You spot a stranger standing in the street. The lighting is dim and the person's face and clothing are indistinct. Who is it? Chances are you will think it is a man - and this is because of a survival reflex, according to a study published yesterday. Psychologists at the University of California at Los Angeles asked male and female students to look at 21 human silhouettes, all of them the same height but with a progressively changing waist-to-hip ratio. The figures began with an obviously female "hourglass" figure and, after incremental changes, ended with an obviously male "hunk" figure. The volunteers were asked to identify the gender of each silhouette, the idea being to identify the point at which they perceived a shift in gender. What was striking, said researcher Kerri Johnson, was a preference to deem a shape to be a man whenever it was ambiguous - or could readily have been taken for a woman. "I was surprised by the size of the effect. It was much stronger … than I imagined," she said. In the natural world, the demarcation between a woman's shape and man's shape comes when the ratio of the waist and hip circumferences is 0.8. But the students, on average, placed the boundary at 0.68. In other words, an identifiable female shape for them was close to the idealised curves of a pinup. Were those errors in the volunteers' perception? Not so, said Johnson, who believes it is a survival mechanism. A man is likelier than a woman to be a bigger physical threat and our default perception is to prepare for risk. "We suspect that this might be for a self-protective reason," she said. "If you are walking down a dark alley at night, a woman poses no great physical threat to you in general. But if you encounter an unknown man, he's more likely to have a physical formidability that could pose some risks."