If defective behaviour is influenced by 'faulty' genes, how responsible should criminals be for their crimes? Susan Greenfield shows how advances in genetics are making demands on our sense of values

Think "health", and nowadays you are immediately on to "science". More specifically, you are thinking "genes". But science has never been, and genomics in particular never will be, a source of values. It is just that the newest, genetic technologies add to the existing dilemma of how to live in a society bereft of religious or uniform cultural values - a society seething with individuals.

I would take individuality as the starting point for a 21st-century value - one that would surely be jeopardised by cloning. But, in fact, nothing could be further from the truth. Already, we have clones - identical twins - and they are still individuals, even though they might look the same. Twenty-first century individuality is under a more insidious threat than simply the prospect of people looking like each other.

The problem with genetic manipulation is that it offers a spectrum of possibilities. It is difficult to know where to draw the line between highly desirable and completely detestable. Take genetic screening. Surely it is a good thing to screen to see if you have a gene that might predispose you towards breast cancer. You would then be able to take precautionary methods such as having more frequent checks. On the other hand, if a foetus turns out to have a single-gene disorder, such as Huntington's chorea, should you have an abortion? Huntington's chorea - a disease characterised by wild, involuntary movements - occurs typically only in middle age. Even when afflicted, an individual is still an individual: should they not have lived at all? But people will still argue - why stop with faulty genes when you could have perfection?

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There is, of course, the more distant prospect of "virtual children". Rather as you might pick colour schemes for a room, you might pick a constellation of genes for a certain personality. But the very assumption on which this strategy would be based is erroneous: that you can tweak a single gene, or replace it - and then, hey presto, you have suppressed an undesirable trait and created a good one. As far as the brain goes, this is nigh on impossible. Although there are a few gene disorders that can be traced back to a single faulty gene, most problems, especially disorders of the brain, have a multi-genetic basis. Even if you can trace the role of a single gene in that disorder, there is not a one-to-one relationship between the gene and the fault. How could shyness or depression be trapped inside a strand of DNA? The gene is indeed an essential part of brain function, but being necessary is not the same as being sufficient. You have 100 billion brain cells, with 1,000,000,000,000,000 connections, but only some 100,000 genes. If every gene in your body were to count for a connection between the brain cells, then you would be out by a factor of 1010. If in the future we proceed, nonetheless, to tweak faulty genes in the hope that we are going to create "better" individuals, then we might be opening a Pandora's Box within the brain, whereby the effects of changing genes would be much more far-reaching than we had ever planned, possibly even functionally global.

But individuality is not at risk just from a more standardised gene pool. Information technology poses just as sinister a threat.

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If we have small children exposed to standardised software, with the same in-your-face images, will they lose the ability to develop individual imaginations? Their lives will be pared down to being parked in front of a VDU, rather than interacting in the strange and wonderful ways most of us enjoy in the outside world. Since brain connections are massively influenced by individual experiences, the information technologist might be tweaking nurture as much as the molecular biologist is starting to tweak nature.

Individuality as a value spawns three more: one is accountability. If we are becoming much more comfortable with linking behavioural traits to genes, and even at being able to explore the impact of nurture on the landscape of our brains and bodies, then how do we decide on the actions for which we have to bear some responsibility?

This problem is already upon us. In a US court, lawyers have advanced the mitigating defence that their client harboured genes for "criminality". The nail-bomber in London, David Copeland, recently asserted that he had a mental problem and therefore could plead diminished responsibility. The two boys who killed the toddler James Bulger are now deemed to be different people, whereas Myra Hindley, despite 30 years' incarceration, is deemed not to have changed. What is the difference? According to what criteria do we say someone has become a new individual? And imagine defending Hitler on the grounds that he was unlucky enough to have had an unpleasant upbringing or was born with the wrong set of genes. When do the genetic and environmental factors count, and when does the whole - the responsible individual - exceed the sum of the biological parts?

This increasingly accurate emuneration of the factors that make us who we are leads to a further value - privacy. No one would dispute the importance of having privileged access to one's own medical records. But, increasingly, this simple idea will be stretched to its limit. It is a moot point whether insurance companies should have access to your genetic profile so they can conduct their profession more effectively and efficiently. Could it not be construed as wrong of you - nay, criminal - to hold back such information once you had it, as it is with other medical information?

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Then there is the promise that tiny nanotechnological devices will, in the future, be embedded inside people's bodies. As a result, we will be able to monitor what is going on in the body more precisely than ever before. And, as people's bodies become more open to the scrutiny of outside observers, so, too, might their thoughts. In a world of virtual discussions and friendships, ideas may take on an entity of their own. Drawing a line between the individual and their thoughts will become a lot trickier.

And so we come to the fourth value - freedom. We strive for freedom from pain and fear and anxiety - and I for one would like the freedom to choose my own death. But will I be "free" to be myself in a world where there is so much potential for manipulation and surveillance of my body?

Individuality, accountability, privacy and freedom: these are not new values, but with the inevitable and unstoppable march of science, they take on a sharper definition and prominence that prompt us to revisit them.

Susan Greenfield is director of the Royal Institution and professor of pharmacology at the University of Oxford. Her latest book is The Private Life of the Brain , published by Penguin, price ?18.99.

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