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TODAY we view TV documentaries about identical twins who, despite being
separated at birth, have had amazingly similar life experiences and grow up to
have similar IQs.

But when we think about what those twins imply, idle entertainment turns into
concern and anger. Must we believe that genes virtually determine IQ, and that
IQ differences between racial groups are caused by genetic differences? For
psychologists, there is a special cross to bear. The race and IQ debate has
created a paradox about nature versus nurture that appears insoluble.

Ever since the American military tested conscripts during the First World
War, it has been known that whites in the US outscore blacks by 15 points on IQ
tests. In 1969, Arthur Jensen, an educational psychologist at the University of
California at Berkeley, shocked liberal American public opinion by arguing that
the racial IQ gap had a strong genetic component.

In 1973, Jensen bolstered his case by publishing a mathematical model. He
plugged in two pieces of data: the 15-point black IQ deficit (one standard
deviation), and a low estimate of the influence of environment on IQ (about
0.33). These implied that for environment to explain the IQ gap between blacks
and whites, the environmental gap between the races would have to be immense
(three standard deviations).

His evidence that environment was overshadowed by genes came from a wide
variety of kinship studies. These included not only identical twins brought up
apart, but also comparisons of identical and fraternal twins each brought up by
their own parents, comparisons of adopted children with “natural” children and
so forth. Most psychologists agreed (and would still agree) with him about the
message of the kinship studies.

The mathematics of Jensen’s model are impeccable. If environment is that
weak, it will take a lot of it to cover a large IQ gap. The implication for
blacks: their average environment would have to be worse—in its effect on
IQ—than the environments of 99 per cent of whites. No social statistic,
from income to occupation to schooling, puts the average black person in such
an impoverished environment.

Enter the paradox.

Jensen’s model did more than imply that environment could not explain the IQ
gap between the races. It ruled out an environmental explanation of a large IQ
gap between any two groups. However, in 1987, one of us, James Flynn, did a
worldwide survey of IQ trends over time and found that the current generation
outscores the previous generation by between 9 and 20 IQ points. The size and
the speed of these IQ gains virtually dictate an environmental explanation
because genes just don’t change that fast. So somehow, Jensen’s model has led us
astray.

To state the paradox starkly: how could solid evidence show both that
environment was so feeble (kinship studies) and yet so potent (IQ gains over
time)?

Two years ago, William Dickens of the Brookings Institution in Washington DC
decided to do some modelling of his own. Building on the work of scholars such
as Sandra Scarr, Uri Bronfenbrenner, Steve Ceci, Carmi Schooler, Judith Rich
Harris and Eric Turkheimer, Dickens has constructed a model that we believe
provides the solution.

This model retains all of the kinship data but reassesses it in the light of
two assumptions. First, that those who have an advantage for a particular trait
will become matched with superior environments for that trait. Secondly, that
genes derive a great advantage from this because genetic differences are
persistent. If your genes make you tall, you retain that advantage throughout
your life. Environment is important, but, as we shall see, the environmental
advantages that persist tend to be selected by our genes.

Take those born to be a bit taller and a bit quicker than average. When they
start school, they are likely to be a bit better at, say, 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 a bit slow or
short or overweight. Your genetic advantage is upgrading your environment, the
amount of time you spend playing and practising. In turn, your enhanced
environment upgrades your skill, so you are much more likely to be picked for
your school team. There you get professional coaching, which makes you even more
proficient.

So thanks to genes capitalising on the powerful multiplying effects of the
feedback between talent and environment, a modest genetic advantage has turned
into a huge performance advantage. And just as small genetic differences match
people with very different environments, so identical genes tend to produce very
similar environments.

Assume that everybody plays basketball and that your skill has as much
influence on your fate as your success at school. In that case, identical twins,
with the same height, weight, natural spring and quickness, would tend to
“create” very similar basketball environments even when they grew up apart.
Their environments would not, of course, be as similar as when they grew up
together. Even so, their basketball histories would tend to be far more similar
than those of randomly selected individuals who grew up apart, one of whom might
be 15 centimetres taller than the other.

Kinship studies of basketball, whether they involved people with identical
genes or different genes, would underestimate the potency of environmental
factors. Playing, practising, being on a team, coaching, would all be attributed
to genes simply because similarities and differences in them tend to accompany
genetic similarities and differences between individuals.

This generation is far more skilled at basketball than the previous one,
undoubtedly due to changing environment rather than genes. What would Jensen’s
model show? If he used kinship data, they would imply that those aspects of
environment not matched with genes (which is all that environment gets credit
for in kinship studies) are feeble—far too feeble to cover a large
basketball performance gap. And his mathematics would demonstrate that the
present generation would have to have an average environment for basketball
better than that of 99 per cent of the previous generation. This, of course,
would seem quite implausible, but now we know where the implausibility lies:
with the conclusions generated by Jensen’s model.

Powerful engine

The differences in cognitive ability that are measured by IQ tests may have
the same dynamics. People whose genes send them into life with a small advantage
for developing 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, and so on. 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 there 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.
Remember those TV documentaries. They show identical twins who, despite being
separated at birth, joined the library club at the same age, won a school prize
at the same age, married spouses with surprisingly similar backgrounds,
practised the same professions. The sensational frequency of the “coincidences”
obscures what is being revealed: the remarkable tendency of similar genes to get
matched to similar environments.

The TV shows, the kinship studies, and Jensen’s model have one thing in
common: when genes get matched with potent environmental influences, genes get
all the credit. The identical genes of identical twins get credit for all of the
potent environmental influences they share. Our model shows why this is wrong.
It hides or masks the true potency of environmental influences on IQ. It makes
environment look too feeble to cause massive IQ gains over time.

The model’s next task is to suggest just how environment performs its
demanding role. Forces affecting the whole of society can provide something that
an individual’s life experiences usually cannot. They provide environmental
trends that are just as persistent over time as the individual’s genetic
endowment, and environmental advantages that operate quite independently of
genetic advantages. After all, the present generation has no better genes for IQ
than the one before. Indeed, the reverse may be true because of lower birth
rates among the more highly educated. Between generations, relatively small
environmental differences 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. Massive gains in the cognitive abilities that IQ
tests measure may have begun with the Industrial Revolution. Test results from
Britain show gains beginning as far back as the generation born in 1872. The
Industrial Revolution has steadily upgraded the quality of schooling, nutrition,
disease control—trends that could have had a profound influence in raising
IQ, at least up to about 1950.

After 1950, some nations such as the US and Britain have provided test
information that allows us to break down global IQ gains into their components.
The gains begin to show a new and peculiar pattern. They are missing or small on
the kind of IQ tests closest to school-taught material (vocabulary, arithmetic
and so forth), but they are huge on tests that emphasise on-the-spot problem
solving (working out what verbal abstractions have in common, finding the
missing piece of a matrices pattern, 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.
Children brought up in the affluence of the 1950s did not develop the
Depression-era psychology of their parents, with its emphasis on the practical
over the “frivolous”. Affluence brought smaller families in which children’s
“whys” were taken more seriously, more professional and semi-professional jobs
in which people were expected to take more initiative. It gave people the time
and energy to make leisure more cognitively demanding, from chess, bridge or
video games to conversation in which people were expected to take ideas and
logic seriously.

We call these products of the Industrial Revolution “triggers”. Models do not
themselves specify ultimate causes and we suggest the above very tentatively.
What the model does do is demonstrate the potency that triggers, whatever they
may be, could gain from seizing control of reciprocal causation between
cognitive ability and environment.

The most dramatic tool at the disposal of these triggers 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 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 being televised, triggered greater
participation in the sport, the average performance rose as one individual after
another played more and got better. Initially, a few people learned to shoot
with either hand, then others imitated 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 raises the average performance
once again. Every rise in individual performance raises the group average, which
forces everyone to raise their individual performance a notch, which raises the
group average a notch higher, and so on.

That’s how 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 may explain 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. You
(and many others) rise to meet their expectations, and the average cognitive
performance jumps again.

Our model incorporates the social multiplier, which means it can show how
quite modest initial environmental changes would be enough to explain huge IQ
gains of 20 points over a single generation. Gains need not go on forever: the
model builds in limits—and it is easy to see how brain capacity or an
affluence that bred decadence could halt or even reverse IQ gains.

The model has a third task to perform. It might well help us make sense of a
whole range of questions posed by IQ data. Why, for example, does the match
between a genetic advantage for IQ and environmental advantages for IQ tend to
become greater with age? Why do children make big IQ gains at the start of an
enrichment programme, make little or no gain during the rest of the programme,
and gradually lose those gains after the programme ends? Why do the IQs of
adopted children, at adulthood, correlate better with the IQs of their natural
parents than with those of their adoptive parents?

And to return to the race and IQ debate, the model suggests how environment
might explain racial IQ differences just as it explains IQ differences between
generations.

Finally, the model has acquired an overriding purpose. Since it
applies—in principle—to the dynamics of any human ability where
there is positive feedback between that ability and environment, we hope it will
bridge the (sometimes bitter) division between hereditarians who think genes are
dominant and environmentalists who think culture is dominant. They are both
right: it all depends on whether genetic differences or environmental factors
seize control of potent processes like the social multiplier.

From psychologists inspired by Hans Eysenck and Robert Plomin to
anthropologists inspired by Franz Boas and Ruth Benedict, our model offers them
all common ground. Perhaps we can all work together to advance our understanding
of human intelligence and other important traits.