Scientists know it takes humans just milliseconds to look at someone's face and judge them good-looking or trustworthy.
Now, a new study finds that certain facial features seem to trigger specific first impressions about a person's character, too.
The shape and size of the mouth, for example, appear directly linked to whether someone seems approachable, while eye dimensions are keys to attractiveness.
The study findings suggest that first impressions are fairly simple, basic and predictable, said co-author Tom Hartley, a lecturer with the department of psychology at the University of York in England.
"Our work shows which features are associated with which first impressions, so we could use this to identify pictures that project the most desirable impressions, or even perhaps tweak images to produce a specific effect," he said.
The researchers validated their research by creating cartoon-like faces that were designed to draw certain kinds of first impressions.
"We wanted to see whether we can estimate people's first impressions of the very varied images we encounter every day, like on the Internet," Hartley said. "The impressions we create through images of our faces such as avatars or selfies are becoming more and more important in a world where we increasingly get to know one another online rather than in the flesh."
Nicholas Rule, an assistant professor of psychology at the University of Toronto in Canada, agreed.
"People are making these decisions all the time now in the context of social media. People have lots and lots of photos of themselves, which are easy to come by," said Rule, who was not involved in the study. "With this flood of images comes a choice about how someone wants to represent himself or herself."
In the study, published July 28 in the Proceedings of the National Academy of Sciences, researchers measured 65 physical facial features, such as eye height and eyebrow width, and tried to develop a statistical model that could accurately predict first impressions.
"By combining them we could explain over half of the variation in human raters' social judgments. This was surprisingly straightforward, and our equations involve only weighting the different physical features and adding them together," Hartley said.
Among other things, a "masculine" face -- determined in part by eyebrow height, cheekbone structure, skin color and texture -- was linked to dominance. More feminine face shapes created perceptions of attractiveness and youth, the researchers found.
"Some of our most important judgments do depend on fairly obvious features: a smile looks friendly, and an angry expression makes you less approachable," Hartley said. "However, many of the features we identified as correlated with social impressions are far less obvious. For example, an unapproachable-looking person has a pale, drawn appearance."
The main problem, as it's always been, is that the cover doesn't always reflect the book.
"The danger is that these instant judgments are inaccurate, that we misjudge people based on their appearance," Hartley said. "Worse still, we're largely unaware of the extent to which our initial feelings toward other people are influenced by appearance, and this makes us prey to unconscious biases."
Evolutionary changes may have given humans the ability to make instant judgments so they could identify potential allies and enemies, not to mention mates, Hartley said.
Along those lines, Hartley suggested that people read the study before they post a photo with their resume or online dating profile.