A genetic gender war is keeping humans from achieving their ideal heights.

Shorter women have more children. And while being on the low end of the measuring stick benefits them on an evolutionary scale, it keeps their brothers from attaining the trait that favors their reproductive success: average height. Researchers in the UK claim to be the first to demonstrate this new explanation for why height variation exists: That one's height depends heavily on one's genes, and that the genetic mechanisms for determining height are similar in men and women, preventing each gender from being able to evolve independently toward their sex-specific ideals.

These same researchers have already come up with objective, if narrowly defined, parameters for "ideal" height. The tendency to have more children makes shorter women, in terms of reproduction, more "fit." For men, it's not good to be too tall or too short: Even though women tend to prefer taller men, those of average height turn out to be the most fit.

Why this may be so is unclear. People with these traits must to some degree be favored by the opposite sex, although other factors, like the age at which people of various heights tend to start having children, may also play a role. But it is nonetheless measurable.

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Using the same people from their previous study -- a population of over 10,000 Wisconsinites who have been followed for over 50 years -- Gert Stulp of the University of Groningen and associates looked at the average height of the subjects and one randomly selected sibling. Controlling for age, birth order, and total number of siblings, they found that shorter sibling pairs had more reproductive success through the sister, while in pairs that were average in height, the brother tended to have more children.

This indicates that ideal height for each gender differs significantly enough for mixed-gender siblings to be reproductively at odds with one another.

It also means that "high-fitness" mothers will tend to produce "high-fitness" daughters, to the detriment of their sons, and vice-versa for fathers.

The same, it has been shown, may go for attractiveness as well: A study from last year found that among siblings who were "physically and hormonally masculine," the brothers were seen as more attractive than their sisters. It's also been suggested that this conflict explains how homosexuality can be perpetuated in the population. The genetic factor that makes gay males very much not reproductively fit benefits their sisters, who tend to have more children than the general population.

If one height were ideal for both males and females, or if height was only an important factor for one or the other, we could have evolved toward an agreed-upon standard by now. Unfortunately, the authors write, "it seems unlikely that intralocal sexual conflict over height will be resolved rapidly." Instead, we have to compromise, and reside, as a population, in the middle-ground between our ideals.