For women looking to pass on their genes, it pays to be short. For men, tall is the ideal. The result? An evolutionary tug-of-war in which neither gender reaches their perfect height.
Those are the results of a new study published today (Aug. 7) in the journal Biology Letters. The research finds that an evolutionary battle of the sexes keeps the genders in an endless feedback loop of height variations across the generations.
"We should not simply assume that when a trait is beneficial for one sex, that selection or evolution will necessarily favor this trait," study researcher Gert Stulp, a scientist at the University of Groningen in the Netherlands, told LiveScience in an email.
In the same way, traits that harm one sex but not the other may not be "weeded out" by natural selection, Stulp said.
"This may even hold for health-related traits, such that genetic underpinnings beneficial to the health of one sex may increase the susceptibility to disease in the other sex," he said.
Why height matters
In modern western societies, studies have found that women who are on the short side tend to have more children. In contrast, average-height men do the best, reproductively speaking, outpacing short and tall men in number of children fathered, Stulp said.
Men and women are sexually dimorphic, meaning there are obvious bodily differences in size and shape between the sexes. But we also share most of our genome, meaning that evolution has a limited toolbox for creating this dimorphism. That can lead to conflict in which evolutionary forces act on males and females in opposite directions.
Given the evolutionary pressure for short women and average men, height seemed a potential area of conflict. Stulp and his colleagues pulled data from a long-running research project called the Wisconsin Longitudinal Study, which has followed more than 10,000 Wisconsin high-school graduates for nearly 50 years.
The researchers took data from pairs of siblings from this study, including 808 brother-brother pairs, 996 sister-sister pairs and 1,718 brother-sister pairs. They averaged the heights across the pairs to get a better sense of average family height — height is highly heritable and is under the influence of multiple genes — and then examined how height related to the number of children in each family. [6 (Other) Great Things Sex Can Do For You]
The results revealed that in short families, where both brother and sister were likely to be below-average height, sisters had more children than brothers. In average-height families, however, brothers had more children than their sisters.
In other words, if you were born into a short family, you'd be more likely to have nieces and nephews through your sister than your brother. If your family was of average height, your brother would be the one more likely to make you an aunt or uncle.
The findings support the idea that the sexes are locked in a push-and-pull battle over height. Here's how it works: Say a woman is shorter than average. This makes it more likely she'll have children and pass on her genes. If she has a daughter and a son, they're likely to be short, thanks to their mother's genes. That's good for the daughter — evolution is pushing her toward her ideal height to pass on her genes — but bad for her son, as he would be more likely to reproduce if he had a few more inches.
This pattern happens over an entire population, not just a single family. What that means is that as the population as a whole gets shorter because of short women reproducing more, everyone is moving away from the ideal height for men. That increases the evolutionary pressure for men, so that taller guys reproduce more than their shorter brethren, pushing the heights of the next generation back into the average range. [5 Myths About the Male Body]
"Because selection in this generation is then likely to be stronger on average-height men, the next generation will again be slightly taller," Stulp said. "This is, of course, to the detriment of women, so that the selection pressure on female height will get stronger to push it back to shorter height again."
This back-and-forth loop between slightly shorter and slightly taller generations will continue as long as evolutionary pressures for men and women remain different, Stulp said.
Many factors play a role in mate choice, Stulp said, and height is only one. Nevertheless, research has found that people do care about height when picking a mate, he said.
"Asking people about their preferences for height and examining, for instance, the role of height in speed-daters, a reasonably clear picture arises: Taller men and average height women are on average preferred," he said. "Particularly women value height in their male partner."
Height is also correlated with income, attractiveness, education, health and longevity, Stulp said.
Other traits may be subject to the same evolutionary battle of the sexes, he said. For example, wide hips are good for women in childbirth, but not ideal for locomotion in men. Perhaps facial masculinity is another example, Stulp said: A macho-faced guy is likely to do well with the ladies, but his sisters who inherit the same traits aren't likely to be as attractive to potential male mates.
"I think it is important to recognize that evolutionary processes occur in contemporary human populations," Stulp said. "Evolution did not stop at the industrial revolution."