London: We take as little as 100 milliseconds to “read” a face for traits such as trustworthiness, but the consequences of such judgements often are far from fleeting, so a group of scientists set out to find out what drives these impressions.

“For many reasons, including the increasingly pervasive use of images of faces in social media, it is important to understand how first impressions arise,” they wrote in the Proceedings of the National Academy of Sciences on Monday.

The shape of a jaw or set of the eyes can lead to long-lasting opinions about someone. The scientists listed 65 facial attributes such as these.

They found that despite the extraordinary variation in human faces, a “substantial portion of the variance in first impressions can be accounted” for in changes of objectively defined features. They created a model that predicted first impressions of faces seen for the first time using a combination of facial attributes that explained 58 per cent of the variance in impressions, they wrote.

The researchers, from the University of York in Britain, used 1,000 faces to figure out the facial expressions that communicate approachability, youthful-attractiveness and dominance. They used all white Western faces to avoid any racial influences in impressions, they wrote. Then they reversed the process and came up with a model that generated faces to produce the predictable impressions.

Their work offers insight into how pictures of a person can be chosen to convey specific impressions, the researchers wrote.

Such first impressions, the researchers said, “are made automatically, likely outside of conscious control.”