The pessimistic scenario. In industrialized nations, people with lower IQs tend to have more children, and to have them earlier, than people with higher IQs. To a substantial extent, IQ is determined by genes. Since low-IQers are breeding faster than high-IQers, the average IQ should be declining over time. The rate of expected deterioration has been estimated to be anywhere from one to four IQ points a generation. By now we should be well on our way to becoming a nation of half-wits.

The optimistic scenario. IQ is also determined by environment. And the very changes that have removed the reproductive advantage that the higher socioeconomic classes once enjoyed over the lower classes--improved nutrition, hygiene, and medicine--have also enhanced cognitive environments across the board. So have better education, urbanization, and a rising average socioeconomic condition. Therefore, mean IQ should go up over time. We should be well on our way to becoming a nation of geniuses.

The pessimistic scenario was first limned by the Victorian scientist Sir Francis Galton. More recently, the specter of declining intelligence was conjured up by Charles Murray and Richard J. Herrnstein in The Bell Curve, which blamed the American welfare state for the unchecked breeding of the lower cognitive classes. With equal conviction, optimists hold out the hope that further environmental improvements might lift the cognitive functioning of society as a whole and eliminate the nagging fifteen-point average IQ differential between American blacks and whites.

So let us pose the question that we all, as sophisticated intellectuals, know to be meaningless: Which is more important, heredity or environment?

Well, it looks as if environment has been decisively winning the contest. Since the 1930s, massive IQ gains have been recorded in the United States, Europe, Canada, Australia, Israel, Brazil, Japan, and urban China--indeed, in every nation where data has been kept. The optimistic scenario seems to be prevailing.

The phenomenon of rising IQs was noticed in the 1980s by James R. Flynn, an American political scientist who teaches in New Zealand. The "Flynn effect" has an especially heartening implication for racial IQ disparities. The mean gap between American whites and blacks today appears to be about the same as the one between whites today and whites in the 1950s. Since the latter gap is patently environmental, not genetic, isn't it likely that the former is as well? And if IQs go up almost automatically over time in response to some unknown environmental cause, shouldn't it be possible to identify this cause and use the knowledge to equalize opportunity?

There, alas, is the rub. As a forthcoming volume from the American Psychological Association titled The Rising Curve: Long-term Gains in IQ and Related Measures makes clear, no one really knows what the mysterious factor X might be.

Contributors to the collection entertain every imaginable hypothesis. Some of the proposed explanations--better nutrition, better education, smaller families, homes full of stimulating puzzles and video games--imply that the higher IQ scores are matched by gains in real intelligence. Others suggest that the gains are a mere artifact, a result of defective tests or greater test-taking sophistication.

My personal favorite in the artifact category is the "Brand hypothesis" (after the British psychologist C.R. Brand), which attributes the gains to sex, drugs, and rock'n'roll. Earlier generations were too straitlaced and scrupulous when taking the IQ test, wasting precious time striving to answer every item correctly. In today's permissive society, kids think nothing of skipping or making guesses on hard questions, thereby getting more right within the allotted time.

But the Brand hypothesis fails to square with the data, as Flynn observes in his contribution to The Rising Curve. Nor, it seems, do the others. The nutrition hypothesis might look strong at first blush, especially considering that IQ gains over the generations have been accompanied by similar gains in average height and brain size. But there is plenty of contrary evidence--such as the fact that children born into the severe Dutch famine of 19441945 in no way missed out on the rising IQ curve. Greater test familiarity? Not likely--the gains began before testing was widespread and have continued into an era when it has become popular. Better education? Impossible--the gains are fully manifested before the age of six.

No one is more perplexed by all this than James Flynn himself. Despite the effect that bears his name, he admits that "one does not see an evolution from widespread retardation to normalcy or from normalcy to widespread giftedness" over the generations. Until we figure out what the Flynn effect really consists of, he says, we can scarcely hope to find its cause.

Meanwhile, as an illustration of just how slippery the issue of aggregate IQ gains can be, let me propose a painless little eugenics program. Some years ago, a well-known public official in California quit his job and moved to Alabama, prompting a local editorialist to observe that this probably raised the average IQ in both states. Such a thing is clearly possible (if, say, the California mean were 110, the Alabama mean 90, and the official's IQ 100). By the same token, it should be possible, by a suitable redistribution of people from state to state, to raise the average IQ of all fifty states. And then wouldn't the average IQ of the whole nation--which is, after all, just the average of the IQs of its constituent states--also go up? And couldn't the same process, carried out internationally, boost the average IQ of the entire world?

If you believe that, perhaps you should consider moving to Alabama.

JIM HOLT