She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (36 page)

Heritability is one of the trickiest concepts in modern biology. It describes variations only across an entire population. If the heritability of a trait in a group of people is 50 percent, that doesn't mean that in any given person, genes and environment are each responsible for half of it. And if a trait has a heritability of zero, that doesn't mean that genes have nothing to do with it. The heritability of the number of eyes is zero, because children are virtually all born with a pair of them. When we walk down the street, we don't pass someone with five eyes, another with eight, and another with thirty-one. If someone has only one eye, it's probably because they lost the other one in an accident or from an infection. Yet we all inherit a genetic program that guides the development of eyes.

As tricky as heritability may be to grasp, it's been a powerful tool for making sense of heredity. Our well-being depends on it, in fact. To a large extent,
heritability feeds the world.

How much food farmers can harvest from a given acre of land depends largely on the traits of the crops they plant. A plant that winds up short may produce a low yield. Taller is better, but only up to a point. If plants have to dedicate a lot of resources to reaching a great height, they'll have little left over to produce the seeds or fruit we want to eat. They may also run a greater risk of toppling to the ground and leaving a farmer with no harvest at all.

If the height of a crop were entirely heritable, that would mean that the differences in the plants' height were entirely due to the differences they inherited from their ancestors. Short plants would always produce short plants, tall plants tall. If the heritability equaled zero, on the other hand, the genes of different plants would have no effect on their variation whatsoever. All the differences would arise from their environment. A field of plants growing with the same rainfall, the same rhythm of heat and cold, the same pests and blights, would all grow to roughly the same height.

To measure the heritability of a trait in a crop, scientists can raise plants under carefully controlled conditions and observe how differently they turn out. They grow genetically identical seeds in precisely controlled greenhouses. They can pot them in identical soil, spray them with identical fertilizer, and measure their growth each day of their existence, down to the millimeter. These studies show that height is strongly heritable in some species, and only moderately so in others. This knowledge has helped plant breeders transform crops through artificial selection. It led to the production of “semi-dwarf” breeds of wheat and rice that produce a better yield than taller varieties, because the wind can't flatten them.

Scientists who study human heritability, on the other hand, don't raise babies in laboratories. They don't measure the mashed peas parents feed their toddlers, down to the microliter. Instead, scientists have to hunt for volunteers to study. They can only gather stray fragments of information about their subjects' lives. Errors can thus creep into estimates of human heritability. If children grow to be as tall as their tall parents, that doesn't necessarily mean that the genes they share are the reason. Instead, they might have spent their childhood in the same growth-favoring environment that their parents did.

When Galton first began studying the inheritance of height and other traits, he recognized how hard it would be to reach firm conclusions. But Galton had an inspired idea: Scientists could take advantage of a natural experiment in human heredity. They could study twins.

Galton had no way of understanding the genetic links that twins share, but he had an intuition that they must share a strong common inheritance.
He loved to share stories about the eerie coincidences in the lives of twins. A pair of twins simultaneously came down with the same kind of eye irritation, even though one was in Paris at the time and the other in Vienna. Another set developed the same crook on the same finger of the same hand. Yet another pair decided to buy surprise gifts for each other. They each chose precisely the same set of champagne glasses.

While Galton acknowledged that the experiences of twins might also influence how they turned out, he considered heredity to be paramount. The shared heredity of twins drove their lives along the same path. Galton believed twins demonstrated that everyone's inborn nature had an overriding influence. It guided people through life the way sticks thrown in a stream travel with the current.

“
The one element that varies in different individuals, but is constant in each of them, is the natural tendency,” Galton said. “It corresponds to the current in the stream, and inevitably asserts itself.”

Other scientists soon began investigating twins more rigorously for clues to heredity. But it wasn't until the 1920s that a German dermatologist named
Hermann Werner Siemens tapped their full power. By then, scientists had come to recognize that fraternal twins and identical twins are genetically different. Fraternal twins develop from two eggs, each fertilized by a separate sperm. Identical twins arise from a single fertilized egg that splits into two embryos. Fraternal twins are thus no more genetically similar to each other than any other pair of siblings, having on average 50 percent of their variants in common. Identical twins, on the other hand, are essentially clones.

Siemens realized that these two kinds of twins were an opportunity to study heritability. Twins grow up in similar environments, from the womb onward. But the genetic closeness of identical twins would make them more similar in highly heritable traits. By comparing the similarities in both kinds of twins, Siemens could estimate the heritability of a trait.

As a dermatologist, Siemens was most interested in skin diseases. Did people develop them simply due to bad luck, he wanted to know, or because of bad genes? He counted up the moles on the skin of twins and discovered that
identical twins didn't develop identical constellations of moles. Those differences told Siemens that the environment had a hand in their development.

But while their moles might not be identical, they did correlate. An identical twin with a lot of moles tended to have a twin sibling with a lot as well. If a twin had only few moles, it was a safe bet the other didn't have many. The moles on fraternal twins were also correlated—but only with half the strength as in identical ones. Siemens concluded that genetic variations played an important part in developing moles, although the environment mattered, too.

Siemens's remarkable study inspired other scientists to use his method to study height. A British researcher named Percy Stocks searched for twins in London's schools and had teachers report on how tall they were. He found that fraternal twins tended to be fairly close in height. But identical twins were closer. The difference between them made it possible for scientists to put a number on the heritability of height. As the studies grew larger, that estimate grew more precise. In 2003, a Finnish researcher named
Karri Silventoinen studied the height of 30,111 pairs of twins. He estimated that height was strongly heritable: 70 to 94 percent in men, and 68 to 93 percent in women.

Even a study as sprawling as Silventoinen's rested on a big assumption: that the environmental influences shared by a pair of fraternal twins are no different from those of identical twins. If a trait is more similar in identical twins than fraternal twins, genes can be the only explanation. Scientists can't know that for sure, however, because twins grow up in the wilds of real life, not in a terrarium. Some critics raised the possibility that parents treat identical twins differently from fraternal ones. Since fraternal twins look different, parents might treat them more like ordinary siblings.

Scientists developed twin studies as a way to study human DNA in an age when it was impossible to examine it directly. Once it became possible to read genetic markers in people's genomes, new ways emerged to measure heritability. Peter Visscher and his colleagues found that pairs of siblings can vary tremendously in their genetic similarity, sharing as little as 30 percent of their genetic variants in common to as much as 64 percent. If a
trait is highly heritable, Visscher reasoned, then it should be more similar in siblings who have more DNA in common.

In 2007, Visscher and his colleagues examined the height of
11,214 pairs of regular siblings. They found that “twin-like” siblings—those who shared more than half of their DNA—tended to grow to more similar heights. Siblings with less genetic similarity were not so similar. The scientists used these correlations to calculate the heritability of height. They ended up with an estimate of 86 percent.

That's an exceptionally high figure. Nicotine dependence has a heritability of 60 percent. The age at which women go into menopause is 47 percent. Left-handedness is at a mere 26 percent. In the world of heritability, height stands tall.

Even a trait as strongly heritable as height, however, can also be drastically shaped by the environment. In his own research, Louis-René Villermé watched the average height change over the course of a few years. During the Napoleonic War, the average height of young French soldiers declined—the result of wartime food shortages, he guessed. After the war's end, the army's average height rebounded a little—thanks, Villermé said, to “
a decrease, however slight, in misery.”

Villermé's insight went neglected for the next 150 years, until a small group of economists led by the Nobel Prize winner Robert Fogel started charting height in different countries over the course of decades. They made a compelling case that height could serve as an economic barometer, recording the well-being of societies. They were the first researchers to discover the huge gap between rich and poor boys in late eighteenth-century England, for example.

Their research also gave statistical heft to the stories that Frederick Douglass and other former slaves told about growing up in the antebellum South. Douglass recounted how the sole piece of clothing he was given at age six was a coarse linen shirt. His diet was gruel, served to slave children with as much dignity as slop to pigs.

This cruelty was based on cold economic reasoning: Since slave children were too young to earn money in the fields, their masters chose not to invest in them. When Fogel's followers analyzed plantation records, they found that enslaved American children were much shorter than free ones. But those records also showed that slaves experienced an extraordinary growth spurt in adolescence. That rapid rise was likely the result of the extra food slave owners gave their slaves once they were old enough to turn a profit.

After some small-scale studies in the 1970s, Fogel and his fellow economists widened their spotlight, carrying out a systematic survey of height through history. They looked at military records of conscripts, prison archives, and any other historical data they could get their hands on. They moved from one country to another and pushed back deeper than before into history. When written records failed the researchers, they measured bones from ancient skeletons.

The longest record of height can be found in Europe, where it stretches back thirty thousand years to the Gravettian culture.
Gravettian men stood on average six feet tall. When agriculture arrived in Europe some eight thousand years ago, people experienced a tremendous drop in stature. Men lost eight inches of height. The drop was likely the result of Europeans switching to a grain-rich diet much lower in protein. For the next seven thousand years, European stature hardly changed, wavering just an inch or two from century to century. In the eighteenth century, the average European man stood just five foot five.

But they were not locked in at that height.
When English people emigrated to the American colonies, men swiftly climbed to five foot eight, becoming the tallest men in the world. By the end of the eighteenth century, American apprentices at age sixteen stood almost five inches taller than poor sixteen-year-olds in London.

In both the United States and Europe, the average height dipped in the first half of the nineteenth century. But then,
starting around 1870—at the time Galton began puzzling over height—people in both Europe and the United States started getting taller. Over the next century, Americans grew about three extra inches on average, hitting a plateau in the 1990s. In
Europe, the boom was even more dramatic. With each succeeding decade, Europeans added about half an inch of average height, and kept growing that way into the twenty-first century. Northern and central European countries were the first to begin this ascent, but the southern regions started catching up by the mid-1900s. Today,
Latvian women have become the tallest women in the world, jumping from about five foot one to five foot seven. Dutch men rose from five foot seven in 1860 to just over six feet tall, making them the tallest men on Earth.

In 2016,
an international network of researchers extended this survey to the world. Over the past century, they found, some countries outside of Europe experienced equally impressive gains. South Korean women experienced the biggest gain, growing eight inches in one hundred years. Among men, Iranians grew the most, now standing six and a half inches taller than they did in the early 1900s. Some people barely grew at all: Pakistani men gained just half an inch. And some countries in Africa, such as Niger and Rwanda, shot up in the first half of the twentieth century only to lose an inch or two after 1960.

Overall, though, the world has gotten much taller. It may be hard to believe that Guatemalan women today—standing only four foot eleven—could have been any shorter in the past. In fact, they have gained four inches since the early 1900s.

Three million years ago, our ancestors in East Africa stood only about a yard high. By 1.5 million years ago,
Homo erectus
grew as tall as five foot seven. Natural selection may have favored genes for greater height because they gave our ancestors long legs that could carry them for long distances across the savanna. Our ancestors kept evolving to greater heights; by 700,000 years ago, they had evolved to our modern stature.