People who encounter the dark silhouette of a stranger are more likely to decide it's a man, a gender bias which is a survival reflex, U.S. researchers say.

The new research, conducted at the University of California, shows that people are more likely to perceive a human silhouette as a man — even if its shape is androgynous or curvy and traditionally more female.

Body shapes that are more commonly associated with women, "are nevertheless categorized as men a significant portion of the time," wrote lead researcher Kerry Johnson in a study published today in the journal of the Proceedings of the Royal Society.

This misperception might be a defence mechanism, the study adds. The biased error in judgement happens more often when people are afraid, it said.

"Compared with women, men are not only physically stronger but also prone to aggression and violent crime. Consequently, unknown men pose a potential threat to perceivers," it said.

So, when someone encounters a stranger in a dark alley or at a distance, they may assume that person is a man in order to be "better safe than sorry."

Better safe than sorry

"Miscategorizing a woman to be a man may unnecessarily compel a perceiver to avoid contact, but it is an error committed in prudence," the study said. "Miscategorizing a man to be a woman, by contrast, is an error that may expose a perceiver to physical risks."

Researchers conducted four separate studies. One study involved participants looking at a variety of computer-generated body shapes with different hip and waist sizes, typical physical cues of one's gender.

'Miscategorizing a man to be a woman, by contrast, is an error that may expose a perceiver to physical risks.'— UCLA study on gender misperception

Another study involved showing participants video clips of people in various situations, such as a woman being chased through a darkened house by a murderer, and judging whether the other person involved was male or female.

Consistently, the results across the studies showed that people were more likely to characterize these unknown persons as men.

"Collectively, these findings support our hypothesis that sex categorizations are functionally biased towards a male percept. Although this bias produces sex-categorization errors, they should not be construed as mistakes .... Sex-categorization biases are functional, even though they are error-prone," the study said.