The report considers behavioural traits such as intelligence, personality (including anxiety, novelty-seeking and shyness), antisocial behaviour (including aggression and violent behaviour) and sexual orientation. The focus is on behaviour within the normal range of variation, rather than diseases or disorders.
Is there a ‘gene for X’?
Some diseases are caused by changes to a single gene, such as cystic fibrosis and Huntington’s disease. But many diseases are not straightforward. For example, heart disease and diabetes are likely to be affected by many genes, and the environment may also play a role. The relationship between genes and behaviour is even more complex. It is widely agreed that genes do have some influence on behaviour but it is likely that many genes are involved in influencing behaviours. Environmental factors will also have an effect.
There are several reasons why it is so difficult to find which genes have an effect on behavioural traits:
- more than one gene may contribute to a trait, with many genes each having a small effect;
- a gene may affect more than one trait (for example in mice, memory and sensitivity to pain have been found to be linked);
- the action of a gene depends on the presence of other genes;
- environmental factors may contribute to a trait;
- genes and the environment interact together in different ways; and
- genes do not have a continuous effect throughout our bodies or for all of our lives.
It is unlikely that variation in just one gene contributes to a behavioural trait. The term a ‘gene for X’ is very misleading and does not convey the complexity of genetic factors. Nor should we overestimate the predictive power of genes. The effects of genes are not inevitable. Genes, like environmental factors, probably just make a behaviour more or less likely to occur. They are part of the cause, but not the only cause.
What is normal?
We use a statistical definition of ‘normal’ to refer to the range of variation, usually about 95% of the population, which does not contain anyone with clinical disorders or diseases. This use of ‘normal’ does not imply any value judgement about different forms of behaviour.
Our focus is on traits that are continuously distributed. These traits are not either present or absent, but are found in everyone to some greater or lesser extent. It is likely that most behaviours lie on a continuous spectrum.
How is the research conducted?
Quantitative genetics: researchers compare different groups of people, for example, identical and non-identical twins, brothers and sisters, families and adopted children. These studies use statistical methods to determine the relative contribution of genetic and environmental factors in influencing behaviour.
Molecular genetics: researchers aim to identify individual genes, and to understand how different gene variants might influence variation in behaviour.
Animal models: researchers use animals to try to examine the effects of particular genes on behaviour. Research is mainly focused on mice and rats, but also primates, birds, fish and fruit flies.
What are the findings so far?
The report describes the scientific evidence so far, with reviews of research on intelligence, personality, antisocial behaviour and sexual orientation. Behavioural genetics is still a highly speculative area of research. A few genetic links have been suggested but, despite the newspaper headings, to date no individual gene has been shown conclusively to influence antisocial behaviour, anxiety or intelligence in the normal range, or sexual orientation.