Genetics of human behaviour

Although Darwin's writings about evolution from a physical or structural perspective are well known, he also wrote about the evolution of human behaviour, a topic much less universally discussed.

He suggested that human behaviour, like physical characteristics, changes or evolves from generation to generation and that behaviour is in large part genetically determined.

Today we are still learning a great deal about human behaviour by observing the behaviours of simpler organisms such as sea slugs, mice and fruit flies. While you may not see an obvious connection, we have more in common with these simpler creatures than is apparent.

Since these simple organisms have less complex nervous systems and shorter life spans than human beings, it is possible to actually see changes resulting from learning and also to observe the effects of genetics on behaviour over several generations. Effects of selective breeding experiments and other forms of genetic manipulation can also be observed over several generations and can teach us a lot.

Genetic predisposition is a known factor in virtually all psychiatric conditions that have been thoroughly studied to date. Heritability varies between disorders from between 20 and 30 per cent for most anxiety disorders, 30 to 40 per cent for major depression, 50 and 60 per cent for alcoholism and 80 per cent or greater for schizophrenia and bipolar disorder.

Clearly there is a strong genetic component in these conditions, and psychiatric disorders are not alone, all human behaviour is under the influence of our genetic makeup.

In almost all psychiatric conditions, the behavioural symptoms are the result of multiple genes, which each account for a relatively small part of the variance and whose effects are cumulative. However, the effects are not always additive and sometimes the effects of one gene may be reduced or cancelled by the presence of another gene. This means that two individuals with the same gene for a particular behavioural characteristic may not express that characteristic in the same way. In fact, one may show the behaviour and the other may not because of the mitigating effects of other genes.

Obviously, this makes the study of complex behavioural disorders very complicated in humans.

Fortunately, there are substantial genetic similarities between human beings and those simpler organisms I mentioned earlier. With this in mind, if there is a gene of particular interest in humans that also exists in these less complex animals, we can figure out what that gene does in the simple organism and then look for similar effects in humans. This allows a sharper focus and an enhanced likelihood of understanding the effects in humans.

Almost all genes that influence behaviour have multiple effects which can be modified by the presence of other genes and by evironmental experiences. While this may sound obvious, it is also true that our genetic makeup often influences our environment as well.

A person born with particular behavioural characteristics will experience a different world than someone without those characteristics. For example, someone with attention deficit disorder is often bullied in school, has much more criticism than his or her peer group and may have more experiences with failure of various sorts than someone without this condition.

As far as we can see from research to date, Darwin was remarkably prescient and the genetics of human behaviour and its influences are substantially similar to that of the rest of the animal kingdom.


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