Hollywood’s most beautiful couple, Brad Pitt and Angelina Jolie, are in the grip of evolutionary forces that made it almost inevitable that their child would be a girl.

According to research, attractive parents are 26% more likely to have a daughter than a son as their first child. It is an inexorable process that has resulted in women becoming increasingly more attractive than men.

This is because of differing ‘evolutionary strategies” that each sex has adopted to survive, claim researchers at the London School of Economics.

While reproductive success for males depends largely on the status of the father (as sons from higher-status families inherit their position and are in turn able to protect and invest in their offspring), daughters’ reproductive successes mostly depend on their youth and attractiveness. “We have shown two things,” said Dr Satoshi Kanazawa, who led the research. “Beautiful parents have more daughters than ugly parents, because physical attractiveness is heritable and because daughters benefit from attractiveness more than sons.

“We have also shown that women on average are more attractive than men, because over evolutionary history the slight bias of beautiful parents to have more daughters has accumulated, so that girls have become more and more attractive than boys.”

Men prefer younger and physically more attractive women for their mates. A potential mate’s status or wealth is far less important for men than her youth and physical attractiveness, argues the report.

The research, in the Journal of Theoretical Biology, analysed more than 20,000 people in America. Researchers rated their beauty according to height, weight and apparent age, all factors that can be used to judge basic attraction levels without subjective viewpoints. Only first-born children were included in the analysis.

Dr Mark Thomas, senior lecturer at the biology department of University College London, said the LSE’s results appeared to “fit in” with the state of research on sexual evolution.

He said the phenomenon was rooted in men’s natural promiscuity, noting: “Females can only reproduce so many times in their lives whereas for men, theoretically, the limit is all of the females in the world times the number of reproductive opportunities (those females) have.”

Besides Pitt and Jolie, who named their daughter Shiloh Nouvel, the Hollywood couple Reese Witherspoon and Ryan Phillippe, whose first-born daughter Ava is six, lend weight to the theory.

The phenomenon does not end with superstars. Less obvious public figures are also boosting the figures.

Boris Johnson, the Conservative MP for Henley and heart-throb to some, said that if beauty did affect the sex of offspring he could take no responsibility for it.

The shadow minister for higher education, who has two daughters and two sons with his wife Marina, said last night: “I’m sure that I’ve made no contribution on the attraction front. All the beauty quotient lies with my wife, which is probably why we have two of both.”