A better grasp of the nature-nurture relationship may offer insights into crime, cancer and more, writes Michael Rutter.

Until recently, behaviour genetics was mainly concerned with quantifying the relative influences of nature and nurture on psychological development and mental disorders. Twin and adoptee studies were primarily used to separate genetic and environmental effects. The findings were consistent in showing the strengths of both nature and nurture. Overall, their effects appeared to be about equal, although genetic factors predominated for disorders such as autism or schizophrenia, while environmental factors did so for those such as crime.

Now it has become clear that to regard nature and nurture as separate and independent is an oversimplification. Effects so often depend on the interplay between the two, both in terms of correlations and interactions.

Correlations arise because genes influence individual differences in exposure to environmental risks through three different mechanisms. First, both parents pass on their genes to offspring and provide their children's upbringing environment. The correlation between genetic and environmental influences reflects the fact that, on the whole, the parents who pass on genes involving an increased risk for mental disorders also tend to provide less stable rearing environments.

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Second, people select and shape their environments through their own behaviour. For example, a child with genetically influenced musical, athletic, or mathematical talents is likely to spend a lot of time in these pursuits. Further development of such talents will therefore be influenced by these environmental advantages, as well as by the child's genetic background.

Third, people's genetically influenced behaviour affects their interactions with others. Antisocial individuals are more likely than others to act in ways that provoke hostility, predispose to the breakdown of relationships and put their jobs in jeopardy.

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These findings carry crucial implications for both genetic and psychosocial research. The message for genetics is that part of the genetic effect lies in its indirect impact on variations in exposure to environmental risks. It therefore involves both nature and nurture. The message for psychosocial research is a parallel one: some of the effects that appear to be wholly environmental are, in reality, partly genetic.

Genetic evangelists have sometimes sought to rubbish psychosocial research on this basis. But their criticisms are unwarranted because the genetic findings show that, usually, only a minority of the supposed environmental effects are genetically mediated, and that genetic analyses have confirmed that environmental risk mediation exists. For example, environmental factors have been shown to account for the differences in outcome within monozygotic (identical) twin pairs who share all their genes.

So much for the correlation between genes and environment. Gene-environment interaction reflects a rather different mechanism. It has been a universal finding in environmental risk research that children - and adults - vary enormously in their responses. Thus, exposure to pollens in the spring gives rise to severe hay fever in some individuals, whereas others are completely unaffected: genetic influences are concerned in that individual difference. Moreover, molecular-genetic research, which studies the effects of individual susceptibility genes, has confirmed that genes and environment work together in relation to risk factors as varied as smoking, head injury and infections. Such disorders are unlikely to occur in the absence of susceptibility genes and are unlikely, too, in the absence of the environmental risk factor. It is the presence of the two that is crucial.

But we still need to ask whether there are major genetic effects that are independent of environmental adversities and whether there are environmental effects on individuals who are not genetically susceptible.

The independent importance of genetic effects has the strongest factual basis. Evidence concerning both schizophrenia and autism, for example, indicates that the genetic risks for these disorders are not dependent on the children encountering environmental hazards. The same probably applies, to some degree, to other psychological features. On the whole, environmental effects are most evident in genetically susceptible individuals. Some environmental effects probably exist that do not require genetic susceptibility but they have not been clearly demonstrated.

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Two other caveats have to be added to the nature-nurture question. First, non-genetic influences do not necessarily involve specific environmental effects - biological development is probabilistic rather than deterministic. In other words, the evolutionarily derived genetic programme specifies a general pattern or plan, but it does not determine what each individual nerve cell (or any other type of cell) does. Chance and general perturbations play a considerable role. Thus, every female has two X chromosomes but only one is active - which one seems to be largely determined by chance. Whichever it is will matter in some circumstances because one X is inherited from the father and one from the mother. General perturbations are widespread in development. Thus most of us have minor anomalies of one kind or another - an extra nipple, an asymmetric skin pattern and such like. Such anomalies are meaningful at a group level - they are more common in twins than in singletons and in children born to older mothers - but no specific environmental factor seems responsible for their occurrence at individual level. Moreover, most anomalies have no functional consequences. Disorders may result from some combination of genetic risk and developmental imperfections rather than from any particular environmental experience.

The second caveat is that although quantifying genetic and environmental effects looks at individual differences, we also need to consider effects on the frequency of disorders or traits. Over the past half-century, there have been huge rises in the rates of substance abuse and crime among young people and in suicide among young males. The speed of these rises points to environmental effect.

But the pattern of research findings indicates that the factors responsible for individual differences in a particular characteristic are not necessarily synonymous with the factors responsible for the level or frequency of that characteristic in the population as a whole. Thus genetic factors are largely responsible for individual differences in height but the huge rise in average height (some 12cm or so) over the past century is almost certainly due to improved nutrition. A high, even a very high, heritability does not mean that a major change in environmental circumstances cannot make a big difference.

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Where does that leave the question of whether we are shaped by nature or the environment? The answer has to be both. However, research findings go further than that in emphasising the interplay between the two; much of the variation between people stems from the synergistic combination of nature and nurture.

In a sense, the question is the wrong one. There are no policy or practice implications in heritability being high or low. What really matters is not the relative strength of genetic and environmental effects but the mechanisms by which they exert their effects. Therein lies the future. Do genetic influences on antisocial behaviour operate via the indirect risks associated with sensation seeking, the more direct risks associated with aggressivity, or the protective effects associated with high anxiety?

Molecular geneticists will play a crucial role in providing the understanding of causal processes. Once molecular genetics has identified one or more relevant susceptibility genes, and once functional genomics research has shown the effects of these genes on proteins and on the biological processes the protein products bring about, it should help narrow down the search for underlying biological causal mechanisms. But research will be successful only if it includes investigation of the interplay between nature and nurture. So research needs to extend beyond the processes operating within the cell to the processes involved in how individuals interact with their environments, and hence to the indirect pathways by which genetically influenced liabilities lead to particular behaviours. This task can be accomplished but success will not be easy and will likely take a long time.

Sir Michael Rutter is professor of developmental psychopathology, Institute of Psychiatry, Kings College London. This article is part of The THES 's Big Science Questions series.

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