Are Genes Important for Determining Intelligence?

Dr. Russell T. Warne

Jun 14, 2025

. . . the psychologist’s typical strategy of partitioning the determinants of behavioral characteristics into separate genetic versus environmental causes is no more sensible than asking which areas of a rectangle are mostly due to length and which to width. (Mischel, 2005, paragraph 17)

Despite the high heritability values for intelligence, there are people who still make a variety of arguments to nullify or deny the genetic influence of intelligence. These people use mental gymnastics to argue that even though heritability can be high, environment still matters far more than genetics in determining someone’s intelligence. While no scientist argues that the environment is irrelevant, the evidence has mounted that genes are a far more important influence on intelligence than many psychologists believed in the twentieth century.

The quote at the beginning of the chapter is from Walter Mischel, one of the most influential psychologists of the twentieth century (E. Diener, Oishi, & Park, 2014; Haggbloom et al., 2002). Mischel studied how environments influence people’s behavior and found that people’s actions were consistent – as long as the environment stayed somewhat consistent. Changing environments, though, often led to changes in behavior. Given this perspective, it makes sense why he would see the value of considering the impact of the environment on people’s behavior. However, Mischel (2005) was mistaken to minimize genetic influences so quickly. High heritability does indicate that genes matter – not just for intelligence, but for many other human traits.

Two to Tango: Genetics and Environment

First, it is important to explain what Mischel gets right: it is true that genes and environment do both contribute to a person’s intelligence level. If a person does not have enough of the versions of genes that boost IQ, then no environment – no matter how favorable – will make a person earn a high IQ on an intelligence test. Conversely, even the luckiest win in the genetic lottery is irrelevant if an environment is extremely negative, such as a childhood marred by long-term iodine deficiency, lead poisoning, famine, neglect, or severe brain injuries. Both genes and environment make their contributions to a person’s intelligence level.

The generally accepted theory is that genes set limits on the possible intelligence levels a person can have. These limits are called the reaction range. Within the reaction range, environmental variables determine the exact intelligence level of a person (Hunt, 2011). Scarr and Weinberg summed up the idea well when they stated, “Genes do not fix behavior; rather, they establish a range of possible reactions to the range of possible experiences that the environment provides” (1978, p. 29). Where within the reaction range a person’s IQ develops is a consequence of the environment. Thus, it does take both genes and environment to produce a trait, and both are important.

Where Mischel (2005) erred was in claiming that these contributions to a trait make it impossible or nonsensical to separate genetic and environmental influence. Yes, it is true that all rectangles get their area from their width and their length, just as it is true that intelligence is always a product of both genetic and environmental influences. But heritability is a measure of the influence of genetic differences on trait differences (i.e., variance, as explained in Chapter 12). Thus, heritability explains relative differences among individuals within a sample. To apply this to Mischel’s (2005) analogy, if genes are similar to a rectangle’s width and environmental influences are like a rectangle’s length, then heritability would be a measure of how differences in width correspond to differences in rectangle area. The fact that every rectangle has length and that this also influences rectangle area does not negate the influence of width.

One leading intelligence scientist used another analogy that makes the same point when she wrote:

The irrelevant truth is that an organism’s development requires genes and environments to act in concert. The two forces are inextricable, mutually dependent and constantly interacting. Development is their mutual product, like the dance of two partners. The irrelevant conclusion is that it is therefore impossible to apportion credit for the pair’s joint product to each partner separately – say, 40% of the pair’s steps to the man and 60% to the woman. The inappropriate generalization is that behavior geneticists cannot possibly do what they claim – namely to decompose [trait] variation among individuals ... This is analogous to saying that it would be impossible to estimate whether differences in quality of tango performances among American couples is owing more to skill variation among the male partners than to skill variation among the female partners (i.e., genetic vs. nongenetic variation) ... (Gottfredson, 2009, p. 38, emphasis added)

Yes, it takes genetics and environment to create a trait – just as it takes two partners to dance a tango. But that does not change the fact that some couples are better dancers than others and that it is possible to identify whether couples vary in their quality more because of the variability in how well the man dances compared to the woman (or vice versa). Thus, everyone agrees that genes and environment matter. Where Mischel (2005) and people in his camp disagree with Gottfredson (2009) and people in her camp (including me) is whether it is possible to talk about the influence of genes separately from the environment. Decades of behavioral genetics research – especially in producing data about heritability – show that it is possible to talk about the differences in genes and the differences in environment and how each impacts differences among individuals in a trait.

Send in the Clones: High Genetic Similarity Among Humans

Another tactic people sometimes use to dismiss the importance of genes on intelligence differences (e.g., Grison, Heatherton, & Gazzaniga, 2017, pp. 301, 302) is to state that high genetic similarity among humans means that genetic differences are trivial in their impact. This belief is based on the fact that humans are about 99% genetically identical. With so much genetic similarity, it can seem like that last 1% can be trivial. If this genetic similarity is high, then it may seem implausible that high heritability can have much influence on the development of a trait.

This viewpoint oversimplifies the relationship between genetic similarity and physical and/or psychological differences. First, the 99% of genes that are identical in all humans are what makes people all belong to the same species and be able to interbreed with one another. Without a high degree of genetic similarity, individuals would belong to separate species. This is apparent when comparing humans with their closest relatives: chimpanzees. The two species are “only” about 96% genetically identical (The Chimpanzee Sequencing and Analysis Consortium, 2005).

The 99% of DNA that is identical in all humans is what creates the similarities among every member of the species. The reason why everyone has a head, two lungs, fingernails, two eyes, a brain, etc., is because the DNA that is identical in all humans encodes for the characteristics that all humans share. Therefore, these traits are genetically transmitted, but they are not considered in discussions of heritability because heritability focuses on differences – not similarities. Heritability values do not apply to traits that are identical in all humans; those traits are genetically inherited, but h² does not describe anything about these traits because there are no trait differences to quantify.

Second, the genes that are identical in all humans cannot contribute to heritability because h2 is a measure of the influence of genetic differences. Therefore, all of the influence in heritability lies in that 1% of genes that differ from person to person (Bouchard, 2014). While this does not seem like much, that 1% corresponds to millions of subtle differences in each person’s DNA (Hunt, 2011). Some of these will have an impact on individual differences in intelligence.

Third, subtle genetic differences can sometimes correspond to big differences in a trait. An obvious example of this is when a person is born with a genetic condition because they inherited two defective recessive versions of a gene. Examples of this type of genetic condition include PKU (discussed in Chapter 12), sickle cell anemia, cystic fibrosis, Tay-Sachs disease, and xeroderma pigmentosum. For all these conditions (and others), possessing two defective copies of a single gene can lead to a major medical condition that drastically affects the person’s life, even if all other genes in the person’s DNA are completely normal. When a defective version of a gene is on an X chromosome, then males are particularly susceptible to genetic disorders because they do not have a second, normal copy of the gene on their Y chromosome. Common disorders of this type include red–green color blindness, muscular dystrophy, and hemophilia.

Even within the normal range of development, subtle genetic differences can result in major differences in traits (Cochran & Harpending, 2009). For example, the 1% of genetic differences among humans is at least partially responsible for the major differences in height, which in adult men range from an average of 143 cm in Pygmies to 184 cm in Dutchmen (McEvoy & Visscher, 2009). The 1% of genes which vary in humans also entirely accounts for the genetic influence on other heritable traits like heart disease, weight, aggressive behavior, and intelligence (Warne et al., 2018).

An example from another species makes it clear that small genetic differences can result in major changes in traits. Domesticated dog breeds differ from one another in approximately 0.15% of their genes, which means humans are over six times more genetically diverse than dogs. Yet, the differences between toy poodles and huskies are very obvious and drastic. In fact, genetic diversity among dogs and related species is so low that many of them (e.g., dogs, coyotes, wolves, foxes, dingoes, and jackals) can interbreed (Wayne & Ostrander, 1999).

To recap, humans do exhibit a high degree of similarity among one another. This does not nullify the impact of genes on intelligence (or any other heritable trait, for that matter). High genetic similarity among humans is necessary for people to all belong to the same species. The comparatively few remaining genetic differences (about 1% of the entire human genome) are the source of all genetic influence on the differences (i.e., heritability) found in any trait. Additionally, that 1% of the genome corresponds to millions of potential genetic differences between any two randomly selected people. It is within these varying portions of people’s DNA that heritability lies. Finally, even subtle differences in genetic makeup can result in major differences in traits. All of these facts combine to show that high genetic similarity among humans does not negate the importance of genetic influences on any trait – including intelligence.

Conclusion

While no one dismisses the importance of environmental influences, genes do have an impact on people’s intelligence. Some individuals who want to ignore the research on the heritability of intelligence have made arguments that dismiss genetic influences. These arguments, though they sound enticing to the uninformed listener, fall apart under scrutiny. Yes, both genes and environment are necessary to create a trait in a person; but heritability is concerned with the influence of genetic differences on trait differences. This emphasis on differences does not negate the importance of the environment, nor does investigating the influence of environmental differences negate the influence of genetics. While humans are extremely similar genetically to one another, this says little more than that they belong to the same species. The genetic differences that do exist are important influences on differences in intelligence – and any other heritable trait.

From Chapter 13 of "In the Know: Debunking 35 Myths About Human Intelligence" by Dr. Russell Warne (2020)

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