Scientists in the field of intelligence theory have argued for decades over whether people鈥檚 intelligence is determined mostly by their genes or their upbringing. Over the past 15 years, however, the puzzle has taken on a new wrinkle: If people鈥檚 intelligence is due mostly to heredity, as many experts believe, why is it that IQ scores have been rising?
Researchers who have looked into the increases are not talking merely about a point here and there. Since the early 1900s, the gains have ranged from 9 to 20 points a generation in the United States, Britain, the Netherlands, and 16 other industrialized nations. (Interestingly, no comparable rise in scores on school-related tests has occurred in many of those same countries.) That amounts to a significant increase, considering that the average IQ as determined by a standard test such as the Stanford-Binet is only about 100. The improvements are much too steep, experts say, to be due to either nature or nurture.
The trouble is that scientists are not quite sure exactly how to explain the gains, which have occurred on a wide variety of intelligence tests. Better nutrition, the increasing complexity of daily life, 鈥渟marter鈥欌�� genes, video games and computers鈥攅ven, goes one theory, the prevalence of mazes and word games on fast-food placemats and cereal boxes鈥攁ll may have played a role in boosting IQ power across time.
Now, though, a pair of researchers think they have the answer. William T. Dickens, an economist at the Washington-based Brookings Institution, and James R. Flynn, the New Zealand researcher who first documented the worldwide rise in IQ scores, say the gains can be explained by both genes and environment interacting.
Their theory is consistent with that of others in the field that nature acts on nurture, and vice versa. The difference is that this duo has developed a mathematical model that describes exactly how it all comes together.
鈥淚t鈥檚 as good an explanation as any,鈥� said Robert J. Sternberg, a Yale University psychologist who studies intelligence. 鈥淎lthough at this point,鈥� he cautioned, 鈥渢here are a plethora of explanations, with not such a good basis for choosing among them.鈥�
Multiplying Skills
To understand the paradox posed by the sharp increases in IQ test scores across generations, some background is in order. The dominant view in the field for years has been that the variance in people鈥檚 IQ scores owes mostly to heredity. In 1996, in fact, a committee of prominent psychologists concluded that genes account for three-quarters of the IQ differences between people by the time they reach their late teens.
Yet Mr. Flynn鈥檚 1987 discovery of the test-score rise was not the only study to question the idea that so much of people鈥檚 brainpower is inherited. There were doubts from other studies, too. For example, if children鈥檚 IQs are mostly fixed at birth, how could some preschool programs, such as Head Start, manage to raise the IQ scores of the children who participated in them鈥攁t least for a time?
Writing last April in the journal Psychological Review, Mr. Dickens and Mr. Flynn argued that the explanation is that people tend to match their biological gifts to their environments. Since genetic differences are persistent, that tendency creates a kind of 鈥渕ultiplier鈥� effect that makes genes seem more important than they really are in determining intelligence.
鈥淭ake those born with genes that make them a bit taller and quicker than average,鈥� write the researchers, drawing an analogy with basketball skills, which also have improved since 1945 in this country. When those taller, quicker children start school, they are likely to be a bit better at basketball, the researchers note.
鈥淏ecause you are better at basketball, you are more likely to enjoy it more and play it more than someone who is a bit slow or short or overweight. That makes you better still,鈥� they write. 鈥淵our genetic advantage is upgrading your environment, the amount of time you play and practice, and your enhanced environment in turn upgrades your skill.鈥�
A similar phenomenon can occur on a societal scale, the authors say. When basketball becomes more popular, more people begin playing it.
鈥淚nitially, a few people learn to shoot with either hand, then others imitate them,鈥� the authors write. 鈥淭hose who want to excel have to learn to pass with either hand and this spreads and raises the average performance once again.鈥� Since people鈥檚 鈥渂asketball鈥� genes don鈥檛 improve greatly from one generation to the next, that means that even a modest environmental 鈥渢rigger,鈥� such as the sport鈥檚 increased popularity, can yield a big difference in performance.
In the case of IQ, the two experts theorize, the environmental trigger could simply be that a complex, technology-driven, urbanized society demands more of the skills that intelligence tests are good at measuring: spatial organization, problem-solving, lateral thinking.
鈥淭here has been a huge cognitive upgrading of what the workforce does,鈥� Mr. Dickens said.
If the theory proves true, it pokes a hole in the controversial idea, advanced by some theorists, that genes are the biggest cause of the IQ test-score gap that separates blacks and whites.
鈥淭his says just because a particular group is showing inferior performance at one time doesn鈥檛 mean they can鈥檛 improve,鈥� said Mr. Flynn, an expatriate American who now teaches political and moral philosophy at the University of Otago in New Zealand. A one-time organizer for the Kentucky chapter of the Congress on Racial Equity, Mr. Flynn began dabbling in the field to try to refute studies in the 1970s suggesting that whites, on average, had a genetic edge on IQ tests.
A Gradual Fade
The researchers say their theory also explains why the IQ gains that poor children make in Head Start fade in elementary school.
鈥淚sn鈥檛 it amazing that once they鈥檙e tossed back into the environments in their ghetto schools that those gains seem to disappear?鈥� Mr. Flynn asked with a hint of sarcasm.
The implications for education are huge, the researchers say. The theory suggests, for example, that the brain is like a muscle that requires constant exercise to reach and maintain a level of peak performance. And the kinds of experiences that enhance IQs apparently have the same effect on people of average intelligence as they do on the future Albert Einsteins of the world.That similar gains have not occurred on tests that measure learning, the researchers say, does not necessarily mean that schools have no effect in raising IQ skills. As Mr. Flynn puts it, schools provide an environment in which children can find the teachers who become their role models and the peers who will play chess and computer games with them.
Although the theory is still young, it has drawn the attention of some friendly critics. One is John C. Loehlin, a professor emeritus of psychology the University of Texas at Austin.
In an article slated for publication later this year in Psychological Review, Mr. Loehlin takes issue with the researchers鈥� mathematical model. The equations, he says, do not adequately account for memory or specify time periods or developmental differences.
The example he gives is of a 5-year-old who learns to ride a bicycle but does not pick one up again until age 20. The exclusion of memory in Mr. Dickens and Mr. Flynn鈥檚 mathematical modelings, he writes, means that the 20-year-old would have to learn bicycle-riding skills from scratch.
鈥淭his isn鈥檛 what we expect to happen in real life,鈥� Mr. Loehlin writes.
For his part, Mr. Dickens says the model鈥檚 lack of specificity was a deliberate attempt to keep it simple. Including more details would not change the overall dynamics of the equation much, he says.
The importance that people attach to the theory, meanwhile, depends partly on whether they believe intelligence tests indeed measure intelligence. The researchers note, for example, that the gains on the tests have been more dramatic on questions measuring spatial skills than they have been on questions designed to gauge verbal intelligence. On that subscale, scores have barely budged.
Other researchers, such as Mr. Sternberg, have shown that such tests fail to measure the kinds of practical skills that predict success later in life.
Mr. Sternberg also makes another point. 鈥淭o the extent that intelligence is the use of one鈥檚 intellect for the common good,鈥� he said, 鈥渨e are seeing astonishingly little of it today鈥攆or all the rises in IQ.鈥�
