…75 percent of the variance [in IQ] can be said to be due to genetic variables…and 25 percent to environmental variation.
—Arthur Jensen (1969)
Being raised in one family, rather than another…makes few differences in children’s personality and intellectual development.
—Sandra Scarr (1992)
NOT SO VERY LONG AGO, scientists who study IQ more or less agreed that intelligence is mostly heritable. Some still consider it to be about 75 to 85 percent heritable, at least for adults. The effects of the environment that children share by virtue of being raised in the same family have often been presumed to be slight, sometimes literally zero by the time the children are adults. Scientists frequently believed, or at least wrote as if they did, that the overwhelming importance of heritability meant that the environment could do little and that social programs intended to improve intelligence were doomed to failure.
But many scientists today consider the heritability of IQ to be much lower than 75 to 85 percent. This environmentalist camp estimates heritability to be .50 or less. (Though, as you will see later, heritability actually differs quite a bit from one population group to another.) And I agree with these scientists—in fact I suspect heritability may be even lower than .50.
In the first section of this chapter, I show why earlier estimates of heritability were so high. Even more important, I review the results of adoption studies showing that raising someone in an upper-middle-class environment versus a lower-class environment is worth 12 to 18 points of IQ—a truly massive effect. This fact places a very high upper bound on the degree to which the environment can influence intelligence. Finally, I emphasize that the heritability of a characteristic places no theoretical limits on the degree to which it can be affected by the environment. The upshot is that the environment counts for a lot in determining IQ and could conceivably account for more if we could think of the right ways to change it.
Some of the notes for this chapter are long. This is because I want to answer the concerns of specialists in the heritability of IQ without derailing the attention and understanding of the general reader. Even without the notes, this chapter is by far the most technical in the book. Please don’t get bogged down in it. Rather, take on faith for the time being my assertions that genes are far from being completely determinative of intelligence and that the environment can made a huge difference to intelligence.
Heritability, Environment and IQ
Laypeople sometimes think of the heritability of a characteristic as the degree to which it is inherited from parents. This encourages the assumption that a heritability estimate for IQ of .80 means that 80 percent of a person’s IQ comes from genes. This is quite wrong. Heritability does not refer to the individual at all, but to populations. The heritability of a characteristic refers to the percentage of variation in that characteristic in a particular population that is due to genetic factors. This contrasts with the percentage of variation in the characteristic due to all other factors. For intelligence, these other factors include prenatal and perinatal biological factors, environmental factors of a biological nature such as nutrition, and social factors such as education and experience. This chapter concerns the most interesting sources of variation—that due to genetics and that due to the environment which is shared among children in a given family but differs between families.
The between-family environmental effect refers to how much difference it makes that a person was raised in one family versus another (with all the various factors that go with membership in different families, such as social class, rearing styles, and religious orientation). The between-family environmental source of variation does not include environmental variation within a family, such as that associated with birth order. Only one child in a family can be the first born, only one can be the second born, and so on. And we know that birth order can be an important factor associated with some characteristics. Other factors such as peer influences and schools attended also differ among children within a given family.
Of course, between-family environmental differences do not include the genetic contribution of the parents. Everyone agrees that there are big average differences in IQ between two randomly selected families, and a substantial portion of those differences is due to genes.
The researchers I call the strong hereditarians hold that IQ is between 75 and 85 percent heritable in the population of developed countries, and that the environmental contribution from all sources is between 15 and 25 percent. Most of the strong hereditarians believe that the between-family environmental contribution (growing up with the Smiths rather than the Joneses) is slight to nil, at least after childhood. They also agree that such environmental contribution as exists is mostly due to variations that occur within the family—for example, the children having gone to different schools, having been treated differently by their parents, and having experienced a different uterine environment.
How do the strong hereditarians reach these conclusions? Have a look at Table 2.1, which summarizes the results of a large number of studies of the correlations between individuals with a given degree of relatedness who were raised either together or apart. A direct estimate of heritability is provided by the figure for identical twins—who have the same genetic makeup—who are reared apart. This figure is .74, and is essentially the one that Arthur Jensen drew upon to derive his estimate of the contribution of genes to IQ. Since the environments of the twins are different, so the logic goes, the similarity between the twins can only be due to genetics (as well as to prenatal and perinatal effects that might have been consequential before the twins were separated and which are generally assumed by the hereditarians to be minimal).
TABLE 2.1 Correlations for individuals with varying degrees of relationship, raised either together or apart
|
Relationship |
Raised |
Correlation |
|
Identical twins |
Together |
.85 |
|
Identical twins |
Apart |
.74 |
|
Fraternal twins |
Together |
.59 |
|
Siblings |
Together |
.46 |
|
Siblings |
Apart |
.24 |
|
Midparent/child |
Together |
.50 |
|
Single parent/child |
Together |
.41 |
|
Single parent/child |
Apart |
.24 |
|
Adopting parent/child |
Together |
.20 |
|
Adopted children |
Together |
.26 |
|
Midparent = average of mother’s and father’s IQ. |
Correlations are based on a summary of 212 different studies and are weighted by the size of the sample. From Devlin, Daniels, and Roeder (1997), except for the correlation for adopted children raised together, which is from Bouchard and McGue (2003).
How do scientists typically arrive at a direct estimate of the role of between-family environmental differences? They examine the correlation between the IQs of unrelated individuals living together. A direct way to estimate this is by determining the correlation between the IQs of adopted children and those of their adoptive parents. Since these people do not share genes, the only way that the IQs of the adopted children can resemble those of their adoptive parents is by virtue of sharing the same environment. The next-to-last line of Table 2.1 shows that this correlation is .20. Some scientists take this as a good estimate of the contribution of the environment to the variation in IQ from one family to another. A different way of arriving at this conclusion is to compare the IQs of unrelated children living in the same family (the last line in Table 2.1). Again, because the children do not share genes, the only way they can resemble one another is through sharing the same environment. This correlation is .26, a slightly higher estimate of the contribution of between-family differences to IQ.
Jensen and other strong hereditarians, however, would not accept a figure for between-family environmental effects that is as high as .20 to .26. This is because when people older than those in the studies summarized in Table 2.1, who are mostly children, are examined, the correlations drop dramatically—sometimes to as low as zero. This is true, for example, for unrelated children brought up in the same household. When they are adults, the correlations run in the vicinity of .05 or less. The usual explanation given for this weak effect on adults is that as people grow older, they select their own environments, and their preference for one environment versus another is largely influenced by genetics. The importance of the early environment, never all that great to begin with, drops way off. This means that the strong hereditarians assign most of the environmental contribution to IQ to factors that differ among members of the same family, such as birth order, rather than to factors that are common to the members of the same family and that differ between families.
To summarize, the strong-hereditarian position is as follows: three-quarters or more of the variation in IQ is genetic; some of the variation in IQ is due to nonshared, within-family environmental factors that a parent cannot do much about; and by adulthood almost nothing of the variation in IQ is due to between-family environmental differences—the difference between randomly chosen family A and randomly chosen family B. So the characteristics of your family, in comparison to the characteristics of the randomly selected Joneses—who might make less money, not read as much to their kids, send them to poorer schools, live in a sketchier neighborhood, and have a different religion—make almost no difference.
By now, if you have children, you could be wondering why you spent good money to move to a more expensive neighborhood with better schools, or for that matter why you squander money on books and orthodontia, waste time driving them to violin lessons and museums, and drain off emotional energy holding your temper so as to set a good example. But you don’t have to accept those high estimates of heritability and low estimates of between-family environmental effects.
The direct estimate of heritability based on the correlation between the IQs of identical twins reared apart makes a tacit assumption that is surely false—namely, that the twins were placed into environments at random. For that to be true, the twins would have to be in environments that differed as much as any two people selected at random from the Google-based U.S. telephone book. But that is not the case. Billy is likely to be raised by people—frequently relatives in fact—not all that different from those raising Bobby. And just how similar the environments are makes a good deal of difference for the correlations in IQ between identical twins reared apart. Developmental psychologist Urie Bronfenbrenner showed that when twins reared apart are brought up in highly similar environments, the correlations between their IQ scores range from .83 to .91. But the correlation now reflects not just the fact that their genes are identical but also the fact that their environments are highly similar. Such a correlation therefore gives an inflated estimate of heritability. When environments are dissimilar to one degree or another, the correlations range from .26 to .67. Since we don’t know just how dissimilar the environments are in most studies of twins reared apart, we can’t know exactly what heritability to estimate from the correlation between them.
Regardless of the degree of similarity in environments, the correlation between identical twins overestimates heritability as compared to other ways of estimating heritability based on the correlations for other kinds of relatives. This could be because the environmental experiences of identical twins who are reared separately in quite different environments are highly similar since they look so much alike or have other characteristics in common that tend to elicit the same sorts of behavior from other people. Or there may be gene interactions that specifically make identical twins more similar but that don’t contribute much to the degree of resemblance of other relatives.
A third source of error in the .75 to .85 estimates is that twins share the same uterine environment. Devlin and his colleagues maintain that this shared-environment factor means that as much as 20 percent should be subtracted from the heritability estimates.
I will present a fourth source of error later, when I discuss the fact that heritability differs greatly from one social class to another—and that twin studies are biased toward including disproportionately large numbers of people from social classes for which heritability is high.
Once corrections are made for all these facts, the estimate based on identical twin correlations is likely to be substantially lower than the .75 to .85 assumed by Jensen and the other strong hereditarians.
Genes as Triggers of Environmental Influences
Developmental psychologists Sandra Scarr and Kathleen McCartney, as well as economist William Dickens and philosopher and IQ scientist James Flynn, propose a further reason why the role of genes is overestimated. Slight genetic advantages can be parlayed into very great IQ advantages because of the way they influence the kinds of experiences an individual has. Consider a basketball analogy. The child who is somewhat taller than average is more likely to play basketball, more likely to enjoy the game, more likely to play a lot, more likely to get noticed by coaches and encouraged to play for a team, and so on. That height advantage is totally dependent on such environmental events for its expression. And identical twins reared apart are likely to have very similar basketball experiences, because they are of similar height, so they are likely to end up with similar skills in basketball. But their similarity in basketball skills is not due to their possessing identical “basketball-playing genes.” Instead, it is due to genetic identity in a narrower attribute that causes them to have highly similar basketball-related experiences.
A similar point can be made about intelligence. A child with a relatively small genetic advantage in, say, curiosity is more likely to be encouraged by parents and teachers to pursue intellectual goals, more likely to find intellectual activity rewarding, and more likely to study and engage in other mental exercises. This will make the child smarter than a child with less of a genetic advantage—but the genetic advantage can be very slight and can produce its effects by virtue of triggering environmental “multipliers,” which are crucial for realizing that advantage. All of this gene-environment interaction (or as geneticists would prefer to say, gene-environment correlation), however, gets credited to heritability, given the way heritability is calculated. This is not wrong, but it leads to an underestimation of the role of the environment.
To make it even clearer why heritability estimates slight the role of the environment, let’s return to the basketball analogy. Suppose a child of average height is encouraged to play basketball, perhaps because her older siblings are players and keep a well-used hoop on the driveway. And suppose another child, of above-average height, has little access to basketball experiences, perhaps because she lives in a rural area and there are no neighborhood kids nearby. The taller child from the rural area is not likely to become much of a basketball player, whereas the average-height child has a reasonable chance of becoming good. Now we have a child with a genetic advantage who is not very good at basketball, and a child with a genetic disadvantage who nevertheless is pretty good at it. Genes count, and given a constant environment they may have a big influence in determining talent. But environmental interventions can greatly influence—even largely override—the effects of genes. This is particularly important for estimates of the effects of the environment on IQ. It is easy to imagine any number of ways in which intellectual pursuits can be made more or less available and attractive.
Tolstoy and Adoption
Now let’s consider how the effects of between-family environmental differences are measured. These differences are estimated by calculating the correlation between the IQs of adopted children and those of their adoptive parents, and the correlation between children in the same family who are not related to each other (usually adopted children). As we have seen, the correlations turn out to be low on average—around .20 to .25. But these numbers only make sense if we assume that the variation in the environments created by adoptive parents is about the same as the variation in the population as a whole. It turns out, though, that adoptive families, like Tolstoy’s happy families, are all alike.
Psychologist Mike Stoolmiller showed that the variation in factors that predict IQ is, for adoptive families, a fraction of what it is for families in general. We know this for two reasons. First, the socioeconomic status (SES) of adoptive families is higher than that of nonadoptive families; the bottom rung of the SES ladder is scarcely represented among adoptive families. Second, there is much less variation in scores derived from a method of assessing home environments called the HOME (Home Observation for Measurement of the Environment). HOME researchers assess family environments for the amount of intellectual stimulation present, as indicated by how much the parent talks to the child, how much access there is to books and computers, degree of warmth versus punitiveness of parents’ behavior, and so on. HOME assessments show that adoptive families rate far above the general run of families in these respects. In fact, adoptive families score at the 70th percentile on average. Just as important, the range on these variables is very restricted compared to the population at large. HOME measures for disadvantaged families are five times as variable as those for adoptive families; that is to say, disadvantaged families differ far more from one another on average than do adoptive families.
Why does restricted range of environmental variation result in correlation estimates that are too low? Because if there is very little variation in one variable being correlated with another, the correlation cannot be very high. Consider the extreme case of variable A, which has no variation: the correlation with variable B is zero. High and low scores on B would alike be associated with the same score on variable A, so the correlation between scores on variables A and B can only be zero. Therefore, if the variation in environments between adoptive families is mistakenly estimated as being higher than it really is, the impact of the environment on IQ will be underestimated.
Because the environmental variation of adoptive families has mistakenly been assumed to be as great as the environmental variation in the population as a whole, the estimates of between-family environment effects are way off. Stoolmiller calculated that if you correct for this restriction of environmental range, as much as 50 percent of the variation in intelligence could be due to differences between family environments. Since we know that within-family variation also makes an important contribution to IQ, this would mean that most of the variation in IQ is due to the environment. (These numbers would hold, though, only for children. We know that heritability goes up with age to some degree, so Stoolmiller’s estimate for the contribution of between-family differences has to be lowered by some unknown amount.)
There Is No Such Thing as the Heritability of IQ (or Anything Else, for That Matter)
So what value should be assigned to heredity’s contribution to IQ? Actually, geneticists say that there is no such thing as a single point estimate for heritability. Heritability is dependent on the particular population and the particular circumstances in which it is examined. For IQ in particular, the nature of the population turns out to be crucial. Psychologist Eric Turkheimer and his colleagues recently showed that heritability is radically dependent on social class. They found that the heritability of IQ was about .70 for children whose parents were upper-middle class but was about .10 for children whose parents were of lower social class. A plausible explanation for this is that higher-SES families are providing excellent conditions for the development of intelligence, and they may not differ much among themslves in these respects. Under these circumstances, the contribution of heredity can be very great. At the extreme, if the environment is completely constant across families, the only possible source of variation is genetic.
Why should the heritability of IQ be so low for lower-SES people? We know from the work by Stoolmiller that the range of environments with respect to the variables that influence IQ is far greater for lower-class families than for middle-and upper-middle-class families. The environment for lower-SES people probably varies from being equal to the most supportive upper-middle-class setting to being pathological in every respect. This means that the environment for this group of people is going to make a great deal of difference to IQ. And in fact the environment almost completely swamps heredity.
So you haven’t wasted your time, money, and patience on your children after all. If you were to average the contribution of genetics to IQ over different social classes, you would probably find 50 percent to be the maximum contribution of genetics. Most of the remainder of variation in IQ is due to environmental factors—those shared within families and differing between families, plus those not shared within families. (The rest—a small amount—is due to measurement error.)
Note also that the Turkheimer findings provide yet another, and crucially important, indication that the very high estimates of heritability are overestimates. This is because they are based largely on twin studies, and there is a substantial bias in twin studies toward middle-class participants since middle-class and upper-middle-class people are easy to contact and to persuade to participate in research projects. Therefore, estimates of heritability for adults are biased upward, and estimates of between-family environmental effects correspondingly biased downward.
Stoolmiller’s claims about the superiority of adoptive parents from the standpoint of encouraging the development of intelligence raise the question of just how big the effects of adoption are. Addressing this issue would provide another way of drawing a bead on the contribution of family environment to IQ. If adoptive families are such an exemplary bunch with respect to the variables that predict IQ, shouldn’t we find adopted children to have IQs that are higher than would be expected from their origins? We certainly would under the hypothesis that environments matter a great deal to IQ.
The Proof of the Importance of Family Environment in Determining IQ
Heritability estimates are based on correlations, and as we have just seen, inferences based on correlations can be misleading. What is needed to test the power of differences between family environments are experiments. As it turns out, there are a large number of natural experiments based on the everyday occurrence of adoption. We can ask if it makes a difference whether children are adopted into a family with highly favorable environmental conditions versus a family with less favorable conditions. Many natural experiments, with many different “designs,” point to the same conclusion: being raised under conditions highly favorable to intelligence has a huge effect on IQ.
Psychologists Christiane Capron and Michel Duyme carried out a “cross-fostering” study with French children. They tracked down children born to low-or high-SES parents who had been adopted by either low-or high-SES parents. The class differences were pronounced: they compared children of poor and working-class parents based on occupation of the father (semi-skilled or unskilled worker having nine years of education or less) with children of upper-middle-class parents (professional or upper-level manager, having an average of sixteen years of education). This design allowed an independent assessment of the contribution to IQ of genes from parents of very low versus very high SES and the contribution to IQ of being raised in a very-low-versus very-high-SES family. As it turns out, both genes and class-related environmental effects are powerful contributors to intelligence.
On average, the biological children of high-SES parents had IQs that were 12 points higher than those of low-SES parents, regardless of whether they were raised by high-SES parents or low-SES parents. (We don’t know how much of this difference was due to genes and how much was due to nongenetic prenatal, perinatal, and immediate postnatal environmental factors, though I don’t doubt that most of the difference was genetic.)
The crucial finding is that children adopted by high-SES parents had IQs that averaged 12 points higher than the IQs of those adopted by low-SES parents—and this was true whether the biological mothers of the children were of low or high SES. So the study showed that being raised in higher-social-class environments produces children with a far higher IQ than does being raised in lower-social-class environments. Equally important, the school achievement of children raised in upper-middle-class environments was much higher than that of children raised in lower-class environments.
Another French study, having a different “natural experiment design,” examined lower-SES children adopted into upper-middle-class families and compared them with their siblings who had not been adopted. The adopted children had an average IQ of 107 by one test and 111 by another, whereas their biological siblings who were not adopted had an average IQ of about 95 by both tests. We therefore get an estimate of 12 to 16 IQ points as the value of being raised in an upper-middle-class environment versus a lower-class environment. The difference in academic achievement between adopted and nonadopted siblings was huge. The school failure rate was 13 percent for children adopted by upper-middle-class parents and 56 percent for their nonadopted siblings.
In another extremely important natural experiment conducted in France, but with yet a different design, Duyme and his colleagues looked at abused, low-IQ children who were tested for IQ adopted at the age of four or five and then retested for IQ at age fourteen. They deliberately searched for children who had been adopted into families of varying social classes. When younger, the children had IQs between 61 and 85—between what IQ testers describe as decidedly retarded and dull-normal levels. The families into which they were adopted were poor (unskilled workers), lower middle and middle class (lower-or middle-level managers, tradesmen, and skilled laborers), or upper middle class (professionals and high-level managers). The effects on IQ of being adopted were very large, 14 points on average. But the social class of the family made a great difference. Children adopted into lower-SES families gained 8 points; those adopted into middle-class families, 16 points; and those adopted into upper-middle-class families, almost 20 points. This gives an estimate of 12 points for the effect of being raised by an upper-middle-class family as opposed to a lower-class family.
Conveniently, from the standpoint of being able to make a confident inference about the effects of the social class of adoptive parents, there was no selective placement of children. That is, relatively lower-IQ children were no more likely to be adopted into lower-SES families than were relatively higher-IQ children. So this study reached the same conclusions as the other two French studies: the difference between being raised in a lower-class environment versus an upper-middle-class environment is 12+ points. Note that the study showed that being raised in a relatively modest lower-middle or middle-class family can give a big boost in IQ over being raised in a lower-SES family, namely, 8 points. Note also that this study gave, if anything, an underestimate of the effects of being raised in a higher-versus a lower-SES family, because, as Stoolmiller showed, even lower-SES families who adopt have parenting practices of a kind that encourage the growth of intelligence.
A review that examined all of the well-designed adoption studies available as of 2005 found that the effect on children’s IQ of being adopted by a middle-or upper-middle-class family as opposed to being left behind in the family of origin (which was generally of lower SES) is 1.17 SD, which translates into an 18-point advantage for upper-middle-versus lower-class upbringing. The review also gave an estimate for the contribution of biological factors—genetic plus prenatal, perinatal, and immediate postnatal factors. This estimate was derived by comparing the biological children of middle-and upper-middle-class families with their adopted siblings. As it happens, the difference was 12 points, the same value Capron and Duyme found.
The crucial implication of these findings is that the low IQs expected for children born to lower-class parents can be greatly increased if their environment is sufficiently rich cognitively.
The review of adoption studies tells a somewhat less optimistic story for school achievement. The adopted children performed only .55 SD better in academic achievement measures than their siblings who were not adopted. On the other hand, they scored only about .25 SD below the general population and even less behind when compared to their classmates.
Before leaving the topic of adoption, I would like to point out that it is customary for strong hereditarians to maintain that the primacy of genes and the low relevance of the environment are established by the fact that there is typically a much higher correlation between biological parents’ IQ and their offsprings’ IQ than there is between adoptive parents’ IQ and their adopted children’s IQ. The hereditarians believe that the environment of the adopted child does little for the child’s intelligence, since differences in adoptive environments do not make for differences in IQ. We can see now how mistaken this conclusion is. The environments of adoptive families are highly similar for the most part, chiefly consisting of stable middle-and upper-middle-class families. Even adoptive families who are of lower SES are high on the parenting practice factors that predict a high IQ. Since variation in adoptive families is relatively slight, very high correlations would not be expected between the IQs of adoptive parents and those of their children. There just isn’t that much difference between the environments of adoptive parents on the dimensions that matter for determining IQ—and if the differences are small, the correlation cannot be big. But there is a huge difference between the adoptive environments on the one hand and lower-SES environments in general on the other, and this results in big IQ differences. So the relatively low correlation between the IQs of adoptive parents and those of their adopted children is nothing but a red herring—it does not contradict in the least the fact that adoptive families are having a huge effect on their adopted children’s IQs.
Finally, since Herrnstein and Murray assert in The Bell Curve that the “consensus” about the average effect of adoption on IQ is 6 points, it is necessary to point out that their evidence was a review by Charles Locurto. The actual value that Locurto gave as an average effect of adoption was 12 points.
The belief that differences between family environments have little effect on IQ has to be one of the most unusual notions ever accepted by highly intelligent people. Judith Rich Harris, the author of the very interesting and best-selling book The Nurture Assumption, premised her work on the assumption that the contribution made by differences between families is virtually nil. In his brilliant book The Blank Slate, Steven Pinker insisted on the same principle. In the best-selling Freakonomics, Steven Levitt and Stephen Dubner were explicit that adoption has little effect on intelligence: “Studies have shown that a child’s academic abilities are far more influenced by the IQs of his biological parents than the IQs of his adoptive parents.” (I wish I could exempt myself from the company of strange believers, but unfortunately for many years I bought—but was deeply puzzled by—the claims of the hereditarians that family environments do not matter much.)
The evidence we have just been looking at concerning the effects of genes versus the environment tells us something crucially important about social class and intelligence. The experiences of the children of the professional and middle classes result in much higher IQs and much lower school-failure rates than is typical for lower-SES children. Moreover, we can place a number, or at least a range, on the degree to which environmental factors characteristic of lower-SES families reduce IQ below its potential: it is between 12 and 18 points. Whatever the estimates of heritability turn out to be, nothing is going to change this fact. So we know that, in principle, interventions have the potential to be highly effective in changing the intelligence of the poor. Interventions could also greatly affect the rate of school failure of lower-class children. The minimum estimate for this reduction is about half a standard deviation. The maximum estimate for this is much higher—one standard deviation, or about the same rate that would be found for middle-class children raised by their own parents.
Note also that it is not just the IQs of lower-SES children that can be affected. One study looked at the IQs of white children who were born to mothers with an average IQ and who were adopted by mostly middle-and upper-middle-class families. The children adopted relatively late had an average IQ in childhood of 112 and those adopted relatively early had an average IQ of 117. This study suggests that even children who would be expected to have an average IQ if raised in an average environment can have their IQ boosted very considerably if they are raised under highly propitious circumstances. Similarly, the cross-fostering study of Capron and Duyme showed that upper-middle-class children can have their IQs lowered if they are raised in poverty. The loss is about 12 points. So children born to poor families are not the only ones who can have their IQs dramatically affected by the environment. All children can.
Heritability Says Nothing about Mutability
Now I can deal a final blow to the idea that high heritability of IQ means that the environment has little effect. The degree of heritability of IQ places no constraint on the degree of modifiability that is possible. This is so important that I need to say it again, more emphatically: the degree of heritability of IQ places no constraint on the degree of modifiability that is possible. All geneticists accept this principle, but hereditarians often acknowledge the principle and then go on to write as if heritability does in fact place limits on modifiability.
To understand why heritability implies nothing about mutability, think about two facts: (a) the heritability of height is about .85 to .90 and (b) gains in average height of a standard deviation or more have appeared in a generation or less in several countries in the world. The average height of thirteen-year-old Korean boys increased by more than seven inches between 1965 and 2005, a difference of 2.40 SDs. The average boy in 1965 would have been painfully short in 2005. The forty-year time span is far too brief for genetics to have played a role in the increase. The increases in height we have seen in many places in the world in the last couple of generations are obviously due to environmental changes of some kind, probably in nutrition.
Or we can think of an even more extreme case: heritability of 1.0 yet massive environmental influence. We can randomly toss the seeds for corn plants into either rich soil or poor soil. Imagine that the heritability of the height of corn plants is 1.0 in rich soil and 1.0 in poor soil. The average height of the two groups of plants can nevertheless be greatly different and will be entirely due to environmental factors.
These examples should make it clear that the heritability of a characteristic within a given population places no theoretical constraint on the modifiability of that characteristic by environmental forces. And that’s good, because, as you will see in the next chapter, despite the moderate heritability of IQ, it is enormously influenced by environmental factors—namely, school and the social changes that have taken place over the last eighty years.