And it suggests that, as time goes on, the beauty gap between men and women will grow.

Researchers demonstrated that beautiful people are 36 per cent more likely to have a daughter than a son as their first-born child, backing the evolutionary theory that parents tend to produce children who benefit from their own attributes.

Selection pressure means that when parents have traits to pass on that are better for boys than for girls, they are more likely to have boys. Such traits include size, strength and aggression, which might help a man compete for mates.

On the other hand, parents with heritable traits that are more advantageous to girls are more likely to have daughters.

Dr Satoshi Kanazawa, an evolutionary psychologist at the London School of Economics, who led the study, said: "Physical attractiveness is good for both men and women, but it is much better for women than for men. So, physically attractive parents bias their offspring sex ratio to have more daughters."

Dr Kanazawa based his conclusions on data from 3,000 Americans, who were assessed for factors that provide an objective measurement of attractiveness, such as symmetry, and characteristics, such as female breasts or male body hair.

When he looked at the sex of the volunteers’ first-born children, he found attractive parents were far more likely to have had girls, it was reported in the Journal of Theoretical Biology.

Dr Kanazawa said: "Because physical attractiveness is heritable, and because physically attractive parents have more daughters and less attractive parents have more sons, over time, the average level of physical attractiveness among women increases relative to men, so that women are on average more attractive than men."

His previous research has shown that scientists, mathematicians and engineers who have systematic "male brains" are more likely to have sons.