There’s no doubt about it, there is something about twins that us non-twins find downright fascinating. We’ve got all kinds of questions (some more ridiculous than others) about the twin experience. Do they know what the other is thinking? Can one feel the pain the other is experiencing? How does growing up as a twin influence their worldview, how does it shape how they experience the world?

Of course, it is probably a stretch to suggest that twins have the power to read each other’s minds. However, that doesn’t mean that being a twin isn’t a wildly unique experience. Being a twin does have a strong influence on a person’s experience.

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Most interesting, perhaps, is the fact that twins offer up a whole new area of research that would be otherwise difficult to study. Thanks to research involving twins, we now have the ability to ask hard questions about inherited and learned traits. More specifically, by closely observing both identical and fraternal twins, we’ve been able to examine the differences between nature and nurture and what other factors seem to influence development.

How Researchers Use Twins to Further Science

Using twins to study inherited and learned traits began in the 1920s, but really took shape over the last several decades. Researcher Walter Jablonski is credited with the first formal twin study in 1922, according to a paper in the Indian Journal of Community Medicine. Using information gathered about identical and fraternal twins, Jablonski studied refraction of the eyes by comparing the differences between the the sets of two. He attempted to explore whether this trait was more influenced by genes or environment.

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Since that first study, twin research has evolved greatly, but the idea remains the same. By noticing the shared and the different traits in identical and fraternal twins, researchers may be able to determine which traits are a part of their genes and which are a part of their nature. The assumption is that twins share an environment, so traits shared by fraternal twins are likely environmental; traits shared by identical twins just might be genetic.

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This research isn’t perfect, according to the American Psychological Association. Using twins to study the heritability of traits is believed to be flawed because it rests on the assumption that twins are raised in equal environments. Researchers aren’t about to let that scare them away from twin studies, however, because they’ve proven to be a great jumping off point for researching everything from the tendency towards shyness to genetic risks of cardiovascular disease.

Obesity: Personal Flaw or Predetermined Condition?

Unfortunately, many people believe that being obese or overweight is an indication of personal flaw. We’re quick to assume that people carry excess weight simply because they don’t put forth the effort to exercise or they eat too much junk food.

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This is one area where twin studies have been incredibly eye-opening. Twin research has been helpful in breaking down false assumptions about obesity and offering up ideas for further research. For instance, research published in the Danish Medical Bulletin explained that genetics may be as much a 60 to 70 percent responsible for obesity based on observing identical and fraternal twins. The same research also found that dietary preferences aren’t just environmental—we inherit some smell and taste preferences—and that exercise can be used to lessen the influence of genetics on weight.

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Further research is needed to further clarify the role of genetics in obesity and determine better treatment options for condition, but twin studies have given us a really great start.

The Heritability of Eating Disorders

In a similar vein, eating disorders appear to have a genetic component as well. Many sufferers report inciting events they believe encouraged the development of their eating disorders, suggesting the environment is responsible for these dangerous mental illnesses. However, research suggests otherwise, according to the journal Psychiatry, and we can thank twin studies in part for this information.

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By studying both identical and fraternal twins and observing the incidence of behaviors associated with eating disorders, researchers were able to document that eating disorders have a significant genetic component. For instance, behaviors associated with anorexia and bulimia appeared to be 32 to 72 percent inherited. And general attitudes about food, dieting, and preoccupation with appearance are also largely inherited traits. This information is important since treatment of eating disorders is uniquely difficult and calls for more research on prevention and treatment.

The Role of Genetics and Class on IQ

Doesn’t it seem like some people are simply born smart? Well, there is science to back this up, and again, we can thank twin studies for their contribution to this discovery.

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A famous study conducted by Eric Turkheimer in 2003 took a close look at IQ, and just how much it is influenced by genetics. By comparing both identical and fraternal twins, Turkheimer and his team were able to ask some tough questions about the heritability of IQ and if these inherited traits are still influenced by environment. More specifically, they wanted to study how class influences the genetics of high IQs.

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What they found was that IQ is an inherited trait, but the level to which genetics influence IQ depends on the child’s environment. For instance, children in wealthy environments, who have access to more opportunities and less stress in their lives, are less limited and therefore more influenced by their genetics. In comparison, children of lower classes are believed to be constrained by their environment, making them less influenced by their genetic predisposition to a higher IQ, according to the American Psychological Association.

Searching for the Gay Gene

Is sexual orientation nature or nurture? This is a question that has created many heated debates in our culture, and it is a question we’re still trying to answer. In 2010, The Archives of Sexual Behavior published the largest twin study on sexual orientation, studying 3,826 pairs of both identical and fraternal twins.

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Although groundbreaking, their research has opened up more questions. They found that genetics did, in fact, appear to influence same-sex attraction. This was determined because both twins in an identical set reported same-sex attraction more frequently than both twins in an fraternal set.

However, these differences were only slight, and researchers were able to make stronger connections between shared environmental factors and same-sex attraction. This confirmed previous research that suggested that same-sex attraction is in small part genetic but largely influenced by environment.

Common Diseases and Genetics

Since 1987, TwinsUK has been publishing data from their repeated twin studies on common diseases. They’ve devoted over three decades to determining which medical conditions are all about the environment, which are all about genetics, and which are a combination of both.

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For instance, in the year 2000, TwinsUK was involved in a study published by the journal Arthritis and Rheumatism to determine if bone mineral density was an indicator of future risk for hip osteoarthritis. In this specific study, they used twins to observe that a higher bone mineral density typically indicated a lower risk of hip osteoarthritis.

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Another example of a twin study being used to make groundbreaking discoveries is the 1998 research on the heritability of blood pressure in the journal Twin Research. This study looked specifically at the influence of stress on inherited blood pressure conditions, observing twins to gather data. Their research suggests that inherited blood pressure conditions persist regardless of whether or not they’re stressed, whereas environmental effects on blood pressure decrease in the presence of stress.

Twins in Space

One of the most recent twin studies, and perhaps the coolest one to date, used astronauts Mark and Scott Kelly, identical twins, to answer questions about how space travel affects the body. While Scott spent a year in space, Mark, who was retired, stayed behind on earth.

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Before Scott’s voyage, both twins underwent a variety of tests, both physical and psychological. Samples of saliva, blood, and urine were also provided. These tests were then repeated after Scott returned and the results were compared. Although the research is still in progress, NASA released a peek at some of what was discovered at the beginning of 2017, revealing that space traveling has varying effects on the human body.

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NASA

For instance, telomeres, which are compound structures on chromosomes, generally shorten as people grow older. Instead of following the predicted pattern, Scott’s telomeres actually lengthened while he was in space. Researchers predict that may have something to do with his controlled environment, as he was eating a very specific diet and exercising regularly.

On the flip side, space travel did have some negative effects on the astronauts. Compared to his brother, Scott experienced a greater decline in bone formation. He also exhibited an increase in inflammation that appeared to be related to the stressors of reentry.

We have a sneaking suspicion that we’ve only scratched the surface of the science of being a twin.

As we make further advances and learn more about genetics, twin studies may become more and more valuable for learning about individual differences in the human body.