Is IQ Malleable?

Dramatic environmental shifts such as adoption make these studies particularly useful for evaluating intelligence malleability. Prior chapters demonstrate that traits can exhibit both high heritability and significant environmental influence simultaneously. Through appropriate dietary interventions for PKU, reducing lead exposure, and addressing iodine deficiency, we can either enhance intelligence or prevent substantial declines. These environmental approaches represent major achievements likely responsible for millions of people being considerably smarter today. Research on adoption indicates that growing up in middle- or upper-class households probably increases IQ by approximately 4–5 points, a meaningful improvement that would significantly benefit people's lives.

Given these successful environmental modifications, some observers conclude that intelligence is highly changeable and that numerous common interventions can boost IQ. Unfortunately, research examining the effectiveness of intelligence-boosting treatments doesn't support such optimism. This chapter explores additional large IQ shifts and explains why they don't necessarily demonstrate intelligence malleability. Subsequent chapters will examine specific IQ-raising interventions.

While the impact of adoption on IQ is undisputed—with even skeptical scholars agreeing adoption produces roughly 5-point increases—what specifically about adoptive families causes this improvement remains unclear. In developed countries, adoptive families typically belong to middle and upper classes, but income itself may not drive higher IQ in adopted children. These families differ from lower-income families in numerous non-economic ways that cannot be isolated and studied individually. Perhaps access to quality services, safer neighborhoods, greater household stability, or particular parenting behaviors each contribute small amounts that combine for approximately 5-point gains. Or perhaps none of these factors actually cause higher IQ.

Since adoptive home characteristics cannot be isolated, what precisely other parents should do to boost their child's IQ remains unclear. The best guidance from adoption research is essentially, "Try making your home environment sufficient for adoption eligibility"—hardly a foundation for targeted intervention.

Environmental effects on IQ extend beyond individuals who've been adopted or avoided PKU or lead poisoning. Throughout the twentieth century, IQ scores rose worldwide at roughly 3 points per decade in the United States. This phenomenon, popularized by philosopher James Flynn and subsequently named the Flynn effect, occurs because these increases cannot be genetic—human gene pools simply don't change that rapidly. The Flynn effect offers definitive proof that environmental changes can dramatically increase IQ scores.

After three decades of investigation, no single cause explains the Flynn effect. Instead, multiple factors seem to operate simultaneously. Increased education represents one probable cause, as an additional year typically raises IQ by 1–5 points. Other suggested contributors include better physical health, reduced lead levels, larger brain sizes, improved prenatal health, and increasingly complex cognitive environments requiring more abstract thinking.

However, optimism based on the Flynn effect is unwarranted. While it demonstrates that environmental improvements can increase population IQ, it offers little guidance for boosting individual IQ. In wealthy nations where people already experience education, health care, and modern living, it's unclear what additional steps could raise IQ further. Since the early 2000s, IQ increases have halted in several industrialized countries—Denmark, Norway, Finland, the Netherlands, and France—suggesting these nations may have reached a saturation point where environmental improvements provide no additional IQ benefits.

At the individual level, IQ scores stabilize between ages 7 and 10. While about 10–15% of children experience 15-point changes or more during childhood, research shows these fluctuations are random, temporary, and lack identifiable environmental triggers. Consequently, no interventions have emerged from studying these fluctuations.

While adoption studies, the Flynn effect, individual fluctuations, and developmental growth initially appear promising for creating intelligence-boosting interventions, all prove to be dead ends. For people already in positive environments—as many in industrialized nations are—current knowledge about environmental causes of high IQ provides minimal guidance for raising intelligence further.





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