Aug 17, 2025·Advanced Topics & Research

Are Genes Important for Determining Intelligence?

Understanding the role of genes vs. environment in intelligence. Discover why heritability research shows genetic factors are more influential than many believe.

Dr. Russell T. WarneChief Scientist

Are Genes Important for Determining Intelligence?

Many people attempt to downplay genetic influences on intelligence, arguing that even with high heritability values, environmental factors remain more significant than genetics. However, mounting evidence suggests genes play a more crucial role in intelligence than many twentieth-century psychologists acknowledged.

Walter Mischel, one of the most influential psychologists of his era, studied environmental influences on behavior and found people acted consistently when environments remained stable. From this perspective, he understood the importance of environmental impact. Nevertheless, Mischel (2005) was wrong to dismiss genetic influences so readily. Elevated heritability indicates that genes matter substantially—not only for intelligence but for numerous human characteristics.

Genetics and Environment: Both Matter

Mischel correctly recognized that both genes and environment contribute to intelligence levels. Without favorable genetic variants, even the most enriching environment cannot produce exceptionally high IQ scores. Similarly, even optimal genetics become irrelevant in severely negative environments involving iodine deficiency, lead exposure, malnutrition, neglect, or serious brain trauma. Both factors are essential.

The prevailing theory suggests genes establish boundaries for potential intelligence levels, called the reaction range. Environmental variables then determine where within this range a person's intelligence develops (Hunt, 2011). Scarr and Weinberg captured this concept: "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).

Mischel's error lay in asserting these contributions make separating genetic and environmental influences impossible or illogical. While every rectangle derives its area from both dimensions, heritability measures how genetic variations influence trait variations. Heritability explains relative individual differences within populations. Applying this to Mischel's analogy: if genes resemble width and environment resembles length, heritability measures how width variations correspond to area differences.

Gottfredson (2009) offered another analogy: development requires genes and environments working together, like dance partners performing a tango. Yet this doesn't prevent determining whether performance variations among couples stem more from male or female partner skill differences. Similarly, while everyone acknowledges that genes and environment both matter, behavioral genetics research demonstrates we can discuss how genetic differences and environmental differences each impact individual trait variations.

Genetic Similarity Doesn't Diminish Genetic Importance

Some dismiss genetic influences by noting humans share approximately 99% genetic identity. This viewpoint oversimplifies the relationship between genetic similarity and differences.

The 99% shared DNA makes humans one species capable of interbreeding. Humans and chimpanzees share roughly 96% genetic identity, illustrating how high similarity defines species boundaries. The identical DNA creates universal human features: heads, lungs, fingernails, eyes, brains. These genetically transmitted traits aren't included in heritability discussions because heritability focuses on differences, not similarities.

All heritability influence resides in the 1% of differing genes (Bouchard, 2014). Though seemingly small, this represents millions of DNA variations (Hunt, 2011), some affecting intelligence differences.

Small genetic differences can produce substantial trait variations. Single defective genes cause conditions like PKU, sickle cell anemia, and cystic fibrosis. Within normal development ranges, that 1% genetic variation partly explains major height differences—from 143 cm in Pygmies to 184 cm in Dutchmen (McEvoy & Visscher, 2009)—and entirely accounts for genetic influences on heart disease, weight, aggression, and intelligence.

Dog breeds differ by only 0.15% genetically, making humans six times more genetically diverse. Yet toy poodles and huskies exhibit obvious, dramatic differences.

High human genetic similarity doesn't nullify genetic impacts on intelligence. This similarity simply confirms shared species membership. The remaining genetic differences constitute all genetic influence on trait heritability. These differences represent millions of potential variations between individuals, and even subtle genetic variations can produce major trait differences. These facts demonstrate that high genetic similarity doesn't negate genetic influences on intelligence or any heritable trait.

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