Vaughan Bell, Beyond Boundaries column, The Psychologist, July 2009

Nature versus nurture is a lie. Music is not melody versus rhythm, wine is not grapes versus alcohol and we are not environment versus genes. We are their sum, their product and their expression. They dance together and we are their performance, but neither is an adversary. The art of understanding this elegant ballet is complex and arcane but you may never realise this from reading the quoted results of genetic studies, because the extent to which a trait is heritable, that is, accounted for by genetics, is usually expressed as a simple percentage.

If you search Google for the phrase “80 percent genetic”, you will discover hundreds of sources that claim that everything from schizophrenia, to height, to intelligence has been found to be four fifths ‘genetic’. Pick any other figure and you can find everyone from psychologists, to politicians, to journalists claiming that this or that is explained by genes to a given percentage. Geneticists know the subtly of this percentage and why these statements, usually lifted from the results of twin studies, are misleading, but clearly many others do not.

Imagine a mental illness is described as being 80% heritable. This is often taken to mean that four fifths of an individual’s risk is down to his or her genes, but this is not the case. What it means is that 80% of the variance in the measured illness was explained by genetic factors in the specific group that was studied. If this seems like a frivolous distinction, bear with me, because it is key in understanding heritability and it becomes crystal clear when tackled as an example.

Imagine that we could study a population where everybody lived in an identical environment. They did the same things everyday; they ate identical foods, had identical relationships and were stressed by identical events. Their lives were carbon copies of each other. A twin study would find that mental illness would be close to 100% heritable, because if the environment is fixed, any difference must be down to genetics. In fact, twin studies would find that everything is close to 100% heritable, for exactly the same reason. To flip our thought experiment on its head, if we only studied genetically identical clones, everything would be 0% heritable, because any difference must be down to the environment.

These figures do not necessarily tell us anything about the potential for a trait to be influenced by nature or nurture, because heritability is rarely an immutable and absolute fact about biology; it is an overall measure of how things are for that group, at that moment. In other words, the process of measuring the influence of genetics is, itself, subject to environmental factors. It captures the dance, not the dancers.