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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