Does the finding that human beings have far fewer genes than expected settle the nature v. nurture debate? Yes, say some scientists. The scarcity of genes, they argue, means experience and environment -- nurture -- must play a stronger role in shaping humans than genes, or nature.

But other experts are equally vehement that the debate is far from concluded.

The passions being raised recall the 1970s heyday of nature v. nurture. Today as then, the debate often appears to be as much about philosophy and politics as DNA.

In a recent New York Times article, Stephen Jay Gould, Harvard zoologist and well-known writer, said the confirmation of only 30,000 to 40,000 human genes, instead of the predicted 140,000, undermines geneticists' "central dogma": that one gene makes one protein, and the sum of such proteins makes a person. He argued human complexity cannot be generated by 30,000 genes and the lower number of genes "marks the failure of reductionism" to explain intricate biological systems. He suggested the finding will work to liberate society from "simplistic and harmful" ideas associated with genetic determinism.

But Steven Pinker, an evolutionary psychologist at the Massachusetts Institute of Technology, says Mr. Gould exaggerates by implying most scientists expected the genome project to uncover more than 100,000 human genes.

Mr. Pinker had earlier estimated humans have about 50,000 genes each, a figure he says is not radically out of sync with the 30,000 to 40,000 found in the genome study. Yet, even with fewer genes, he claims, there is still substantial evidence humans have many innate instincts and abilities, as well as built-in tendencies for intelligence, aggression, shyness and other traits.

"No one can say whether 30,000 genes are too few, since no one knows how many genes it would take to build a language instinct, or a fear of snakes, or a moral sense," Mr. Pinker notes. "The task is to figure out how that information is coded in the genome, whether the number of genes is 30,000 or 75,000."

One theory holds that the large amount of non-genetic information in the genome, so-called junk DNA (99% of our total DNA), plays an important role in the control and timing of gene functioning. This DNA, it is thought, could be a crucial cog in the cellular mechanisms that differentiate a cat from a human. Another theory speculates that human genes interact in a complex, not yet understood manner, essentially multiplying their ability to carry out and control large numbers of biological functions. Still another conjecture is that scientists have simply miscounted.

Joseph LeDoux, professor of neuroscience at New York University, believes Mr. Gould's attack on genetic causality to be timely and justified. "Because of the abundance of genetic research being done, the perception has been that everything is genetic," he says.

Mr. LeDoux says evidence of fewer human genes weakens but does not eliminate the argument for a genetic basis to human behaviour and learning.

"Nature or nurture is a false dichotomy," he says. "All learning is based on genetically based learning programs. Experience however figures prominently in what we learn." He claims environmental stimuli such as strong parental bonding or malnutrition alter brain synapses, resulting in modified or learned behaviour, a process called brain plasticity.

The contentious question at the centre of the nature or nurture debate, however, is not whether genes or the environment both play a role in human development, but precisely how much is a result of genes, and how much due to experience. It is a question, to many people, tinged with moral and political meaning.

Edward O. Wilson, a famous Harvard biologist, addressed this question in his 1975 book Sociobiology. He estimated that as much as 50% of a person's personality traits, intelligence and behaviour could be accounted for by genes. The book provoked vehement criticism. A group called Science for the People said Mr. Wilson's theories "provide a genetic justification of the status quo and existing privileges for certain groups according to class, race and sex."

Mr. Wilson's book charged the modern "nature or nurture" debate with as much political as scientific significance, leading nurture advocates to take the side of the left, while nature backers are seen to be more closely linked with views at the centre or right of the political spectrum. For instance, while a nurture backer might argue that crime can be eliminated only by reducing poverty, a nature advocate might say decreasing crime requires better policing and harsher penalties.

The legacy of this politicized version of the debate, Mr. Pinker suggests, has led to many misconceptions about genetic-based models of human development and behaviour. One of these, he says, is that genes imply determinism.

"Genes are interested in propagating themselves, so they have created a brain interested in sex," he says. "The brain can override the genes by using a contraceptive. Without a contraceptive, the agenda of the brain and genes coincide."