Such a rapid rate of increase—a height gain of 20 centimeters (8 inches) in 150 years—suggests that the cause may be environmental. For example, nutrition (such as the heavy consumption of dairy products), better medical care, or low levels of social inequality. This trend may have also happened in other Western populations, but it leveled off much earlier than in the Netherlands. In the U.S. for example, male height has only increased 6 centimeters (2.4 inches) across the same time span—though Americans used to tower over the Dutch by 5 to 8 centimeters. One idea is that natural selection favors this taller stature, acting in concert with whatever environmentally induced changes were promoting the height increase.
To investigate, a team led by Gert Stulp from London School of Hygiene and Tropical Medicine examined three decades’ worth of data on height and fertility of 42,612 people aged 45 years and up. The data came from the LifeLines study, which followed 167,729 people living in the northeast region of the Netherlands.
From 1935 through 1967, taller than average men and average height women had more children—compared to shorter men and both taller and shorter women. This was despite the fact that taller people were having their first child at an older age. Additionally, taller men and women experienced higher child survival, which further helped increase their reproductive success.
This higher fertility, the researchers say, was at least partly because tall men and average height women were more likely to be in a current relationship and to produce more offspring within those relationships. One exception is that once taller than average women are in an established relationship, they end up with an even higher reproductive success—though this happens at a later age.
You might speculate that height is somehow correlated to attractiveness, education, and income. However, “there is much variation in what men and women want,” Stulp tells the Guardian. “When it comes to choosing a mate, height tends to have (only) a small effect, which is not very surprising given the many other, more important, traits people value in their mate.”
Both natural selection and environmental conditions, the team concludes, may help explain why the Dutch are so tall. The researchers didn’t present direct evidence of selection, rather their inference is based on the fact that genes affect both height and fertility. After all, 80 percent of the variation in human height in Western populations is due to genetic differences between individuals. The team is now gathering more data to figure out how much the height increase has to due with natural selection.