It only takes a fraction of a second for us to decide whether we find someone attractive and you'd be amazed what we may discover in that time.
Scientists at the University of Aberdeen and St Andrews University have found that masculinity and femininity are important factors when we're checking someone out.
The faces in our Sex ID survey were manipulated in various ways to find out exactly what people look for in a potential partner.
Masculinity and femininity
Studies have shown that men tend to prefer feminine faces. But women tend to vary in what they find attractive. Some women prefer masculine men while some women prefer feminine men.
This appears to be partly due to hormonal factors. Women demonstrate stronger preferences for masculine men during the fertile phase of the menstrual cycle than at other times. It is also partly due to a woman's own attractiveness. The more attractive a woman rates herself, the stronger her preference for masculine men.
Some of the faces seen in the test were manipulated to appear more masculine (eg a square jaw and pronounced brow) while others had been manipulated to be feminine (eg large eyes and full lips).
Scientists hope to learn more about our preferences for masculine and feminine faces by studying people's responses to the Sex ID questions and their performance on the faces task.
The Sex ID test was originally an online experiment. We've stopped collecting data now and many of the questions have been removed from the test, but we've kept the fun stuff!
Secrets of the Sexes
A BBC One television series Secrets of the Sexes explores brain sex differences. Find out more about the programmes.
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Find out about the BBC One television series Secrets of the Sexes
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