As part of NASA’s "Year in Space" project, Kelly and his Earth-bound identical twin brother, retired astronaut Mark Kelly, provided samples of blood, saliva and urine and underwent a barrage of physical and psychological tests designed to study the effects of long-duration spaceflight on the human body.

Studies of identical and fraternal twins have long been used to untangle the influences of genes and the environment on particular traits. Identical twins share all of their genes, while fraternal twins only share 50 percent. If a trait is more common among identical twins than fraternal twins, it suggests genetic factors are partly responsible.

"Twins studies are the only real way of doing natural experiments in humans," says Tim Spector, a professor of genetic epidemiology at Kings College, London. "By studying twins, you can learn a great deal about what makes us tick, what makes us different, and particularly the roles of nature versus nature that you just can't get any other way.”

Spector is director of the TwinsUK Registry, which includes data from 12,000 twins and is used to study the genetic and environmental causes of age-related complex traits and diseases. He estimates that twins research is currently being conducted in more than 100 countries, and that most of those projects draw upon information contained in large databases such as the TwinsUK Registry.

While it may be a while before we see results from the astronaut twins, researchers are hopeful that the opportunity will yield some unique insights into human health. Here are some examples of what we've learned from past twins studies—both famous and infamous:

The Birth of Eugenics

Victorian scientist Francis Galton, a half-cousin of Charles Darwin, was one of the first people to recognize the value of twins for studying the heritability of traits. In an 1875 paper titled "The History of Twins," Galton used twins to estimate the relative effects of nature versus nature (a term that Galton himself coined). But his firm belief that human intelligence is largely a matter of nature led him to a darker path: He became a vocal proponent of eugenics (another term that he coined) and the idea that "a highly gifted race of men" could be produced through selective breeding.

Genes and I.Q.

In 2003, Eric Turkheimer, a psychology professor at the University of Virginia, took a fresh look at the research on the heritability of I.Q., which relied heavily on twin studies. Turkheimer noticed that most of the studies that found I.Q. is largely due to genetics involved twins from middle-class backgrounds, and he wondered what the pattern was among poorer people. When he looked at twins from poor families, he found that the I.Q.s of identical twins varied just as much as the I.Q.s of fraternal twins. In other words, the impact of growing up poor can overwhelm a child's natural intellectual gifts.

Genetic Basis for Everyday Diseases

Working with data and biological samples in the TwinsUK Registry, Spector and his colleagues have shown in more than 600 published papers that many common diseases such as osteoarthritis, cataracts and even back pain have a clear genetic basis to them. "When I started in this field, it was thought that only 'sexy' diseases such as cancer were genetic," Spector says. "Our findings changed that perception."

Heritable Eating Disorders

One of the newer twin registries to come online, the Michigan State University Twin Registry (MSUTR) was founded in 2001 to study genetic and environmental influences on a wide range of psychiatric and medical disorders. One of the most surprising findings to come out of the group's research is that many eating disorders such as anorexia have a genetic component to them.

"People thought for the longest time that it was due entirely to culture, the media and social factors,” says MSUTR co-director Kelly Klump. "Because of twins studies, we now know that genes account for the same amount of variability in eating disorders as they do in schizophrenia and bipolar disorder. We would have never known that without twins studies."

The Genetics of Obesity

A classic twin study conducted by geneticist Claude Bouchard in 1990 looked at the importance of genes for body-fat storage. Bouchard, now at Louisiana State University, housed a dozen lean young male twins in a dormitory and overfed them by 1,000 calories a day for three months. Although every participant was heavier by the end of the experiment, the amount of weight and fat gained varied considerably, from 9 pounds to 29 pounds. Weight gain within pairs of twins was much more similar than weight gain between different twin pairs, and the twins in each pair tended to gain weight in the same places, whether it be in the abdomen, buttocks or thighs.

Untangling the "Gay Gene"

Numerous twin studies have attempted to elucidate the importance of genes in sexual orientation. In 2008, researchers led by Niklas Langström, a psychiatrist at the Karolinska Institute in Stockholm, drew upon the treasure trove of twin data contained in the Swedish Twin Registry, the largest in the world, to investigate genetic and environmental influences that determine whether or not a person is gay. The scientists found that genetics accounted for only 35 percent of the differences between identical and fraternal gay men and even less—roughly 18 percent—in gay women.

The study, one of the most comprehensive to date, indicates that a complex interplay of genetics and environmental factors work together to shape people’s sexual orientations. But like other twins studies on this controversial subject, Langström’s study was criticized for possible recruitment bias, since only 12 percent of the males in the Swedish registry were included in the study.

Twins Reared Apart

In 1979, Thomas Bouchard conducted what is perhaps the most fascinating twin study yet. Then director of the Minnesota Center for Twin and Family Research, Bouchard looked at identical and fraternal twins separated in infancy and reared apart. He found that identical twins who had different upbringings often had remarkably similar personalities, interests and attitudes. In one of the most famous examples, Bouchard came across twins who had been separated from birth and reunited at the age of 39.

"The twins," Bouchard later wrote, "were found to have married women named Linda, divorced, and married the second time to women named Betty. One named his son James Allan, the other named his son James Alan, and both named their pet dogs Toy."

But MSUTR's Klump is quick to point out that Bouchard's findings are not proof of genetic determinism. "What they show is that we we enter the world not as random beings or blank slates,” Klump says. “As we walk through life, we have a lot of free choice, but some portion of that free choice is probably based on things that we're really good at and things that we like to do. Bouchard's study tells us that there is a dynamic interplay between what we like, what we want and the environments that we choose."

Article published March 2016 Smithsonian