Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved

James R. Flynn and William T. Dickens
William T. Dickens University Distinguished Professor of Economics & Social Policy - Northeastern University

April 1, 2001

The following is a summary of the article published in Psychological Review, the full article can be found by visiting the Psychological Review website.

Darwin’s Origin of Species sparked the modern debate about genes versus environment in explaining differences between human individuals and groups. Ever since, the pendulum of scientific opinion has swung back and forth with consensus always out of reach. For the last 15 years, psychologists have been plagued by a paradox that suggests that environment is both feeble and overwhelmingly potent.

The paradox emerged from a debate about race. US whites outscore US blacks on IQ tests by 15 points. Does that gap have environmental causes or is it partially due to genes? In 1973, Arthur Jensen constructed a model that applied kinship data to group differences in IQ. Evidence from kinship studies showed identical twins separated at birth and raised in different homes grow up with very similar IQs. The fact that they have identical genes provides an obvious explanation. Jensen argued that fully 75 percent of IQ variance between individuals was due to genetic differences (a value which sits in the middle of the range recently endorsed by a select committee of the American Psychological Association for adult IQ). Jensen’s model showed that a purely environmental explanation of the black/white IQ gap meant that the environment of the average US black must be as unfavorable for the development of IQ as the lowest one percent of white environments measured in terms of their effects on IQ. That simply did not seem possible.

Jensen’s model seemed to preclude a purely environmental explanation for any large IQ gap between groups. Then, in 1987, Flynn showed that in nation after nation, the current generation outscores the last generation by some 9 to 20 IQ points. The gains are greatest on those tests often called the best measures of intelligence. Their size and speed dictate an environmental explanation. Flynn applied Jensen’s model. An environmental explanation meant putting the current generation within the top one-tenth of one percent of the last generation in terms of environmental quality. What was known to be true was shown to be impossible.

How could solid evidence show both that environment was so feeble (kinship studies) and yet so potent (IQ gains over time)?

Dickens has proposed a model that we believe solves the paradox. It assumes that people who have an advantage for a particular trait will become matched with superior environments for that trait; and that genes can derive a great advantage from this because genetic differences are persistent. A genetic advantage remains with you throughout life, while environmental differences tend to come and go, unless sustained by the steady pressure of genes.

Take those born with genes that make them a bit taller and quicker than average. When they start school, they are likely to be a bit better at basketball. The advantage may be modest but then reciprocal causation between the talent advantage and environment kicks in. Because you are better at basketball, you are likely to enjoy it more and play it more than someone who is bit slow or short or overweight. That makes you better still. Your genetic advantage is upgrading your environment, the amount of time you play and practice, and your enhanced environment in turn upgrades your skill. You are more likely to be picked for your school team and to get professional coaching.

Thanks to genes capitalizing on the powerful multiplying effects of the feedback between talent and environment, a modest genetic advantage has turned into a huge performance advantage. Just as small genetic differences match people with very different environments, so identical genes tend to produce very similar environments—even when children are raised in separate homes.

In other words, kinship studies of basketball, no matter whether they involved people with identical genes or different genes, would underestimate the potency of environmental factors. Playing, practicing, being on a team, coaching, all of these would be credited to genes—simply because differences in them tend to accompany genetic differences between individuals. Genes might seem to account for as much as 75 percent of variance across individuals in basketball performance. If someone showed that the present generation was far more skilled at basketball than the last (as indeed they are), Jensen’s math would prove that it was impossible. It would show that those aspects of environment that are not correlated with genes (which is all that environment gets credit for in kinship studies) were very feeble. So feeble that the present generation would have to be within the top one percent of the last in terms of quality of environment for basketball.

The cognitive ability differences measured by IQ tests may have the same dynamics. People whose genes send them into life with a small advantage for these abilities start with a modest performance advantage. Then genes begin to drive the powerful engine of reciprocal causation between ability and environment. You begin by being a bit better at school and are encouraged by this, while others who are a bit ‘slow’ get discouraged. You study more, which upgrades your cognitive performance, earn praise for your grades, start haunting the library, get into a top stream. Another child finds that sport is his or her strong suit, does the minimum, does not read for pleasure, and gets into a lower stream. Both of you may go to the same school but the environments you make for yourselves within that school will be radically different. The modest initial cognitive advantage conferred by genes becomes enormously multiplied.

Once again, just as different genes are matched with very different environments, so identical genes will be matched with very similar environments. You and your separated identical twin will get very similar scores on IQ tests at adulthood. Using Jensen’s model, genes will get credit for all of the potent environmental influences you both share. And environment will appear so feeble that it could not possibly account for the huge IQ advantage your children enjoy over yourself. Our model shows why this is a mistake. It shows that kinship studies hide or ‘mask’ the potency of environmental influences on IQ. Therefore, they do not really demonstrate the impossibility of an environmental explanation of massive gains over time.

The model’s next task it to suggest just how environment performs its demanding role. Social forces affecting the whole of society can provide something that an individual’s life experiences normally do not. They provide environmental influences that are just as persistent over time as the individual’s genetic endowment, and that are not at the mercy of genes. After all, the present generation has no advantage in genetic quality over the last, indeed, it is often argued that the reverse is true due to the lower fertility of the more highly educated. So between generations, the mask slips and environmental forces stand out in bold relief. Relatively small environmental differences between generations gain enormous potency just as small genetic differences between individuals did: They seize control of the powerful reciprocal causation that exists between cognitive ability and environment.

No one knows for certain what environmental trends caused massive IQ gains but we can suggest a scenario consistent with their history. There is indirect evidence that massive gains in the cognitive abilities IQ tests measure began in Britain as far back as those born in 1872. They probably began with the industrial revolution and were there waiting for IQ tests to be invented to measure them. The industrial revolution upgraded years and quality of schooling, nutrition, disease control, all things that could have had a profound influence in raising IQ, at least up to about 1950.

After 1950, in nations like the US and Britain, IQ gains show a new and peculiar pattern. The are missing or small on the kind of IQ tests closest to school-taught material like reading and arithmetic. They are huge on tests that emphasize on-the-spot problem-solving, like seeing what verbal abstractions have in common, or finding the missing piece of a Matrices pattern, or making a pattern out of blocks, or arranging pictures to tell a story.

Perhaps the industrial revolution stopped demanding progress in the basics and started demanding that people take abstract problem-solving more seriously. Post-World War Two affluence may be the key. It brought a dilution of the pragmatic depression psychology, smaller families in which children’s whys were taken more seriously, work roles in which people were expected to take more initiative, more energy for making leisure more cognitively demanding, whether devoted to chess or bridge or video games or simply to conversation in which people were expected to take ideas and logic seriously.

We call these products of the industrial revolution that may have set massive IQ gains rolling ‘triggers’. The model itself does not specify ultimate causes and we suggest those listed very tentatively. What the model does do is demonstrate the potency triggers would gain from seizing control of reciprocal causation between cognitive ability and environment. The most dramatic tool at their disposal is the ‘social multiplier’. This posits that when something raises the average performance of society, that rise becomes a powerful cause in its own right, and raises the average performance further, and raises it further, until the original rise is greatly multiplied.

The most potent facet of our environment is other people. When something, perhaps the popularity basketball got from television, triggered greater participation in basketball, the average performance rose as individuals played more and got better. Initially, a few people learn to shoot with either hand, then others imitate them. The rise in average performance feeds back into a new challenge for each individual. Those who want to excel have to learn to pass with either hand and this spreads and raises the average performance once again. In other words, every rise in individual performance raises the group average, which forces everyone to raise their individual performance a notch higher, which raises the group average a notch higher, and so on. Even a modest environmental trigger of enhanced performance can become potent by seizing control of the social multiplier—and cause huge performance gains in a relatively short time.

The same kind of reciprocal causation explains IQ gains. Environmental triggers raise the cognitive demands of work, family interaction, leisure, and everyday conversation. Those who respond by upgrading their cognitive performance raise the average cognitive performance. Then the rising average affects your employer, family, and friends and they demand or expect more, and you (and many others) rise to meet their expectations, so the average cognitive performance jumps once again, and so on, and so on. The model quantifies this process and shows that quite plausible initial environmental changes would be enough to explain huge IQ gains—gains of 20 points over a single generation.

The model has a third task. It offers an explanation for a whole range of other phenomena that have proved baffling. Why people’s genes seem to count more for IQ as they age. Why enrichment programs boost IQ a lot at the start, then little more, and then see their effects fade away after children leave the program. Why cross-racial adoptions do not raise the IQs of black adoptees to the white average. Why certain methodologies produce nonsense results, such as showing that group IQ differences known to be environmental in origin have a genetic component. And to return to the race and IQ debate, it shows that environment could explain racial IQ differences just as it explains IQ differences between generations.

Finally, the model has an overriding purpose. In principle, it applies to the dynamics of any human ability where there is positive feedback between that ability and environment. We hope it will reconcile social scientists who have divided themselves, sometimes with bitterness, between hereditarians who think genes dominant and environmentalists who think culture dominant. They are both right: It all depends on whether genetic differences or environmental factors seize control of potent processes like the social multiplier. We hope that our model will allow them all, from the psychologists inspired by Sir Cyril Burt to the anthropologists inspired by Franz Boas, to find common ground, and work together to advance our understanding of human intelligence and other important traits.