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We are not yet able to cure disorders by genetic engineering. But we do sometimes respond to disorders by adopting eugenic policies, at least in voluntary form. Genetic counselling is one instance, as applied to those thought likely to have such disorders as Huntington’s chorea. This is a particularly appalling inherited disorder, involving brain degeneration, leading to mental decline and lack of control over movement. It does not normally come on until middle age, by which time many of its victims would in the normal course of things have had children. Huntington’s chorea is caused by a dominant gene, so those who find that one of their parents has it have themselves a 50 per cent chance of developing it. If they do have it, each of their children will in turn have a 50 per cent chance of the disease. The risks are so high and the disorder so bad that the potential parents often decide not to have children, and are often given advice to this effect by doctors and others.

Another eugenic response to disorders is involved in screening‐programmes for pregnant women. When tests pick up such defects as Down’s syndrome (mongolism) or spina bifida, the mother is given the possibility of an abortion. The screening‐programmes are eugenic because part of their point is to reduce the incidence of severe genetic abnormality in the population.

These two eugenic policies come in at different stages: before conception and during pregnancy. For this reason the screening‐programme is more controversial, because it raises the issue of abortion. Those who are sympathetic to abortion, and who think it would be good to eliminate these disorders will be sympathetic to the programme. Those who think abortion is no different from killing a fully developed human are obviously likely to oppose the programme. But they are likely to feel that elimination of the disorders would be a good thing, even if not an adequate justification for killing. Unless they also disapprove of contraception, they are likely to support the genetic‐counselling policy in the case of Huntington’s chorea.

Few people object to the use of eugenic policies to eliminate disorders, unless those policies have additional features which are objectionable. Most of us are resistant to the use of compulsion, and those who oppose abortion will object to screening‐programmes. But apart from these other moral objections, we do not object to the use of eugenic policies against disease. We do not object to advising those likely to have Huntington’s chorea not to have children, as neither compulsion nor killing is involved. Those of us who take this view have no objection to altering the genetic composition of the next generation, where this alteration consists in reducing the incidence of defects.

If it were possible to use genetic engineering to correct defects, say at the foetal stage, it is hard to see how those of us who are prepared to use the eugenic measures just mentioned could object. In both cases, it would be pure gain. The couple, one of whom may develop Huntington’s chorea, can have a child if they want, knowing that any abnormality will be eliminated. Those sympathetic to abortion will agree that cure is preferable. And those opposed to abortion prefer babies to be born without handicap. It is hard to think of any objection to using genetic engineering to eliminate defects, and there is a clear and strong case for its use.

But accepting the case for eliminating genetic mistakes does not entail accepting other uses of genetic engineering. The elimination of defects is often called ‘negative’ genetic engineering. Going beyond this, to bring about improvements in normal people, is by contrast ‘positive’ engineering. (The same distinction can be made for eugenics.)

The positive–negative distinction is not in all cases completely sharp. Some conditions are genetic disorders whose identification raises little problem. Huntington’s chorea or spina bifida are genetic ‘mistakes’ in a way that cannot seriously be disputed. But with other conditions, the boundary between a defective state and normality may be more blurred. If there is a genetic disposition towards depressive illness, this seems a defect, whose elimination would be part of negative genetic engineering. Suppose the genetic disposition to depression involves the production of lower levels of an enzyme than are produced in normal people. The negative programme is to correct the genetic fault so that the enzyme level is within the range found in normal people. But suppose that within ‘normal’ people also there are variations in the enzyme level, which correlate with ordinary differences in tendency to be cheerful or depressed. Is there a sharp boundary between ‘clinical’ depression and the depression sometimes felt by those diagnosed as ‘normal’? Is it clear that a sharp distinction can be drawn between raising someone’s enzyme level so that it falls within the normal range and raising someone else’s level from the bottom of the normal range to the top?

The positive–negative distinction is sometimes a blurred one, but often we can at least roughly see where it should be drawn. If there is a rough and ready distinction, the question is: how important is it? Should we go on from accepting negative engineering to accepting positive programmes, or should we say that the line between the two is the limit of what is morally acceptable?

There is no doubt that positive programmes arouse the strongest feelings on both sides. On the one hand, many respond to positive genetic engineering or positive eugenics with Professor Tinbergen’s thought: ‘I find it morally reprehensible and presumptuous for anybody to put himself forward as a judge of the qualities for which we should breed.’

But other people have held just as strongly that positive policies are the way to make the future of mankind better than the past. Many years ago H. J. Muller expressed this hope:

And so we foresee the history of life divided into three main phases. In the long preparatory phase it was the helpless creature of its environment, and natural selection gradually ground it into human shape. In the second – our own short transitional phase – it reaches out at the immediate environment, shaking, shaping and grinding to suit the form, the requirements, the wishes, and the whims of man. And in the long third phase, it will reach down into the secret places of the great universe of its own nature, and by aid of its ever growing intelligence and co‐operation, shape itself into an increasingly sublime creation – a being beside which the mythical divinities of the past will seem more and more ridiculous, and which setting its own marvellous inner powers against the brute Goliath of the suns and the planets, challenges them to contest.⁵

The case for positive engineering is not helped by adopting the tones of the mad scientist in a horror film. But behind the rhetoric is a serious point. If we decide on a positive programme to change our nature, this will be a central moment in our history, and the transformation might be beneficial to a degree we can now scarcely imagine. The question is: how are we to weigh this possibility against Tinbergen’s objection, and against other objections and doubts?

For the rest of this discussion, I shall assume that, subject to adequate safeguards against things going wrong, negative genetic engineering is acceptable. The issue is positive engineering. I shall also assume that we can ignore problems about whether positive engineering will be technically possible. Suppose we have the power to choose people’s genetic characteristics. Once we have eliminated genetic defects, what, if anything, should we do with this power? […]