LONDON - When Hollywood's golden couple, Brad Pitt and Angelina Jolie, had their first child, it was a girl. When Tom Cruise and Katie Holmes had a baby, and Gwyneth Paltrow and Chris Martin had a baby
, they also were girls.
Coincidence? Perhaps not.
Research from the London School of Economics indicates that physically attractive couples are 36 percent more likely than unattractive couples to produce a girl as their first child.
The research - led by evolutionary psychologist Satoshi Kanazawa and published in the Journal of Theoretical Biology - is based on the study of 3,000 young American adults in 2001 and 2002 who were taking part in an investigation called the National Longitudinal Study of Adolescent Health.
After hours of face-to-face discussions, interviewers ranked the attractiveness of participants using a five-point scale ranging from "very unattractive" to "very attractive."
Asked whether rankings were subjective, Kanazawa argued that physical attractiveness is an objective quantitative measure, just like height and weight.
"Standards of beauty are both innate and culturally universal, and everybody agrees on who is beautiful and who is ugly just like they agree on who is tall and who is short," he said. The biggest factor is facial features, he said.
He said there are computer programs that can precisely measure someone's physical attractiveness.
Kanazawa then compared the percentage of boys and girls born to the participants who were very attractive with the sex ratio of babies born to everyone else. He discovered that 56 percent of first babies born to very attractive parents were girls, while fewer than half of the babies born to parents in each of the other categories were girls.
Why does this occur? Kanazawa said that the study supports the evolutionary theory that parents tend to produce offspring who benefit from their own attributes.
Parents who have traits likely to be more beneficial to boys - such as large size, strength, and aggression - are more likely to have boys. Parents who have traits likely to be more beneficial to girls - such as physical beauty - are more likely to have girls.
Kanazawa believes that men value physical appearance more than women do when seeking a partner, and so beauty, in general, is a better attribute to pass on to girls than to boys.
"Physical attractiveness is good for both men and women, but it is much better for women than for men," he said.
Mark Thomas, senior lecturer at the biology department of University College London, told the Sunday Times newspaper that Kanazawa's findings do seem to conform with research on sexual evolution.
Kanazawa said his research also explains why women, on average, are better-looking than men.
"Because physical attractiveness is heritable - and because physically attractive parents have more daughters and less attractive parents have more sons - the average level of physical attractiveness among women increases over time relative to men," he said.
"In my study I demonstrate that more men than women are average-looking, while more women than men are either attractive or very attractive," he said.
Kanazawa said that this is the evolutionary consequence of attractive women having more daughters.
Previous research by Kanazawa published in 2005 showed that parents engaged in professions such as nursing, teaching, and counseling - or those with empathetic "female" brains - were more likely to have daughters. Those engaged in more masculine professions such as math, science, and engineering were more likely to have sons.
At the time, researchers explained that babies of people with masculine professions might encounter more testosterone in the womb and so would be more likely to be male.