Subtitles section Play video Print subtitles Hey smart people, Joe here. I am a descendent of giants. With a grandfather, dad, two uncles, and an aunt all towering over 6 feet tall. Siblings and cousins, too. We are the people you don't want to stand behind at a Bon Jovi concert, but we're also the people who can get that thing off the top shelf for you. At 6'3", I turned out to be the short one in the family. Now that I have a son myself, I'm wondering if he's going to be tall too. Is height only written in our genes, or is there something else that determines how tall we get? These days, the average American woman is about 5'4", while the average Joe American male is about 5'9". But… human height has had its ups and downs over the centuries. Three million years ago, our ancestor Australopithecus only stood about 4' tall. One-and-a-half million years later , Homo erectus, the first early human to use complex tools, reached up to 5'7". And by the Stone Age, men of the Gravettian hunter-gatherer culture in Europe stood at an average of 6 feet. Most of the historical data we have is for male height, because . . . reasons. Then agriculture happened. When Europeans switched to a lower-protein, higher-grain diet, men gradually lost 8 inches in height, on average. And they stayed that way for thousands of years. By the time the 18th century rolled around, the average European man was only 5'5" inches tall. But when those Europeans emigrated to America, their kids grew up to be 5'8" inches tall on average. A huge jump in just a generation. During the Industrial Revolution heights took a dip due to urban crowding and disease, but soon after, the human height boom continued and continues today. Every decade for the past couple centuries Europeans have grown an average of about half an inch. Today, Dutch men are the tallest people in the world, with an average height of just over 6 feet—back to where those Gravettians started 8 millennia ago—and, almost as tall as me. These fluctuations of height, sometimes within a single generation, show that our environment determines a big part of how tall we are. But people in different regions, and different families, show us that height has genetic causes too. So which has a bigger role, nature? Or nurture? In the early 19th century, scientists first noticed a correlation between people's heights and their wealth — people from poor backgrounds tended to be shorter than people who were more well-off. Instead of asking whether someone's upbringing might influence their height, many scholars at the time decided tallness was a physical mark of "superior" humans. Francis Galton — who would later become infamous for popularizing eugenics—was the first scientist to conduct a large-scale, systematic study of height. He precisely measured the height of thousands of people as part of a sort of scientific side show. But Galton's results were confusing. Parents' heights often didn't predict the heights of their kids. The heights of siblings on the other hand, were much closer. This inspired scientists to look at height in twins. Studying twins can teach us a ton about how genes and environments influence human attributes. Fraternal twins can be as different genetically as any other pair of siblings, with the added advantage of being exactly the same age. Identical twins are genetically, well, identical. So we can see how much genetic carbon copies end up differing. And twins separated at birth offer a window into what happens when genetically identical individuals grow up in very different environments. Turns out that twins, especially identical twins, tend to be close in height—but not exactly the same. Twin studies, like history, show us genes can only be part of the story when it comes to height. So, how big a part? In 2007, scientists compared height and DNA between more than 11,000 pairs of siblings and found that, across humans, 86% of height variation can be explained by genetics. As traits go, this is very high; for comparison, genetics only explains about 26% of left-handedness. So we should be able to predict a person's height from his or her DNA right? Well, not so fast. We know genes make a huge difference, just not which genes. So far, scientists have identified about 800 genes that influence height, but many of them only make a tiny contribution. Take HMGA2, one of the first genes linked to height. Having one copy of the “tall” version only “lifts” a person about an eighth of an inch, so even if you inherit a copy from both of your parents, that still only gains you a quarter of an inch at most. Altogether, the 800 height genes we know of can only explain 27% of how height varies between people. There's clearly lots of genetic influence we don't understand. Maybe the effects of some genes add up in unexpected ways — genes may interact in combinations, where four and four makes sixteen, not eight. If we could just study the DNA of all the 7 billion people on Earth, maybe we would find all the genes that affect height. Or maybe we'd find that scientists have overestimated the contribution of genetics. Because our environment, defined by health and diet, certainly has a hand in shaping our height. South Koreans today are more than an inch taller than North Koreans, despite minimal genetic differences. Clearly, one's diet during childhood, is crucial in determining adult height. That's why humans shrank with the switch to agriculture and again during the Industrial Revolution. Today, most scientists agree that nature and nurture combine to shape our height. Some even propose calling height an “omnigenic” trait — one that nearly all our genes influence in some way. For now, the only surefire way to know how tall you'll end up… is just to wait and see. Stay curious! Hey guys, I want to take a second to tell you about "REINVENTORS". It's a new show from PBS Digital Studios and KCTS 9 in Seattle. We'll introduce you to the scientists and tinkerers in the specific North West make cutting edge of green technology. They'll try edible plastic, so you don't have to. And bring you to unexpected places like a garage in Seattle with a nuclear reactor in it. You can check out REINVENTORS and subscribe to them at the link in the description.