Has genetics won the IQ debate?

The idea that genes determine intelligence is fashionable again. The role of the environment is being badly neglected, argues Toby Andrew

"Does any serious scientist any longer doubt that heredity contributes importantly to individual differences in intelligence?" This was the question put by Professor Robert Plomin of the Institute of Psychiatry in London in a recent issue of the scientific journal Behavioural Genetics.

The Medical Research Council certainly takes his ideas very seriously; it has awarded Professor Plomin and others pounds 2.6m to identify the genes that underpin intelligence. Last week, at London Zoo, the Galton Institute organised a conference on the same topic called the "Biological and Social Aspects of Intelligence".

Historically, much of the research purporting to show that intelligence is inherited has depended upon studies of rare cases of identical twins reared apart from each other. Many of the studies have been deeply flawed in terms of scientific rigour. More fundamentally, the criticism has been that such studies conflate valid scientific concepts (such as genes) with terms that are less well defined (such as intelligence, or social class).

At last week's conference, studies were presented which claimed to address the criticisms levelled at the methodology of earlier twin and adoption studies. Professor Thomas Bouchard, head of a long-running twin study at the University of Minnesota, asserted in the keynote Galton address: "Since Francis Galton's brilliant outline [of the genetic basis to intelligence], there has been a steady incremental advance. All the evidence points in the same direction and that is genetics." Professor Bouchard claims intelligence is 70 per cent inherited, whereas Professor Plomin believes the figure is nearer 50 per cent.

Professor Plomin argues that the "nature vs nurture" debate has become sterile and that, in the Nineties, the two camps have started to come together. The task today is to study how genes and environment interact, not how one takes precedence over another. Traits that are genetic need not be immutable, because the environment can be modified so that the genes never become manifest.

For example, people with a defective gene which codes for an enzyme that breaks down the amino acid phenylalanine will tend to become mentally retarded. But this is not inevitable - if fed a diet low in the amino acid, no mental handicap develops. Genetic expression depends as much upon the environment as the genetic make-up.

However, before we all rush to subscribe to Behavioural Genetics, it is worth noting that despite recognition by behavioural geneticists that an inherited trait is not immutable, deterministic assumptions do seem to emerge on closer inspection. For example, there was a consensus at the Galton conference that social class is a product of the genetic make-up of individuals. No one questioned Professor Plomin's public avowal of "socio-economic status being genetically mediated". Likewise, conference delegates appeared to agree with the opening speaker, Chris Brand from Edinburgh University, who declared that "IQ accounts for all social stratification in society". When the American politics lecturer Charles Murray said the same thing in relation to race in his book The Bell Curve, there was an outcry in the US.

It seems that genetic accounts of group differences are intolerable to polite society, but the genetic basis to individual difference is implicitly accepted as common sense.

More strangely, it appears that quantitative genetic analysis now assumes the gene is the only active component in the equation. Professor Plomin states that quantitative measures previously thought to reflect environmental factors should now be recognised in large part to be genetic in origin as well. For example, he claims that in the past psychologists have studied the child as a passive recipient of the environment. The new insight today is that a child can actively create the environment by shaping how his parents respond to him. This, in turn, is influenced by the innate intelligence and personality of the child.

In the model, the individual is graced with being able to shape his surroundings, but even this active element in shaping interpersonal relationships is reduced to the functioning of genes. Such a narrow focus precludes the possibility that society shapes our intelligence. In behavioural genetics, the individual is viewed as logically prior to the society we live in.

The twins study being pursued by Professor Bouchard has led in the past to claims that "religiosity" is heritable. This is scarcely credible - is there really a protein produced by such a gene that would turn people to religion? But as one informed critic of hereditarian ideas, Professor Leon Kamin of Princeton, has noted, there has been a discernible "rise of neurogenetic determinism" in recent times. Or, as Professor Bouchard says, "the table has now been turned. The onus is now on environmentalists to demonstrate the social basis to intelligence."

Toby Andrew is a social science statistician.