Human nature is on the verge of changing forever. The cause of this extraordinary and terrifying development is almost commonplace. We are slowly but surely abandoning both our lay and our professional reservations about 'designer babies'. Growing government investment in genetic research — another £50 million was pledged last June — is matched by increasingly liberal public attitudes towards genetic 'advances'. As molecular biologists discover more and more about the genetic foundations of human traits, we become less willing to accept the accidents of heredity, and ever more eager to bring human nature itself within the ambit of personal choiceIt is true that some biologists are still warning us about how the complexity of the human genome makes it difficult or impossible to manipulate such sophisticated traits as cognitive abilities or emotional temperament. But uncertainty has never been an insuperable obstacle to scientific curiosity. What was once impossible to envisage has already become a realistic prospect. No longer, for example, is it difficult to imagine gene therapy being used on embryos to cure them of serious diseases, in the full knowledge that this prenatal modification of the genome may alter the non–clinical profile of the future person. Measuring such human traits as aggression, humour, athleticism or intelligence, may also seem a fruitless exercise, but scientists still claim they have found the genes 'for' these traits anyway. In the under–regulated industry of reproductive 'medicine', where the consumer rights of would–be parents are increasingly seen as inviolable, this means that there will be ample opportunities for the marketing of 'potential' enhancements and modifications to future children, with no absolute guarantee of success a precondition for trade.

But there is little sign that any serious consideration has been given to what these developments might mean for our society, culture and relationships. One does not need to subscribe to a crude form of genetic determinism, in which the manipulation of genes gives scientists the power to programme the fate of the new born child, in order to find human genetic engineering ethically troubling. The problem with designer babies is not that adults will produce for themselves a predetermined product, which is lacking in autonomy and uniqueness. It is, rather, that the parents of such a child, having been seduced by the ideology of genetic determinism, will think they have ordered a pre–designed being. It is this expectation, which will be all the more robust when mediated by the morally neutral actions of the white–coated scientist, which is likely to constitute the biggest impediment to the resulting child's attempt to become a self–determining individual.

Consider sex selection. Since it has become possible to sort sperm into cells with either X or Y chromosomes, and thus determine, with a relatively high level of success, the future sex of a child before it has actually been conceived, the legalisation of sex selection, as a means of addressing 'unbalanced' families, is now being given serious consideration. Only a moment's reflection is required, however, to realise that the relationship between sex selection and a balanced family, like the relationship between genes and traits, is far from direct. People who want to balance their families are really not thinking in terms of sex, but of gender. They don't just want a child with a penis who will develop facial hair and a deep voice; they want the definitive heterosexual features, behaviours, tastes and styles which will mark that person out as the polar opposite of his (its) sister.

Yet because gender is socially rather than biologically constructed, it can also be resisted, subverted, and reworked. Choosing, with scientific assistance, to conceive a female child, in other words, will not prepare you well for "a six–foot–tall, 300–pound daughter who wears nothing but denim and boots", to quote Barbara Katz Rothman. Making sex selection a reproductive right will in fact nourish our cultural intolerance of all the ambiguous, fluid and unique features of people's personalities and preferences — elements of identity which naturally fall on a spectrum between the purified extremes of masculinity and femininity.

Gender selection neatly shows exactly what is at stake in human genetic engineering. It shows that the imperfect link between DNA and behaviour will not deter those who are committed to biological determinism (even our own HFEA has formally consulted the public about the desirability of sex selection for family balancing); and it also makes concrete the argument that if we grant adults the right to predetermine the genetic inheritance of their children, we are effectively inviting, or at least legitimising, their effort to consolidate their biological investment by controlling those social and environmental variables which genetics leaves untouched.

After all, who in their right mind would buy for their child the genes of an Olympic athlete, then allow that child to pursue an interest in music or books?

Of course, liberal democratic societies already grant parents the right to shape the nutritional, cultural, social and material environment of their children. So, isn't the right to modify the 'inner environment' of one's offspring, to manipulate the genetic influences on their development and behaviour, consistent with our existing system of parental liberties?

The German philosopher, Jürgen Habermas, provides a powerful answer. In a recently translated book, The Future of Human Nature, he argues that what is jeopardised by the current trend in human biotechnology is nothing less than the "self–understanding of the species". A common thread running through all our ethical traditions, from early religious doctrines and metaphysical worldviews to modern forms of humanism, is a vision of 'man' who, though capable of enormous cultural diversity, is everywhere fundamentally the same.

This distinction between what we commonly are, organically, and what we are made, by the contingencies of culture, society, and technology, is now, however, under threat. Habermas suggests that our ability to see ourselves as free, as beings capable of "making a new beginning", derives in part from the fact that our own beginning lies beyond human disposal. It is because we have, until now, perceived new human life as a gift of nature (or God), that we have been able to distinguish between the processes of socialisation and the bodily person who is subject to these influences. By making this distinction we assume a continuity of self over time, and we resist treating ourselves as if we were passive products of our society and culture.

The prenatal modification of the human genome, on the other hand, will endanger this moral self–understanding, this sense of being the author of one's life. It is Habermas's fear that the resulting fracturing of identity, in which the person's designer is perceived as the co–author of his or her life, may fatally disrupt the subjective preconditions for that person's participation in a moral community.

Habermas believes that the moral and political accomplishments of contemporary liberal societies, the formal principles of egalitarian individualism, cultural pluralism, and universal human rights, depend for their stability on a prior ethical consensus that we are all, as members of the same species, free and equal beings guided by responsibility and reason. This consensus breaks down not only through the experience of the genetically programmed person, but also in the eyes of the programming parents who, because their genetic investment has a lifetime of functioning, will produce a child they will never be able to look upon as an equal.

As well as driving a wedge between the generations, human genetic engineering may sever the ties which bind different cultural communities, often with irreconcilable values and beliefs, to a common political space in which participants share their species–specific capacity for reasoning and moral judgement. With the prenatal programming of the human genome, the power to transfigure culture into biology arises, allowing each community to reproduce itself by creating offspring who are genetically predisposed to the values it seeks to transmit. Instead of the 'clash of civilisations' — which always implies a conflict over different ways of being human — we will have a 'clash of natures', and perhaps even a 'clash of species'. This is why biotechnology is such a disturbing threat not just to the humanist worldview, but to humanity itself.