facts in a double helix. There are certainly real genetic differences between human populations. And the scientific study of these differences can help unravel the roots of disease, develop new medicines, unpick the details of deep human history ­ and perhaps even tell us something about the nature of intelligence.

But such genetic differences are not the same as racial differences. The kinds of populations that are useful for scientific research are very different from the kinds of populations we call "races".

We know, for instance, that sickle cell anaemia is a black disease. Except that it isn't. Sickle cell is a disease of populations originating from areas with high incidence of malaria. Some of these populations are black, some are not. The sickle cell gene is found in equatorial Africa; in parts of southern Europe; in southern Turkey; in parts of the Middle East; and in much of central India.

Most people, however, know that African Americans suffer disproportionately from the trait. And, given popular ideas about race, most people automatically assume that

what applies to black Americans also applies to all blacks and only to blacks. It is the social imagination, not the biological reality, of race that turns sickle cell into a black disease.

Racial thinking divides human beings into a small set of discrete groups, often defined by skin colour or appearance, views each group as possessing a fixed set of traits and abilities and regards the differences between these groups as the defining feature of humanity. All these beliefs run counter to scientific views of population differences.

But if Watson's ideas of race were warped, so were the arguments of many of his critics. For the Science Museum, Watson went "beyond the point of acceptable debate". Really? Two years ago, the then Harvard chancellor Larry Summers caused outrage by suggesting in a speech that evolved brain differences, rather than gender discrimination, may explain why men dominate science.

The evolutionary psychologist Steven Pinker was asked whether Summers's comments had put him beyond the pale of legitimate academic discourse. "Good