Is IQ Malleable?

Dr. Russell T. Warne

Jun 15, 2025

Being raised by different parents changes not only a child’s family environment, but also all factors contributed by the neighborhood, peer, and school environments. These often-drastic environmental changes make adoption studies a particularly powerful method to assess the malleability of intelligence. (Sauce & Matzel, 2018, p. 34)

Chapter 12 shows that high heritability and strong environmentally driven change are both possible in the same trait. By providing an appropriate diet to people with PKU, reducing lead exposure to children, and treating iodine deficiency, it is possible to either increase intelligence or prevent large drops in intelligence. These environmental interventions are major successes that are probably responsible for millions of people being smarter today than they would have been otherwise. Adoption studies also show that being raised in a middle- or upper-class family home probably raises IQ scores by as much as 4–5 points, an increase that would have major positive impacts on people’s lives (see Chapter 11).

Because of the success of these environmental interventions, some people believe that intelligence is highly malleable and that it is possible to increase IQ through other common interventions. An example of this is at the opening of the chapter, where Sauce and Matzel (2018) argue that a handful of large, environmentally induced changes in IQ must lead to the conclusion that intelligence is a trait that is changeable and that – with available interventions – many people can become smarter. Unfortunately, studies that investigate the effectiveness of treatments to increase intelligence do not support this high level of optimism. This chapter will discuss some other large changes in IQ scores and why they do not necessarily mean intelligence is malleable. In Chapters 15 and 16, I will discuss the results of specific interventions to raise IQ.

Adoption Studies

The impacts of adoption on IQ are unquestioned, and even scholars who are very skeptical of the effectiveness of interventions to raise IQ agree that adoption causes an increase in IQ by about 5 points (e.g., Jensen, 1998). However, it is not clear what it is about adoptive families that causes their adoptive children to be smarter than if they had stayed with their birth parents. As stated in Chapter 11, in industrialized nations, adoptive families are usually middle- and upper-class, but that does not mean that income is the cause of higher IQ for adopted children. These families differ from low-income families in many ways that are not economic, and it is impossible to isolate these differences and to examine them individually to determine why adopted children experience an IQ increase. Perhaps having access to high-quality services (e.g., health care, preschool, K-12 education) increases IQ. Maybe it is the nicer, safer neighborhoods that these families live in. Perhaps the greater stability of the homes – with less divorce and greater likelihood of having two parents in the family – makes adopted children smarter. Maybe parental behavior (for example, more parent–child interactions, parents involved in the child’s education) increases IQ. Perhaps they each make their own little contributions that then combine to increase IQ by approximately 5 points.

Or maybe it is none of these characteristics that have a causal impact on higher IQ at all. There is no way to know for sure with the current evidence. Scientists can’t force adoptive parents to eliminate a potentially beneficial piece of their home environment just to see if it reduces the impact of adoption on IQ scores. That would not be ethical, and few parents would agree to be of a study like that.

Because the characteristics of adoptive homes are not possible to isolate, it is not clear what, exactly, other parents should do to increase their child’s IQ. The best possible answer based on adoption data is, “Try to make your home environment good enough that you would be eligible to adopt a child.” This is not an effective foundation for a targeted intervention to raise IQ.

Up, Up, and Away: The Flynn Effect

Environmental impacts on IQ scores are not limited to individual people who have been adopted, or escaped the ravages of PKU or lead poisoning. Worldwide, IQ scores drifted higher over the course of the twentieth century. This means that people today perform better on intelligence tests than their grandparents did. This phenomenon was publicized by philosopher James Flynn, who knew that when intelligence tests were created, the average IQ score was set to be 100. Yet, as time passed, that average gradually increased at a rate of 3 points per decade in the United States (Flynn, 1984). Later, Flynn found this increase in IQ scores was a regular phenomenon that occurred in many other countries (Flynn, 1987), and later research has confirmed his work (Pietschnig & Voracek, 2015).

IQ score increases of 3 points per decade is a lot. This would indicate that a person with an average IQ of 100 from 1970 who had traveled through time to 2020 would only score 85 compared to a twenty-first-century population. A person of average intelligence from 1920 if transported through time to today would score 70, which is about the average IQ for people with Down’s Syndrome and the approximate cutoff for being diagnosed with an intellectual disability. Projecting this trend further back in time produces results that simply don’t match reality if these IQ score increases were the result of a real rise in intelligence. For example, someone from the era of the American Revolution (almost 250 years ago) would score approximately 25 on a modern intelligence test. This score is so low that the average person would not be expected to master language or feed themself. But it is simply not possible that the average person from the 1770s would have a disability this severe. After all, this is the generation that produced George Washington, Benjamin Franklin, and the other Founding Fathers. If their fellow Americans were really so disabled, someone would have probably mentioned it in surviving documents.

So, IQ scores were increasing, but clearly actual intelligence was not – or at least, not as quickly. Because Flynn was a philosopher, he was willing to consider implications of rising IQ scores that were almost unthinkable to psychologists. Flynn rocked psychology by suggesting that this was evidence that intelligence tests really did not measure intelligence. He also argued that group comparisons of intelligence scores were meaningless and that studies on intelligence and the aging process were invalid (Flynn, 1984, 1987).

Flynn brought so much attention to the increasing IQ scores that Herrnstein and Murray (1994) called it the Flynn effect, a name that has since stuck. Ironically, Flynn was not the first to notice the increasing IQ scores, and he never claimed to have discovered the phenomenon. What Flynn did was discover how universal and regular the increases in IQ scores were. He also asked some tough questions about intelligence tests that psychologists were not always prepared to answer. Before Flynn, psychologists just took the idea that intelligence tests measure intelligence for granted. When they did discuss the increasing IQ scores, it was with the belief that it was a quirk of test construction (e.g., Garfinkle & Thorndike, 1976), and not with the understanding of how far-reaching the increases were (e.g., Thorndike, 1975; Tuddenham, 1948).

Table 14.1 Percentage of Americans, ages 25 and over, with a high school diploma or bachelor's degree, 1940-2017

	1940	1960	1980	2000	2017
High school diploma	24.5%	41.1%	68.6%	84.1%	89.6%
Bachelor's degree	4.6%	7.7%	17.0%	25.6%	34.2%

Source: National Center for Educational Statistics, 2017, Table 104.10.

The Flynn effect is relevant to the debate about the malleability of intelligence because these IQ increases cannot possibly be genetic in origin. Human gene pools just do not change quickly enough for IQ scores to increase 3 points every decade (Ceci, 1991). The Flynn effect provides incontrovertible evidence that IQ scores can increase dramatically due to changes in the environment. After Flynn’s articles in the mid-1980s, the race was on to discover the cause of the Flynn effect because discovering it could hold the key to creating interventions that make people smarter.

After more than three decades of research, it is clear that there is no single cause of the Flynn effect (Mackintosh, 2011). Instead, several causes seem to act at the same time to increase IQ scores in a country. One highly likely cause is increased education (Hunt, 2011). Compared to previous generations, people who live in industrialized nations in the twenty-first century are much more educated. This is apparent in Table 14.1, which shows that a far greater percentage of Americans graduate from high school and college in the twenty-first century than did in 1940. An additional year of education causes IQ to increase 1–5 points (Ritchie & Tucker-Drob, 2018), and disruptions in school attendance seem to lower IQ in children (Ceci, 1991). However, the degree to which this effect is cumulative is not clear. In other words, there is no guarantee that each additional year of school keeps adding 1–5 IQ points until people stop going to school. Still, it would be very surprising if school – where people are taught knowledge and how to think – did not contribute to the Flynn effect.

Other suggested causes of the Flynn effect include improved physical health. Blood lead levels are lower in industrialized nations (see Chapter 12). Additionally, brain size in the UK and Germany is larger today than it was a generation ago (Woodley of Menie, Peñaherrera, Fernandes, Becker, & Flynn, 2016), which may be important because brain size is positively correlated with intelligence (see Chapter 3). Birth weight – a measure of prenatal health – has increased (e.g., Surkan, Hsieh, Johansson, Duckman, & Cnattingius, 2004), probably due to better medical care and healthier behavior from pregnant women, such as lower smoking rates during pregnancy. Because the time before birth is very critical in brain development, this may result in the Flynn effect being apparent even in very young children (e.g., Bassok & Latham, 2017).

One intriguing theory about the Flynn effect is that environments have become more cognitively complex – that is, they require more thinking to navigate in. As the industrial revolution and then the computer revolution took hold, navigating life required more brain power. As people went to school for longer periods of time and learned how to reason and think better, they were better able to think abstractly. The more complex environment ensured that they would have to use these skills in daily life. The unintended side effect of all this abstract thinking is higher performance on intelligence tests compared to previous generations. James Flynn summarized this theory well:

The ultimate cause is the trend to modernity, caught but only partially so by the industrial revolution ... The intermediate causes are the spinoffs of modernity, such as better nutrition, smaller family size, hothouse “education” of preschoolers, new parenting, more formal education, far more creative work and leisure, the new visual culture, and urbanization. (Flynn, 2013, p. 856, emphasis removed)

Flynn sees all these consequences of industrialization and modernization as creating this increase in IQ. This would explain why the twentieth century saw such large gains in IQ in many countries as they adopted these innovations. It is because of the dramatic gains in IQ scores (which, it must be remembered, can only have an environmental cause) that many people see the Flynn effect as evidence that treatments can increase intelligence.

However, this optimism is misplaced. While the Flynn effect shows that improvements in the environment can increase IQ in populations, it doesn’t say much about what to do to help boost IQ in individuals. Sure, the Flynn effect suggests that to raise IQ people need to go to school, be healthy, and live in the modern world. But in wealthy countries, almost everyone experiences these conditions to some degree. Based on the Flynn effect, it is not clear what more anyone can do to raise IQ in a country like the United States or in other wealthy nations. On the other hand, developing nations that implement reforms to improve public health, education levels, and modernize the economy are seeing larger IQ gains than anything seen in industrialized nations in the twentieth century (e.g., Daley, Whaley, Sigman, Espinosa, & Neumann, 2003; Liu & Lynn, 2013; Wang & Lynn, 2018). Many nations are already implementing these changes to their societies for reasons that often have little do with an explicit attempt to raise IQ, such as a desire to increase economic growth or to develop the local education system.

Since the early 2000s, there has been a new development in research on the Flynn effect. The increase in IQ has stopped in some countries: Denmark (Teasdale & Owen, 2000), Norway (Sundet, Barlaug, & Torjussen, 2004; Bratsberg & Rogeberg, 2017), Finland (Dutton & Lynn, 2013), the Netherlands (Woodley & Meisenberg, 2013), and France (Dutton & Lynn, 2015). Additionally, the Flynn effect has slowed down and may stop soon in Germany, Austria, the United States, Australia, and the United Kingdom (Pietschnig & Gittler, 2015; Russell, 2007). These countries are all industrialized and wealthy, with access to all the technological, societal, and cultural changes that modern society brings to a nation. The countries also have widespread access to a quality education, and some provide universal health care to their citizens. These countries may have reached (or may soon reach) a saturation point where environmental improvements provide no additional boost in IQ. If this is true – as some experts believe (Rindermann, Becker, & Coyle, 2017) – then there is little more that Flynn effect-driven improvements in the environment can do to raise IQ for most people living in these countries.

Stepping Into the River Twice: Individual Growth and Fluctuations

At the individual level, IQ scores seem to stabilize between the ages of (approximately) 7 and 10. Thereafter, “Small changes are common, large changes are rare” (Jensen, 1980b, p. 283). However, large changes in IQ do occasionally happen, even in people who are experiencing typical development. About 10–15% of children experience a 15-point change or more during their childhood after their peers’ scores have stabilized (Baldwin & Stetcher, 1922; Burks, Jensen, & Terman, 1930; Jensen, 1980b, 1998; Moffitt, Caspi, Harkness, & Silva, 1993). Some people see these exceptions as evidence that IQ can be changed.

Moffitt et al. (1993) published the best study on IQ fluctuations in childhood. The researchers administered an intelligence test to almost 800 children at ages 7, 9, 11, and 13. A total of 13% of children had a large IQ fluctuation from one test administration to another, but a change was “variable in its timing, idiosyncratic in its source and transient in its course” (Moffitt et al., 1993, p. 455). In other words, the fluctuations were random, and after a large change the child’s IQ score would then rebound to about its previous level at the following testing. Additionally, there seemed to be no triggering event for these changes. “For every child who had a life event linked with IQ change, we found at least five children who had experienced that same event, but with no measurable effect on IQ” (Moffitt et al., 1993, p. 491). So, while IQ changes do happen after age 7, they only occur in a minority of children, the changes usually are temporary, and there does not seem to be any apparent environmental cause. Earlier research on IQ fluctuations showed similar results (Baldwin & Stetcher, 1922, pp. 36–39; Burks et al., 1930, pp. 41–61). As a result, no intervention to improve IQ has come out of the study of children with large IQ fluctuations, and that fact seems unlikely to change.

But the stabilization of IQ seems confusing because children do get smarter over time. Children’s mental ability increases as they go to school, learn new knowledge, and have new experiences. This is why a typically developing 10-year-old will be smarter than a typically developing 5-year-old, and it seems to contradict the stability of IQ in childhood. (If IQ is so stable, it seems odd that any learning would be possible.) As a result, some people believe that the change that is apparent from age to age can lead to clues on how to change IQ.

However, this thinking mixes up two concepts: (1) IQ as a measure of someone’s performance on a test of g compared to their peers and (2) individual absolute changes in knowledge or ability. An IQ score is the relative standing compared to one’s peers, and that does not change much after middle childhood. In other words, a child with an IQ of 85 at age 10 is going to have a similar IQ at age 20 because their rank order compared to members of that person’s peer group does not change drastically. Age-to-age development (where someone’s knowledge or abilities grow) is where the real change happens. But as everyone in the same age group experiences that change, the relative standing of individuals – i.e., their IQ score – changes little (Rindermann, 2018, p. 57). Thus, there are two kinds of change, and because they are different, age-to-age change does not impact IQ and is irrelevant for creating an intervention to raise IQ compared to a person’s peers (Gottfredson, 2009).

Conclusion

All four of these changes in ability – adoption studies, the Flynn effect, individual fluctuations in IQ, and absolute growth in knowledge and intelligence – appear to offer hope for creating interventions that would increase intelligence. However, all four are dead-ends for providing information that could result in interventions that make people smarter. Adoption studies show that growing up in a middle- or upper-class home is beneficial for a person’s IQ, but it is not clear how this IQ boost occurs or what specific elements of these homes cause adopted children to have IQ score increases. The Flynn effect is a population-level phenomenon (not an occurrence for individuals), and wealthy nations are already reaping the IQ-related benefits of industrialization. The Flynn effect also provides little guidance about what other environmental changes or treatments citizens of wealthy nations should experience in order to raise their IQ. The fluctuations in IQ that are sometimes seen in individuals seem to be random and temporary – two characteristics that do not form a useful foundation for interventions. Finally, while the absolute change in knowledge and intelligence across the lifespan is a real phenomenon, it does not provide information about raising IQ because IQ is not a measure of absolute intelligence. Rather, it is a measurement of an examinee’s intelligence compared to that person’s age mates – who are also experiencing the same development changes. The absolute changes arising from development and the relative changes required to raise IQ are not interchangeable.

Other chapters in this book show that environmental influences are important for determining intelligence. The Flynn effect and the interventions discussed in Chapter 12 (i.e., treating PKU, eliminating iodine deficiency, and preventing lead exposure in children) show that massive improvements in IQ are possible. However, these examples are very specific, and for people already in positive environments – as many people in industrialized nations are – current knowledge about the environmental causes of high IQ provides few clues about how to raise IQ.

From Chapter 14 of "In the Know: Debunking 35 Myths About Human Intelligence" by Dr. Russell Warne (2020)

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