In 1990, Thomas J. Bouchard, Jr. and his colleagues (including esteemed twin researcher and PT blogger Nancy L. Segal) at the University of Minnesota published a striking finding: About 70% of the variance in IQ found in their particular sample of identical twins was found to be associated with genetic variation. Furthermore, identical twins reared apart were eerily similar to identical twins reared together on various measures of personality, occupational and leisure-time interests, and social attitudes.
Bouchard's study, along with many others, has painted a consistent picture: genes matter. The studies say nothing about how they matter, or which genes matter, but they show quite convincingly that they indeed do matter. Genes vary within any group of people (even among the inhabitants of middle-class, Western society!), and this variation contributes to variations in these people's behaviors. This finding should not be understated; it counters many a prevailing belief that we are born into this world as blank slates, completely at the mercy of the external environment. Because our psychological characteristics reflect the physical structures of our brains and because our genes contribute to those physical structures, there are unlikely to be any psychological characteristics that are completely unaffected by our DNA.
The authors of many twin studies have claimed that the environments experienced by twins (or any two siblings) do little to create differences in intelligence and personality. In fact, it turns out that genetic variation is correlated with variations in people's environments, a finding that some theorists have interpreted to mean that genes help to create the environments (see earlier post)! The idea here is that certain environments set off an appetite in the genes that nudges individuals to engage in certain experiences, and the environment then responds in a reciprocal fashion that reinforces an individual's nature.
To be sure, twin studies have received much criticism. Indeed, the proliferation of advanced statistical techniques (such as structural equation modeling) and the implementation of additional controls have allayed some of the concerns, but certainly not all.
Nevertheless, our point here isn't to rehash all of these criticisms. Instead, we are more concerned with how findings from twin studies are often misunderstood, misinterpreted, and blown out of proportion. Not just be the media, but even by serious scientists who get their work published.
To put things in perspective, I teamed up with the well-known developmental psychologist David S. Moore to list 8 facts about genes, twin studies, and the heritability statistic that may come as a surprise to many people-even biologists! We hope these facts will help clarify past and future misunderstandings.
1. Genes, by themselves, can't determine anything
Twin studies partition the variance in nature and the variance in nurture. This allows researchers to determine whether differences in genes or differences in the environment in a particular population are associated with more of the differences in observed behavior.
In reality, all biological and psychological characteristics are constructed during development, when genes interact with local environmental factors that can be influenced by the broader environment. Therefore, gene-environment interactions are understood to drive the development of all of our characteristics. Naked DNA (or RNA) is simply not sufficient to produce psychological or biological traits.
Therefore, when it comes to understanding the development of a trait in a particular person, nature can never be separated from nurture. As Matt Ridley has put it: "[Genes] are devices for extracting information from the environment. Every minute, every second, the pattern of genes being expressed in your brain changes, often in direct or indirect response to events outside the body. Genes are the mechanisms of experience."
2. Parents matter, and will always matter
Somehow, the finding that the shared environment plays only a small role in creating personality differences in adults (see Judith Rich Harris's work for a good review) sometimes translates in the media as: parenting doesn't matter. This couldn't be farther from the truth.
Take the most essential element: a child needs to be raised in a family, almost any kind of family, to develop the ability to speak a language. Since every single person in twin studies checks that box (i.e., is raised in a family of some sort), this factor never varies and thus does not predict differences in ability to speak a language. But does this mean that the variable "has a family" doesn't matter in determining whether or not a person develops the ability to speak a language? Of course not! That's like saying that water has no influence on a fish's development because all fish live in water. Just because a variable doesn't vary doesn't mean it has no causal impact on a particular outcome.
The parenting factors that are statistically associated with differences between individuals should never be confused with the parenting factors that cause the development of a trait within an individual. Genes could "account for" 100% of the variability in a trait in a particular twin study, but this does not mean that environmental factors are therefore unimportant in the development of the trait; parents still matter and will always matter.
It turns out that parenting matters, just in a way different than originally assumed. Genes matter to the extent that they support parenting--because like any other behavior, parenting behaviors are influenced by the genes--and parents matter to the extent that they support the expression of genes.
3. Heritability depends entirely on context
None of the twins in Bouchard's study were reared in real poverty, raised by illiterate parents, or were mentally retarded. There is reason to believe that under more dire circumstances, the heritability of IQ would be significantly lower than that reported by Bouchard. After all, if everyone was raised in an identical environment, variations in their psychological characteristics couldn't possibly be accounted for by anything other than variations in their genes (since there would be no variations in their developmental environments); the more variation in environments that twins in twin studies are exposed to, the lower the heritabilities we should expect to find.
In one study, Eric Turkheimer and colleagues studied 320 pairs of 7-year-old twins who were raised in extreme poverty. Among the poorest, the shared environment accounted for most of the differences in IQ (60%), and the genes accounted for very little; consequently, in this study, the heritability of IQ was reported to be close to zero! Among the richest, however, the heritability of IQ approached what Bouchard found: variations in the genes accounted for most of the differences in IQ scores, and the shared environment accounted for very little of the variance. This study points to the fact that estimates of heritability depend on the sample that is studied, and the environment of that sample.
Turkheimer's study should also be a reminder that just because something is heritable doesn't mean it's immutable. Remember the Flynn effect (see earlier post)? That's a reminder just how much the environment matters, even after completely controlling for genes (by looking at IQ changes across generations).
