Women think more feminine-looking men are most likely to be committed in a relationship.

More masculine-looking men are just after a quick fling, according to over 18,000 women who took part in a BBC Television Megalab '99 experiment.

The experiment aimed to find out why previous research had shown that more women preferred feminine-featured male faces, like Leonardo DiCaprio, than more masculine male faces, like Arnold Schwarzenegger.

Who placed which lonely hearts advertisement?

The answer, the Megalab '99 experiment suggests, is that women associate different personality characteristics with the different faces. Masculine-faced men are seen as dominant but cold and untrustworthy. However, feminine-faced men are seen as warm and kind.

Megalab viewers were shown two male faces, A and B. The faces are very similar but A has slightly more masculine features: a squarer jaw, more prominent brow, thinner lips and smaller eyes.

They were then read a lonely hearts advertisement and asked to choose which man they thought had placed it.

Half of the UK's television viewers were read this: "Attractive professional back in town for short period, likes cinema, pubs and sport. WLTM F for fun and laughter."

The other half heard this: "Caring, attractive professional, likes cinema, pubs and sport, seeks special lady to love and cherish forever."

The results showed that 66% of women who heard the second advertisement, in which a long-term relationship is sought, believed the more feminine-looking man had placed it.

In contrast, only 56% of the women who heard the first advertisement, in which a quick fling is suggested, thought the more feminine-looking man had placed it.

The experiment shows that women's choice of partner may result from what characteristics they think faces show.

Butch but dishonest

Previous research by the experiment's designer, Professor Dave Perrett of St Andrews University, has revealed that making faces more masculine made them look more dominant, but they also appeared colder and less honest.

By contrast, the feminised faces were perceived as kinder and more trustworthy and therefore perhaps more likely to be good partners and parents.

The masculinity of a man's face is determined by his genes (through the action of testosterone), as is the symmetry, another important factor in attraction.

But the Megalab experiment suggests that though purely biological factors are important for humans when choosing a mate, cultural factors also play a major role.

This might also explain some human physical characteristics: the relatively small difference between human males and females, and the relatively feminine appearance of modern man compared to our ancestors.