"Some girls are bigger than others," Morrissey sang. "Some girls' mothers are bigger than other girls' mothers."
Popular singer Smith's fans believe that Morrissey was referring to breast size, but the lyric also implies that overall size is strongly influenced by genes. But why are some girls taller than others? One reason might be puberty. At sexual maturity in females, there is a wave of sex hormones that closes the epiphyses (the ends of the long bones in our bodies), so that girls who mature late will generally be taller because they have had more time to grow. Short women generally reach puberty earlier than tall women -- whose bodies spend more energy on growing than on entering puberty.
Regardless of the timing of puberty, most men are taller than women. Why is this, in an age where most of us are well-fed and have access to good medical care?
"Sexual dimorphism" is how biologists refer to size differences between the sexes. In mammals, the largest sexual size difference is found in elephant seals, where bulls are up to 10 times bigger than the cows. The most successful male elephant seals are the biggest ones, best able to fight and maintain a harem of females -- and inseminate them all.
Are humans subject to the same laws? If sex in the city is the same as sex in the jungle, tall men should be more successful at getting mates and having children than short men.
That is what Daniel Nettle, of the departments of biological sciences and psychology at England's Open University, set out to test.
He used data from Britain's National Child Development Study, an ongoing, longitudinal investigation of social economic and health outcomes for all the children born in the United Kingdom during a single week in March 1958. The data, which covers nearly 10,000 men and women, showed that the taller the men were, the less likely they were to be unmarried or childless in 2000, at age 42.
But Nettle also looked at the data for women and found that while taller-than-average men were more successful at attracting a mate and having children, the same was true for shorter-than-average women.
This selective force is the reason why height differences persist in humans in the modern age.
Writing in the Proceedings of the Royal Society (B), Nettle shows that there is a positive relationship between male stature and reproductive success. Men of average height (for that group of British children born in 1958, the average is 177 cm) were less likely to have children than a taller man of 186 cm. Taking into account other complicating factors such as health, socioeconomic status and education level, the difference, says Nettle, appeared to be due to taller men's greater ability to attract a mate.
Women were more likely to have children if they were between 151 cm and 158 cm, well below the average of 162 cm. (The average woman was 15 cm shorter than the average man.)
"It seems that tall men and petite women are favored in evolutionary terms, even in a modern population, so the height difference between men and women is unlikely to disappear," said Nettle.
The results corroborate earlier research showing that taller men are rated as more attractive than those of average height, but it's the first study to examine the relationship between size and reproductive success in women.
"In the past, hunting ability and status in social groups might well have been correlated with height," said Nettle in an e-mail interview. "Also, being tall is an honest signal that you have been successful in staying healthy and nourishing yourself. Someone who has managed to grow to above-average height must have good genes or be good at operating in the environment."
That explains why tall men are attractive, but what about short women? Why are they more attractive than tall women? Very short and very tall women had low reproductive success, mainly, Nettle found, because their height is more likely to be caused by conditions such as gigantism and warfism. Men apparently prefer women of below-average height because they are (subconsciously) perceived as more fertile. The reason is that in our antelope-hunting, savanna-living past, a short woman was more likely to have reached sexual maturity.
"Men still find themselves attracted to women who show the hallmarks of fertility, even if, in the modern day and age, they might not have children with them for many years and perhaps not at all," said Nettle.
"The psychological mechanisms of attractiveness are vestiges of life under past conditions."
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