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Anyone who has sat on a modern jury trying a case of rape knows about the DNA test. The matching of DNA in semen from the rapist with that found on the victim is a powerful piece of evidence. But it does not prove that rape was committed. For that, other evidence - to do with the state of mind of both parties - is required.

It is the same, but more so, with the charged debate over genetic predisposition to crime (pages 16-18). Some people, particularly on the right and particularly in the United States, where violent crime is horrifically ordinary, are attracted to the idea that criminals are born and not made, that the violence is chemically not socially rooted. Others, particularly on the left, reject such ideas, fearing their use to collapse whole rafts of welfare programmes intended to ameliorate bad conditions for those living in squalor and poverty. Neither is in fact denying all truth to the other. The debate now, as it was before genetic testing, is still about the balance between nature and nurture; about which questions should have priority for investigation; and about what follows from any findings. Meanwhile, the US is transfixed by the trial of O. J. Simpson and we are only spared a similar circus surrounding Rosemary West by rigid reporting restrictions. Even if there is a genetic predisposition to violence - and it is that rather than culturally determined crime with which biological studies are concerned - few, except Mr Mobeley, maintain that that is all there is to understanding crime. The contingencies that drive people to wickedness have perennial fascination.

Reductionism, whether genetic or otherwise, is in many ways a satisfying philosophy. It promises to explain which bit does what, "how things work", and how they can be repaired or altered. Who does not marvel at the decoding of the molecular structure of DNA, or the more recent discovery of a new form of carbon in addition to diamond and graphite, the so-called "buckyballs" or buckminsterfullerenes (page 21)? And who does not feel excited at the unparalleled possibilities opened up by genetic engineering? Starving populations may be fed, fatal cancers banished.

Some suggest that human consciousness can be understood in the same way. The argument is gathering pace. On the basis of a study of brain damage, Francis Crick (of DNA-decoding fame) gave it a brisk stir when he wrote recently in The Astonishing Hypothesis: "Free Will is located in or near the anterior cingulate sulcus." And added, as if to prove that he was serious: "In practice, things are likely to be more complicated. Other areas in the front of the brain may also be involved." This week the Royal Society held a briefing for science writers on consciousness research, and to warn against undue expectations of progress (page 3). One speaker noted, for example, that consciousness research has not yet even established which feature or features of the brain enable it to produce conscious experiences - let alone which brain tissue is responsible for which aspect of the human personality.

Reductionism has given us modern science and still has much to give us. As it advances, helped and limited by available and developing technology, new detail as to how we function will emerge. As detail is uncovered, new light will be shed on old questions, moving the boundaries of generalisation and speculation in ways unsettling to those whose intellectual territory is invaded. That territory will be stoutly defended and so it should be. Those who deal in the details, will need constantly to be reminded of the immense complexity of systems. The recent Japanese earthquake with its social consequences, and the humbling failure of seismologists to predict it on the basis of plate tectonic theory, is a reminder of the need to study systems - organisms, societies, the environment - as well as the constituent atoms, molecules and genes. If, for example, it turns out that a genetic propensity to violence is triggered only by certain social or environmental conditions - to locate such genes, while useful, will hardly be sufficient.

The variety of academic disciplines now contributing to the debate around these issues suggests that opportunities are opening up for collaboration between disciplines on a scale unfamiliar in recent decades. This has implications for the organisation of research: to split specialisms into isolated units could be a mistake, while to locate research in universities where many disciplines rub shoulders should be productive. In this case, the whole really is greater than the parts. Drinking coffee in the senior common room has perhaps never been more important - nor the time available for it more limited.