Ideally, any potential date deserves a fresh look, unaffected by what you thought of the last person you saw. But new research suggests that we may not be giving prospects a fair chance when we switch or swipe from one profile to another on dating apps and Web sites.

In a study described in March in Scientific Reports, female subjects saw men's faces on a screen for 300 milliseconds—about the length of a very short view on a dating app such as Tinder. After each face, they judged it attractive or not. The researchers found that faces were more likely to be judged attractive when they followed other attractive faces. Two factors caused this pattern: a response bias, in which one presses the same key as last time, and a perceptual effect mostly likely caused by the short interval allowed for processing the faces.

Previous studies have shown contrast effects, in which people in photographs look uglier when viewed next to portraits of attractive strangers. But in the new study, the exposure was so brief that an individual face was not fully processed, and thus it took on qualities of the previous face. Jessica Taubert, one of the lead authors of the paper and a researcher at the University of Sydney, advises online daters: “Be mindful that your brain has limited cortical resources.” In other words, slow down!

In another new paper, in the Journal of Experimental Social Psychology, researchers asked whether contrast effects occur when judging personality. Participants viewed two dating profiles. When the first person came across as uncaring (“I get bored talking about feelings and stuff”), the second person, who was nice but unattractive, seemed much more appealing. In real profiles, people might not appear as blatantly callous as in this study, but other personality traits could be turnoffs that bias viewers' later decisions.

So whether on Tinder or OkCupid, it pays to clear your head and try to view each profile as a unique individual—before rushing on to the next one.