After 100 Years of Bitter Debate, Is the Secret of Human Nature about to Be Revealed at Last.?; as a Fascinating New Book Claims to Settle the 'Nature versus Nurture' Debate Once and for All

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GRAFFITI on a wall proclaims: 'My mother made me a homosexual.'

Underneath, some wag has scrawled: 'If I give her the wool, will she make me one too?' It is an old and rather lame joke, but it does illustrate, in the crudest of terms, one of the longest, most heated and unresolved debates in the history of science: what is it that determines an individual's innermost nature?

Is it our upbringing, and the influence of our parents and environment, that makes us gay or straight, criminal or lawabiding, clever or stupid?

Or are these things fixed in our genes from the start, entirely unaffected by external forces? For 100 years, warring camps of theorists have slugged it out to decide whether 'nature' or 'nurture' is the key. But new evidence suggests that we may have to rethink the terms of the whole debate.

An important new book by science writer Matt Ridley shows that a host of human traits - from aggression to intelligence - may not be down to our genes or our upbringing, but instead may be caused by a fascinating interaction between the two.

This radical new approach helps explain the latest theory about gayness, which the Mail reported earlier this month. According to American research, boys with older brothers are more likely to be homosexual than those with sisters, younger brothers or no siblings at all.

The reason, as I'll explain later, lies in a remarkable interplay between genetic factors and the environment of the growing foetus in its mother's womb.

It's an explosive subject, because - like many aspects of the nature versus nurture debate - the causes of homosexuality are not just a matter of scientific speculation. They are hugely controversial in political terms, too.

The belief that an individual's makeup is fixed by their genes has become associated with the warped pseudo-Darwinism that inspired the Nazis.

Hitler and his followers believed that by exterminating anyone they saw as carrying 'tainted' genes - such as Jews, gays, gypsies and the mentally or physically handicapped - they could purify the population and create a master race.

This combination of poisonous prejudice and a blind faith in the power of 'nature' led to some of the worst excesses of the 20th century. But an unthinking belief in the opposing view has political implications of its own.

For example, it was assumed for a long time that gayness was a purely environmental thing: something about their upbringing making boys fancy other boys rather than girls.

Even though there was never any real evidence to back it up, this ' nurturist' argument found moral support among many gays, who were unhappy with the idea that their sexuality was foisted on them by their DNA.

But it was also seized upon by the homophobic, who argued that if being gay was just to do with upbringing, there was no reason why the condition - effectively a lifestyle choice - could not be 'cured'.

There are wider ramifications, too. If the nurture principle is correct, and our genes are largely irrelevant to the kind of people we become, then that makes us all essentially equal - the clarion call of the Left ever since the days of Karl Marx.

Every newborn baby would be born as a 'tabula rasa', literally a blank slate, upon which the ideas and experiences of a lifetime wait to be written.

THE IMPORTANCE of this is that it would make us infinitely malleable through social engineering. Simply by tinkering with our environment - from health care to income levels, and from schools to the media - a supposedly benign government could make us all fit its template for the ideal society.

It's a belief that can lead to its own kind of fascism, as the author Aldous Huxley knew all too well. Many people seem to think that his novel Brave New World was about the creation of a genetic underclass and an enhanced-DNA elite. …