Genetic Expression of Intelligence Influenced By Environment, Especially in Childhood

“Reading is fun!” by Isaac Wedin is licensed under CC BY Interview with:
Bruno Sauce, PhD and
Louis D. Matzel, PhD
Department of Psychology, Program in Behavioral and Systems Neuroscience
Rutgers University
New Jersey, USA What is the background for this study?

Response: Scientists have known for decades that intelligence has a high heritability, which means that much of the individual differences in IQ we see in people are due to genetic differences. Heritability is a value that ranges from 0.0 (meaning no genetic component) to 1.0 (meaning that the trait is completely heritable). For example, the heritability of breast cancer is estimated at 0.27; the heritability of body mass index is 0.59; and the heritability of major depression is 0.40. In comparison, the heritability of IQ is estimated to be as high as 0.8 – quite a high value!

More recently, however, there have been studies showing that intelligence has a high malleability: the studies cover cognitive gains consequent to adoption/immigration, changes in IQ’s heritability across life span and socioeconomic status, gains in IQ over time from societal and scientific progress, the slowdown of age-related cognitive decline, the gains in intelligence from early education, differences in average IQ between countries due to wealth and development, and gains in intelligence that seem to happen from working memory training.

Intelligence being both highly heritable and highly malleable is seemingly paradoxical, and this paradox has been the source of continuous controversy among scientists.

Why does it matter? Because IQ predicts many important outcomes in life, such as academic grades, income, social mobility, happiness, marital stability and satisfaction, general health, longevity, reduced risk of accidents, and reduced risk of drug addiction (among many other outcomes). A clear understanding of the genetic and environmental causes of variation in intelligence is critical for future research, and its potential implications (and applications) for society are immense. What are the main accomplishments of your study?

Response: In our study, we started by considering relevant research in this field, and created a framework to integrate these findings. From that, we advanced a frequently overlooked solution to the high heritability/high malleability paradox of intelligence:

Intelligence is a trait with unusual properties that create a large reservoir of hidden gene–environment networks, allowing for the contribution of high genetic and high environmental influences on IQ. Gene-environment correlations and interactions are usually obscured by typical research approaches, and end up inflating estimates of the “genetic” contribution to intelligence. For example, the heritability of IQ gets much higher as we age: from 0.2 in children to 0.8 after 50 years of age. This is likely due to the accumulated effect from the environment interplaying with our innate genetic differences (and illustrates the misconception that “heritability” is purely an estimate of the role of genes in the establishment of a trait).

Recognizing the powerful role of gene-environment interplays in intelligence changes the way scientists, therapists, and educators usually look at this topic. There is much to be gained, for example, if interventions to make people smarter and to treat learning/cognitive disabilities are tailored to different genetic backgrounds.

Our proposed gene-environment solution also has novel implications for other enduring problems in the field, including our inability to identify intelligence-related genes (also known as IQ’s “missing heritability”), and the loss of initial benefits from early intervention programs. Programs such as Head Start appear to have transient effects on intelligence, often dissipating within a year or two after completion. The problem, we argue, is that participants typically return to the impoverished environment from which they were selected, an environment wherein access to cognitive challenges are necessarily restricted. This mismatch ends up breaking down gene-environment interplays that were active during the program, and makes it difficult to maintain or amplify any benefits. In other words, programs such as Head Start might become quite effective if more opportunities are given to those children to match their IQs with more challenging environments as they grow older. What should readers take away from your report?

Response: The two most important lessons to take away are:

1) Gene-environment interplay probably plays an important role in making some people smarter than others. And neither the genes nor the environments take priority. As we write in the report, the environment that humans created has as much potential information and sources of changes in IQ as the DNA sequences that were created by evolution. The “potentiality” for intelligence in the genes of humans from 100,000 years ago (if they were transported in a time machine to modern age) is similar to the potentiality for intelligence in the environment of modern civilization (if a human ancestor of a different species, such as an Australopithecus, was raised here and then returned to his peers).

2) Intelligence is an extremely complex trait, and there is a lot we still don’t know about how it develops, what the specific interventions to increase IQ (such as perhaps brain training games) might be, and what the limits of treating learning/cognitive disabilities are. If someone is too sure about the causes and limits of intelligence, he/she is either misinformed or ideologically blinded. Or both.


Bruno Sauce, Louis D. Matzel. The paradox of intelligence: Heritability and malleability coexist in hidden gene-environment interplay.. Psychological Bulletin, 2018; 144 (1): 26 DOI: 10.1037/bul0000131

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Last Updated on January 26, 2018 by Marie Benz MD FAAD