"It ends up putting a lot of importance on physical appearance, and that I think is leaving out a large section of the population," said Twenge, who teaches psychology at San Diego State University . "For a lot of folks who are of average appearance, marriage and stable relationships was where they were having sex." Unlike in face-to-face meetings where "you can seduce someone with your charm," she said, dating apps are "leaving some people with fewer choices and they might be more reluctant to search for partners at all."