclude or include people for the next level of contact. Bruch wondered: Is mate selection like a job interview process, where the person with the best combination of positive factors wins? Or is it more like a Survivor-style reality show, where contestants are picked off one by one for a single failing?

When it comes to the early stage of dating, it seems to be all about the deal breakers. For one, prospective daters were wary of proceeding sight unseen. If a profile did not include a photo, for example, both men and women were 20 times less likely to even look at the rest of the person's profile. Smoking was another big deal breaker, associated with a 10-fold drop in interest. But the biggest deal breaker of all turned out to be age, at least for women. All other factors being equal, women overall were 400 times less likely to browse the profile of a man significantly older than herself. But that changed with age. Whereas 20-year-old women were 10 times more likely to ignore a man 10 years her senior, 45-year-old women were nearly 10% more likely to browse the profile of a man 55 or older compared with a man her own age. Perhaps unsurprisingly, men in their 40s tend to be more interested in younger women.

Other differences between the sexes emerged. "Women care quite a bit more about the height of their partners than vice-versa," Bruch says. In pairings where men were about 17 centimeters (or about 6 inches) taller than the woman, the woman was about 10 times more likely to browse the guy’s profile, whereas the man was about three times more likely to browse hers. "That men care about height at all is, we suspect, a function of their realizing they may get rejected if they aren't quite a bit taller than their potential mates," she adds. But when it came to body weight, men were less likely to browse the profile of a woman who was heavy-set, whereas women showed little aversion to—with some showing even more interest in—heavier-set men. These patterns also generally held for the second step, messaging, but with smaller effects. People were harshest at the browsing stage.

The results convince Ken-Hou Lin, a sociologist at the University of Texas, Austin, who also studies online dating. "The science is absolutely solid." He suspects that deal breakers are more important at the early stage of mate selection when people are winnowing down a pool of candidates. "I expect positive selection to kick in at a later stage of the search," he says. Lin hopes that other dating sites will release similar data, because website design could play a bit part in how people make decisions. For example, says Lin, "Tinder doesn't allow users to search, and emphasizes the photos much more than [personal] attributes, which might reduce the deal breaker effects." Then again, perhaps that simply shifts the deal breakers to a person's appearance instead.