The Greatest Psychological Debate of All?
The influence of genes and environment on intelligence
Posted Mar 23, 2018
Foreword: Fellow lab member and expert in animal intelligence, Mary Flaim, takes over the blog today in a piece discussing the historically controversial yet highly relevant topic of the nature of human intelligence.
Most of psychology has been put forth as a debate between nature versus nurture. Are we the result of our genes or the environment that we grow up in? The answer to this question is that typically both nature and nurture influence our behavior. However, the debate over which factor has the most influence on intelligence still rages on.
First, how we measure intelligence should be explained. What IQ scores measure almost perfectly is the underlying ability that drives performance on all cognitive tasks 1. Spearman was the first to discover that performance on one test positively correlated with performance on another, even if the tests were on very different things. He determined that one underlying factor was partially responsible for this positive test correlation2. He called this factor g, and all intelligence test scores are a proxy for this ability since it cannot be measured directly. Typically, the best proxies for g contain a wide variety of cognitive tasks, which is why most intelligence tests have many different subtests.
In the nature camp, a well-known book was published in the late 90s, The G Factor3. This book stated that IQ scores were due primarily to genetic factors and relatively resistant to environmental interventions. The research used heritability estimates gained from both twin and adoption studies. In twin studies, it has been consistently shown that identical twins are more likely to have the same IQ score than fraternal twins. This indicates that genes influence IQ scores more, because the environmental impact for each twin pair should be similar, but the genetic similarity is different. If the environment strongly impacted IQ, then identical and fraternal twins should be equally likely to have the same IQ score, since the environment is highly similar to each pair. In adoption studies, it has been shown that the child’s IQ score is more similar to their biological mother then their adopted mother. More recent studies have been looking at the entire human genome to determine if there are particular genes that are related to IQ scores4.
In the nurture camp, Dr. James Flynn found that as intelligence tests have been given to multiple generations all over the world, IQ scores have been increasing with every generation. The increases were particularly high for the subtests that did not rely on language or "general knowledge" that is usually attained with Western schooling. This finding indicated that some of the subtests that were originally used had a cultural bias, stacking the deck in favor of whoever had the resources to obtain a formal education. It also indicated that environmental factors were impacting IQ scores — things like prenatal care, nutrition, and increased schooling. Additionally, recent studies have shown that not all environmental factors were taken into account when looking at adoption studies. When children who were adopted are compared to their non-adopted siblings, their IQ scores are higher, indicating that the change in environment was beneficial to a large degree5.
So which really has a greater impact on what your IQ score ends up being? The jury is still out. No specific gene has been associated with high or low IQ scores4. Many environmental interventions have so many components that it is impossible to determine which part of the intervention is responsible for the increase in IQ scores. With these interventions, it can also be difficult to tell where environment ends and genes begin. Our increased understanding of epigenetics suggests our environments have the ability to change the expression of our genes. Thus, if the environmental impact improves gene functionality in general, which factor do we attribute the end result to? Most likely there are a large number of factors, both genetic and environmental, each playing a small role that changes throughout our lifetime3,6. One day, it may be possible to have a cohesive timeline of when IQ scores are most heavily impacted, but much more research needs to be done.
Ultimately, no matter what factors are responsible for your IQ score, it is just one of many points of assessment. At the group level, individual IQ scores do not correlate with how well groups make decisions. The quality of group decisions was best predicted by the equality of how much each group member got to contribute and by members’ abilities to pick up on social cues7.
At the individual level, an IQ score is an imperfect predictor of academic achievement8 — and even less accurate in predicting happiness9. If you want to know your IQ score, that’s great. Just remember that it’s a number, not a life sentence.
Mary Flaim is a third-year Ph.D. student at UCLA in the learning & behavior area. She obtained a B.A. in psychology at Bowling Green State University. Mary has extensive experience studying animal cognition and is currently working on creating a cognitive test battery for pigeons to investigate individual differences in intelligence.
 Farmer, R. L., Floyd, R. G., Reynolds, M. R., & Kranzler, J. H. (2014). IQs are very strong but imperfect indicators of psychometric g: Results from joint confirmatory factor analysis. Psychology in the Schools, 51(8), 801-813.
 Spearman, C. (1904). General intelligence, objectively determined and measured. American Journal of Psychology, 15, 201-293
 Jensen, A. R. (1998). The g factor: The science of mental ability. Westport, CT: Praeger.
 Luciano, M., Wright, M. J., Duffy, D. L., Wainwright, M. A., Zhu, G., Evans, D. M., ... & Martin, N. G. (2006). Genome-wide scan of IQ finds significant linkage to a quantitative trait locus on 2q. Behavior genetics, 36(1), 45-55.
 Van Ijzendoorn, M. H., Juffer, F., & Poelhuis, C. W. K. (2005). Adoption and cognitive development: a meta-analytic comparison of adopted and nonadopted children's IQ and school performance. Psychological Bulletin, 131(2), 301.
 Haworth, C. M., Wright, M. J., Luciano, M., Martin, N. G., de Geus, E. J., van Beijsterveldt, C. E., ... & Kovas, Y. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15(11), 1112.
 Woolley, A. W., Chabris, C. F., Pentland, A., Hashmi, N., & Malone, T. W. (2010). Evidence for a collective intelligence factor in the performance of human groups. Science, 330(6004), 686-688.
 Duckworth, A. L., & Seligman, M. E. (2005). Self-discipline outdoes IQ in predicting academic performance of adolescents. Psychological Science, 16(12), 939-944.
 Furnham, A., & Petrides, K. V. (2003). Trait emotional intelligence and happiness. Social Behavior and Personality: an International Journal, 31(8), 815-823.