A smile gives the first impression that a person is approachable, while large eyes signal youthfulness, and dominance is linked partly to a masculine face shape, scientists say.
A computer system that mimics the human brain has identified which facial features most influence how others first perceive a person.
The system found that mouth shape and area were linked to approachability - a smiling expression is a key component of an impression of approachability.
When it came to youthful-attractiveness, eye shape and area were important, in line with views linking relatively large eyes to a youthful appearance, 'Live Science' reported.
Dominance was linked with features indicating a masculine face shape, such as eyebrow height, cheekbones, as well as colour and texture differences that may relate to either masculinity or a healthy or tanned overall appearance.
For the study, researchers at the University of York in England found 1,000 photographs of people on the Internet, and showed them to six volunteers.
The participants rated their first impressions of the people in the photos on social traits such as trustworthiness and dominance.
These images were typical of pictures seen every day, ranging widely in angle, lighting, ages, expressions, hairstyles and so on.
Each face was broken down into 65 physical features, such as the shape of a person's jaw, mouth, eyes, cheekbones or eyebrows.
The researchers then analysed these faces using an artificial neural network, a kind of artificial intelligence computing system that mimics how the brain works.
They had the neural network attempt to learn which facial physical features might be linked to first impressions of social traits.
This model suggested "that given enough data, we can accurately gauge people's likely impressions of a given image," said study co-author Tom Hartley, a cognitive neuroscientist and psychologist at the University of York.
The researchers then created a model that generated cartoon faces depicting the typical characteristics of someone judged as having certain social traits.
They compared the results with those of 30 human judges, and found that these cartoon faces usually gave the first impressions they were designed to give.
Hartley suggested that future research might be able to use these findings "to select an image which conveys a desirable impression, perhaps even automatically."
However, he noted that the researchers looked only at Caucasian faces in this study, to avoid possible confounding effects of race, though they are currently conducting cross-cultural studies to find out how culture impacts the results.
The study is published in the journal Proceedings of the National Academy of Sciences.
