Identical twins share all of their genes, but do not necessarily share identical environments. Image from wikimedia.org. Photograph by Christopher Michel
Nature or nurture? This is a question that is commonly asked by journalists and lay people. This seems like a reasonable question. There was a period of time when scientists asked it, too, but this time is over.
In modern biology, all traits are understood to be the result of the interaction between genes and environment. This goes for both physical and psychological traits.
But not all traits are equally the result of genes and environment. Some things are caused almost entirely by genes, with little input from the environment, and some are the opposite, with environment being almost entirely responsible for the trait. In one way or another, however, all traits require both. Biologists talk about this in terms of “heritability.” Heritability is a measure of the extent to which differences within a trait across individuals is the result of differences in genes across those individuals. It is expressed as a number between 0 and 1, where 0 is completely caused by the environment, and 1 is completely caused by genes. As a decimal, heritability can easily be converted into a percentage, which is a little easier to conceptualize. A heritability of 0.8 means that 80% of the variation in the trait across individuals is the result of genes. A heritability of 0.2 means that 20% of the variation is caused by genes. Here are some examples:
Consider height. It is unremarkable to observe that tall parents are likely to have tall children, and short parents are likely to have short children. I know a couple who are both over 6’1”, and their child is in the 95th percentile for height. But height has a heritability of only about 0.5. If you look at the variation of height across many people, only about half of that variation is due to genes, and the rest is due to environment. Tall parents can have short children if those children don’t get enough to eat, don’t get the right nutrients, or are exposed to a lot of infectious disease. Likewise, short parents can have tall children if the children grew up in a better environment than the parents.
Type II diabetes has a heritability of about 0.7, if you develop it before age 65. Genes play a big role in how likely you are to develop type II diabetes, and the rest depends on how much sugar and trans fat you eat. As they say, genes can load the gun, and environment can pull the trigger.
Estimates of the heritability of IQ range form around 0.2 to 0.8, although some recent studies have suggested that it is closer to the 0.8 end. Just as with height, IQ can be negatively affected by nutrition and infection.
The leading theory of personality claims that there are five dimensions to personality: agreeableness, openness to experience, extraversion, emotional stability, and conscientiousness. The heritability of these five dimensions ranges from around 0.4 to 0.6.
These traits that I list above all have fairly middling amounts of heritability, but there are also traits that are higher or lower. Eye color has a heritability of 0.8. The haircut you have right now has a heritability of close to zero, but it is probably not exactly zero.
Heritability calculations come mostly from twin studies. I won’t bore you with the computational details, but if you look at how likely identical twins are to share a trait vs how likely fraternal twins are to share the trait, you can get a pretty good idea of the genetic contribution to that trait. Identical twins share 100% of their DNA, meaning that all differences between them must come from environment.
There are also other things that go beyond just heritability. Even traits that have no direct genetic contribution rely heavily on genes. Take playing baseball, for example. Modern baseball has only been around for a couple hundred years — not enough time for evolution to have any impact on it. But playing baseball still relies on both genes and environment. Most obviously, playing baseball requires having been taught how to play the game — there is no genetic component to this. Any human of typical ability can be taught to play baseball. In addition to this, there are many traits with a clear genetic influence that are essential to playing baseball. A person must have a certain level of eye-hand coordination, interest in competition, and approximately two arms, two legs and two eyes. Certain musculature is required to be able to swing a bat and run. Cognition is required to understand the rules, and memory is required to remember them. All of these traits have genetic components. As a rather extreme example, a dog cannot play baseball. They lack the range of motion in their arms, the ability to stand easily on their hind legs, and hands that can grip a bat. Even if you tried to teach a dog to play baseball (environment) it would be unable to because of its anatomy (genes). On the other side, a perfectly functioning human is extremely unlikely to spontaneously begin playing baseball with no exposure to the game.
There are no traits that are entirely due to genes or entirely due to environment, making the “nature or nurture” question the wrong question.
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ChristopherEppig 
July 1st, 2015 at 11:00 am
You are right. Identical twins are 100 DNA and all difference between them is due to the environment. This explains why twins who are raised apart can be more alike than when they are raised together. Raised together they sometimes search for differences so they are not so “alike”, and may change preferences is clothing, work, or even partners, but still very similar. Examples on identical twins that are raised apart are the Jim Twins. One grew up in the states and other in Europe. Both wore the same outfits when they met each other for the first time. Both drove the same car, had a wife with the same name, and did the same job. But can this also prove the homosexuality is due to the environment? For many identical twins, one is gay and the other is not. I personally know three sets Identical twins that one is gay and the other is not. Having minored in Psychology in my undergrad, with my special education degree, and taking a few more psychology classes when receiving my Masters, with Learning Disabilities, plus being an identical twin myself. I am very interested in the topic. Lastly, the twins you picture for this topic are twins which in my opinion choose to be this alike and have done everything in their power to stay so alike. Thank you for sharing cousin!
July 6th, 2015 at 7:40 am
Hi Tammy, thanks for reading!
1) Just as identical twins like to look for things about themselves that are different, people like to highlight ways in which identical twins are the same. There are many notable examples of identical twins, separated at birth, who have uncanny similarities: in one example, both were firefighters, had the same haircut, and both wore mustaches. In another, both wore rubber bands around their wrists and dipped buttered toast in their coffee. These may be the result of shared genetics, but they may also be the result of coincidences. For this reason, it is better to look at aggregates of traits, rather than individual examples, when comparing similarities and differences among twins. Otherwise, you run the risk of a type of confirmation bias called the “sharpshooter fallacy.”
2) Like all traits, homosexuality is due to a combination of genes and environment. There are certainly environmental influences (such as birth order), but there are also genetic influences. Researchers have had a hard time nailing down the rate of concordance of homosexuality among identical twins, but it is much higher than you would expect if only environment were at play:
Let’s say the rate of homosexuality is about 5% (this is a high estimate, but for the sake of this example it is fine). If there were no genetic component to homosexuality, the likelihood of both identical twins being homosexual is about 0.25%. Some studies have found a real rate of concordance at more than two orders of magnitude higher than this (more than 25%).
3) I agree that the identical twins in the picture probably make an effort to look alike. What we look like has less to do with genetics than most people tend to believe — this is why identical twins tend to look less identical as they age.