Is Raising IQ Possible?

Russell T. Warne

Jun 13, 2025

If we assume intelligence is primarily the result of innate (hereditary) factors, we will likely conclude it is fixed and unchangeable. (Zimbardo, Johnson, & McCann, 2017, p. 221)

Chapter 11 stated that heritability of intelligence for adults in positive environments in industrialized countries is about .80. A high h² value like .80 indicates that differences among adults’ IQ scores are strongly related to the differences in their genes. This fact can lead some people – like the textbook authors that I quote above – to feel fatalistic about the likelihood of raising intelligence. The logic goes something like this:

1. Genes are fixed before a person’s birth.

2. Those genes are very important in determining a person’s intelligence level.

3. Therefore, the environment is unimportant.

4. Because people cannot change their genes, intelligence cannot be changed.

This chapter is about how this logic has flaws in it. That being said, I am a realistic optimist. My description of heritability and its meaning showed that heritability does place limits on the influence of the environment. However, high heritability does not rule out the possibility of environmental changes that can increase intelligence – sometimes substantially.

Examples

High Heritability and Effective Interventions. There are two classic examples of traits in humans that have high heritability, but which also have effective environmental interventions that can improve the lives of people. The first is myopia (i.e., nearsightedness), which has very high heritability – .75 to .88 in one typical study (Dirani et al., 2006). But there are simple interventions that correct this highly heritable trait: eyeglasses and contact lenses. Thus, it is possible to change the environment to improve people’s functioning, even if a trait is highly heritable.

The second example is a disorder called phenylketonuria (PKU). People with PKU are unable to metabolize an amino acid called phenylalanine, which is found in chicken, egg whites, nuts, some seafood, potatoes, and many other common foods. PKU is caused when a person inherits two copies of a defective version of a gene on Chromosome 12; everyone who has both copies of the defective gene is born with PKU, which means h2 for the trait is 1. As children develop, untreated PKU causes intellectual disability and other neurological problems. However, by eating a special diet of foods with little or no phenylalanine, people with PKU develop completely normally. This is more proof that high heritability does not mean that people are condemned to live the life dictated by their genes.

Successful Interventions to Raise IQ. The examples of nearsightedness and PKU demonstrate that high heritability of a trait is compatible with effective treatments. That does not automatically mean, though, that the same principle applies to intelligence. Nearsightedness has a simple cause – usually an eyeball that is too long – and the treatment is so simple that eyeglasses were invented in the Middle Ages. PKU is caused by a single gene, and the biochemistry of how the metabolic system of people with PKU malfunctions was discovered in the twentieth century, which led to an effective treatment (Kevles, 1995). In contrast, intelligence seems to be far more genetically and biologically complex than either of these examples. Within the range of normal development, there are likely to be thousands of genes that could potentially lower intelligence, and how these genes act in brain development and functioning is often not clear.

Nevertheless, there has been progress with finding ways to increase intelligence in people. One of the most successful started in the 1970s when scientists noticed that children with high levels of lead had lower IQ scores (about 4–5 points) than children with low lead levels. This was true even if the children with high lead levels appeared to be in good health and showed no outward symptoms of lead poisoning. These differences could not be explained by family or parental variables, such as socioeconomic status, parental attitudes towards school, and many other potentially confounding variables (de la Burdé & Choate, 1975; Landrigan et al., 1975; Needleman et al., 1979). Because of these results and other potentially negative consequences of lead poisoning, the United States government took steps to reduce lead exposure in children and adults. Lead was banned as an ingredient in paint, dishes and cookware, toys, and products marketed towards children in 1978, and lead was banned from new plumbing systems in 1986. Leaded gasoline was phased out gradually until it was banned in 1996. Industrial sites are now required to emit less lead into the atmosphere, and contaminated sites are being cleaned up. Other countries are following this trend, which is reducing lead levels in the atmosphere worldwide.

As a result, Americans have lower concentrations of lead in their bodies; in the 1970s studies, a low level of lead in a child’s blood was 20 μg/dL, which is the standard unit for measuring lead levels in blood (Landrigan et al., 1975; Needleman et al., 1979). In 2016, only 0.50% of American children had blood lead levels of 10 μg/dL or higher, and 3.5% had blood levels of 5 μg/dL or higher (Centers for Disease Control, 2018, p. 8). These decreases in blood levels are encouraging, and during that time, intelligence test scores have risen in the United States by about 9–12 points, though this increase in IQ is probably not due solely to reductions in lead exposure. (Chapter 14 will discuss this increase in IQ scores in more detail.) It is likely that reducing lead levels in children’s bodies increases IQ. However, there is no known safe level of lead in the body, and even low concentrations of lead are associated with lower IQ (Huang et al., 2012).

Another successful intervention to raise intelligence is to cure a child’s iodine deficiency. People with low iodine suffer from thyroid and neurological problems. Giving iodine supplements to people with an iodine deficiency cures this health problem, and – in children – raises IQ by about 8 points (Protzko, 2017a). Two billion people worldwide suffer from iodine deficiency, mostly in southern Asia and Sub-Sahara Africa, and these people are at risk for lower IQ and intellectual disabilities. In fact, iodine deficiency is the most common cause of preventable intellectual disability in the world. The good news is that iodine deficiency is inexpensive to cure, costing about 2 to 5 cents per person per year, which makes treating iodine deficiency the most cost-effective way of raising intelligence. The most common method of increasing a person’s iodine intake is by adding iodine to salt to create iodized salt. Nutrient supplements to provide iodine are also available (M. B. Zimmerman, Jooste, & Pandav, 2008).

In the United States and other industrialized nations, severe iodine deficiency is very rare because of the widespread use of iodized salt. There is no evidence that providing iodine to people who already have enough of the nutrient will raise intelligence. Indeed, too much iodine in a person’s diet can cause health problems, though these are less severe than the problems arising from an iodine deficiency (M. B. Zimmerman et al., 2008).

Reconciling Heritability and Environmental Interventions

The examples above prove that high heritability does not exclude the possibility of an effective intervention that changes a trait. But, from a theoretical perspective, the paradox remains: heritability demonstrates the strength and importance of genes, but large changes in IQ are possible. How to reconcile these two facts?

The answer comes from the statistical basis for h2 and intervention changes. Heritability is based on variance, whereas the effectiveness of interventions is based on the averages. The Introduction explained that the average is a measure of the score of a typical person in a sample, whereas the variance is a measure of how much people’s scores differ from one another (Warne, 2018). Because these statistics measure two different characteristics of a sample, it is possible for genes to act on the variability of IQ scores (via heritability), while an environmental treatment, like improving blood lead levels, can impact the average. The two influences on IQ scores can act independently of one another.

Figures 12.1 and 12.2 show how this is possible. The first graph is a set of imaginary data showing the relationship between blood lead levels and IQ scores in a sample that has high exposure to lead. The second graph shows the same two variables in a sample from the same community after blood lead levels have been reduced by 9 μg/dL. The group with the lower lead exposure has an average IQ that is 4 points higher than the group with the high lead exposure. Within each sample, the correlation between the two variables is the same (r = -.34). The two groups also have the same standard deviation (2.53 for lead levels and 15 for IQ scores) and variances (6.40 for lead levels and 225 for IQ scores).

The 4-point average IQ difference between the two groups shows that lowering blood levels in this community has a beneficial impact on intelligence test scores. However, the variability in IQ scores (as shown by the equal standard deviations and variances in the two samples) within each group is the same because the lower lead levels impacted all community members equally, which did not change how much scores within each group differ from one another.

Figure 12.1 Hypothetical data showing the relationship between blood lead levels and IQ scores in a group of 20 individuals with high lead exposure. The average blood lead level is 14.68 μg/dL (standard deviation = 2.53, variance = 6.40). The average IQ score is 96 (standard deviation = 15, variance = 225). The correlation between the two variables is r = -.34.

Figure 12.2 Hypothetical data showing the relationship between blood lead levels and IQ scores in a group of 20 individuals with low lead exposure. The average blood lead level is 5.68 μg/dL (standard deviation = 2.53, variance = 6.40). The average IQ score is 100 (standard deviation = 15, variance = 225). The correlation between the two variables is r = −.34.

One of the causes of the IQ variability within each group is genetic variability, and the percentage of IQ variance that is due to genetic variance is measured by h² values. Both groups can have high heritability, but that would only reflect the impact of genes within each group. Because the h² values cannot give information about environmental characteristics outside the sample, the potential impact of much lower (or much higher) lead levels than what group members experienced cannot be measured by h². On the other hand, the difference between the groups – where the higher average IQ is seen in the group with low lead exposure – is a purely environmental impact on the groups’ averages, and it acts independently of the genetic impact that operates within each group. This example is a good reminder that heritability measures the influence of genetics under current environmental conditions; change that environment radically enough, and the trait may change.

The example here is idealized, but it is not completely unrealistic. Height is highly heritable (h² values between .87 and .93 for men, and between .68 and .84 for women in one typical study; Silventoinen et al., 2003), and yet, in many countries, adults are taller today than in the past (Komlos, Hau, & Bourguinat, 2003; Komlos & Lauderdale, 2007). The gains in nutrition and health care that have caused people to be taller today have not reduced the importance of genetic differences in determining height differences within modern populations.

A similar situation really does occur with intelligence. IQ scores are much higher today than in the earliest days of intelligence testing (Flynn, 1984, 1987; Pietschnig & Voracek, 2015). Chapter 14 will discuss this increase and its potential causes in more depth. But the important message is that IQ scores are influenced by genes (as seen by the high heritability values) but also that environmental changes – like reduced exposure to lead or a proper diet for people with PKU – can also increase IQ.

Conclusion

Untreated PKU, lead poisoning, and iodine deficiency all have one thing in common: they all have large detrimental effects on intelligence (Hunt, 2011). This is in spite of the strong influence of genes, where h2 in many studies is approximately .50. The examples in this chapter show that environmental variables can have a strong influence on heritable traits – including intelligence. By making important changes to the environment, individuals can see remarkable boosts in their IQ scores. However, this does not mean that every treatment will be as effective or as inexpensive as the treatments in this chapter. Chapters 14–16 will discuss the limits of other interventions that are often proposed to increase IQ scores in healthy populations.

From Chapter 12 of "In the Know: Debunking 35 Myths About Human Intelligence" by Dr. Russell Warne (2020)

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