Over the past few decades, a plethora of research has been conducted regarding the causes and consequences of IQ. Although there are many attributes that influence an individual’s well-being, few are as well established as the role of intelligence. Individuals who possess higher levels of IQ are more successful in multiple aspects of life (Kuncel et al 2004, Gottfredson 1997).
For example, high IQ individuals enjoy superior job performance (Hunter 1986, Schmidt Hunter 1992, Schmidt Hunter 2004, Hunter Schmidt 1996, Ree Earer 1992) and higher social status (Singh-Manoux Richards Marmot 2003) compared to their lower IQ counterparts. Additionally, high IQ has also been connected to superior levels of creativity (Silvia 2008, Nusbaum Silvia 2011, Kim 2005, Silvia 2008). Considering the significant effects of intelligence on quality of life, it is of little surprise that the topic of IQ has garnished a high degree of attention by both scientists as well as the popular media.
Intelligence is Highly Heritable
One factor that has long been suspected of contributing towards IQ is genetics. One finding that has been consistent across the scientific literature is that IQ appears to be highly heritable (Neisser et al 1996, Bouchard 2013, Heutink Verhuls Boomsma 2006). In fact, a twin study by Heutink Verhuls Bloosma concluded that heritability of IQ was 30% at age 5 and increased to 80% by age 12 (2006). Additionally, some studies discovered at least some level of connection between different genetic variants and cognitive functioning (Johnson et al 2015, Deary Spinath Bates 2006).
Genes Influence Brain Structure
One possibility is that particular genetic mutations can lead to more favorable developments in brain regions and chemicals that contribute to cognitive ability. According to Thompson et al, the structure of certain brain regions related to intelligence, such as Broca’s area, Wernicke’s area, and the frontal region, were nearly 100 percent correlated in identical twins (2001). The extreme similarity of these brain regions between identical twins lends support to the theory that the development of IQ is likely influenced by a genetic component.
Genes Influence Brain Chemicals
Along with the size and shape of the brain, compounds such as brain-derived neurotrophic factor (BDNF) may also play a role. According to Tang et al., different alleles of genes that encode for BDNF have been correlated with differing levels of working memory (2008). Consequently, it is of little surprise that working memory, which is a major contributor to IQ (Kyllonen Christla 1990, Oberauer et al 2008, Suß et al 2002, Colom et al 2004), has also been shown to be highly heritable (Friedman et al 2008). Because both the structure of the brain and production of BDNF can affect executive functioning, this research provides insight into some of the primary neuroscientific mechanisms in which genes contribute towards IQ.
The Role of Environment
Although genetics certainly play a role in the development of IQ, other additional factors have also been connected to intellectual development. For instance, when children are young, healthy cognitive development has been significantly correlated with effective parenting styles and high levels of communication (Baumrind 1967, Svendsen 1983, Raikes et al 2006, Dornbusch et al 1987).
Amazingly, adoption studies have found that children from low socioeconomic (SES) backgrounds who are adopted into higher SES homes ultimately experience a dramatic rise in IQ (Van Ijzendoorn Juffer Poelhuis 2005, Duyne Dumaret Tomkiewicz 1999, Lucurto 1990). These findings would not be possible if genes were the sole determinant of intellectual ability.
Can Environmental Factors be Manipulated to Improve Childrens’ IQ?
In order to capitalize on the ability of environmental factors to favorably influence IQ, multiple controlled trials have been conducted in hopes of improving childrens’ IQ through the use of behavioral interventions. Fascinatingly, research by Protzko Aronson Blair has found that training mothers how to speak with their children more frequently or to read books more often leads to superior gains in IQ compared to children who do not receive this training (2013).
Additionally, when children spend more time in school, this leads to additional gains in IQ as well (Brinch Galloway 2012). Clearly, multiple factors other than genetics are highly impactful towards cognitive development.
The Complicated Interaction between Nature and Nurture
Although environment has been proven to play a major role in the development of IQ, many experts still believe that genetics is a primary factor. Although there is a strong hereditary component of IQ, the role of genetics in the development of cognitive ability is significantly more complicated than often portrayed.
For example, research has found that when children raised in homes of high SES status are compared to each other, the connection between genetics and IQ does indeed to be quite strong. Amazingly, however, when comparing children raised in homes of low SES, suddenly the connection between genetics and IQ appears to be negligible (Tucker-Drob et al 2011, Tucker-Drob Bates 2015, Turkheimer et al 2003).
If IQ were strictly determined by genes, then the strong correlation between genes and cognitive development would be present in all circumstances, including the low SES homes. Clearly, these studies demonstrate the hereditary link is not universal, which provides evidence that environmental factors play a much larger role than previously believed.
Genes May Promote Interest Towards Cognitively Demanding Tasks
One explanation that reconciles these seemingly disparate findings is that rather than genetics directly promoting intelligence, genes instead influence an individual’s appeal towards intellectually stimulating activities. In individuals who are raised in high SES households, intellectually stimulating opportunities are readily available, and those who regularly engage in these activities will gradually develop superior cognitive abilities.
Conversely, households that are low in SES may lack sufficient opportunities for children to experience cognitively engaging activities. Therefore, children who possess these inclinations are not provided with equal opportunity to develop their cognitive functions. As evidenced by a landmark analysis from Dickens Flynn, a large quantity of research exists to substantiate this explanation (2001).
In fact, recent research has identified specific gene variants that explain large correlations between identical twins readiness to seek and engage in cognitively-demanding tasks (Ksiazkiewicz Ludeke Krueger 2016). This body of evidence suggests that rather than exclusively affecting biological factors, the genetic influences of IQ are largely behavioral.
Significant amounts of research suggest that rather than genetics inherently determining intelligence, it is actually an individual’s motivation for cognitive tasks that are influenced by genetics (Dickens Flynn 2001).
Conclusions
Overall, the connection between genetics, environment, and IQ appears to be complicated and nuanced. While popular media may portray IQ in an overly simplistic manner, the reality is that both genes and environment interact together to influence IQ.
References
Wighton, K. (2015). Intelligence ‘networks’ discovered in brain for the first time. Imperial College London News. Retrieved from http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_22-12-2015-10-13-44
Kuncel, N. R., Hezlett, S. A., & Ones, D. S. (2004). Academic performance, career potential, creativity, and job performance: Can one construct predict them all?. Journal of Personaliy and Social Psychology. 2004 Jan; 86(1): 148-161.
Gottfredson, L. S. (1997). Why g matters: The complexity of everyday life. Intelligence, 24(1), 79-132.
Hunter, J. E. (1986). Cognitive ability, cognitive aptitudes, job knowledge, and job performance. Journal of vocational behavior, 29(3), 340-362.
Schmidt, F. L., & Hunter, J. E. (1992). Development of a causal model of processes determining job performance. Current directions in psychological science, 1(3), 89-92.
Schmidt, F. L., & Hunter, J. (2004). General mental ability in the world of work: occupational attainment and job performance. Journal of personality and social psychology, 86(1), 162.
Hunter, J. E., & Schmidt, F. L. (1996). Intelligence and job performance: Economic and social implications. Psychology, Public Policy, and Law, 2(3-4), 447.
Ree, M. J., & Earles, J. A. (1992). Intelligence is the best predictor of job performance. Current directions in psychological science, 1(3), 86-89.
Singh-Manoux, A., Richards, M., & Marmot, M. (2003). Leisure activities and cognitive function in middle age: evidence from the Whitehall II study. Journal of Epidemiology and Community Health, 57(11), 907-913.
Silvia, P. J. (2008). Another look at creativity and intelligence: Exploring higher-order models and probable confounds. Personality and Individual differences, 44(4), 1012-1021.
Nusbaum, E. C., & Silvia, P. J. (2011). Are intelligence and creativity really so different?: Fluid intelligence, executive processes, and strategy use in divergent thinking. Intelligence, 39(1), 36-45.
Kim, K. H. (2005). Can only intelligent people be creative? A meta-analysis. Prufrock Journal, 16(2-3), 57-66.
Silvia, P. J. (2008). Creativity and intelligence revisited: A latent variable analysis of Wallach and Kogan (1965). Creativity Research Journal, 20(1), 34-39.
Neisser, U., Boodoo, G., Bouchard Jr, T. J., Boykin, A. W., Brody, N., Ceci, S. J., … & Urbina, S. (1996). Intelligence: Knowns and unknowns. American psychologist, 51(2), 77.
Bouchard, T. J. (2013). The Wilson effect: the increase in heritability of IQ with age. Twin Research and Human Genetics, 16(05), 923-930.
HEUTINK, P., VERHULS, F. C., & BOOMSMA, D. I. (2006). A longitudinal twin study on IQ, executive functioning, and attention problems during childhood and early adolescence. Acta neurol. belg, 106, 191-207.
Johnson, M. R., Shkura, K., Langley, S. R., Delahaye-Duriez, A., Srivastava, P., Hill, W. D., … & Rotival, M. (2015). Systems genetics identifies a convergent gene network for cognition and neurodevelopmental disease. Nature neuroscience. 2016 Feb;19(2):223-32
Deary, I. J., Spinath, F. M., & Bates, T. C. (2006). Genetics of intelligence. European Journal of Human Genetics, 14(6), 690-700.
Baumrind, D. (1967). Child care practices anteceding three patterns of preschool behavior. Genetic psychology monographs, 75(1), 43-88.
Svendsen, D. (1983). FACTORS RELATED TO CHANGES IN IQ: A FOLLOW‐UP STUDY OF FORMER SLOW LEARNERS. Journal of Child Psychology and Psychiatry, 24(3), 405-413.
Raikes, H., Alexander Pan, B., Luze, G., Tamis‐LeMonda, C. S., Brooks‐Gunn, J., Constantine, J., … & Rodriguez, E. T. (2006). Mother–child bookreading in low‐income families: Correlates and outcomes during the first three years of life. Child development, 77(4), 924-953.
Dornbusch, S. M., Ritter, P. L., Leiderman, P. H., Roberts, D. F., & Fraleigh, M. J. (1987). The relation of parenting style to adolescent school performance. Child development, 1244-1257.
Protzko, J., Aronson, J., & Blair, C. (2013). How to make a young child smarter: Evidence from the database of raising intelligence. Perspectives on Psychological Science, 8(1), 25-40.
Brinch, C. N., & Galloway, T. A. (2012). Schooling in adolescence raises IQ scores. Proceedings of the National Academy of Sciences, 109(2), 425-430.
Tucker-Drob, E. M., Rhemtulla, M., Harden, K. P., Turkheimer, E., & Fask, D. (2011). Emergence of a gene× socioeconomic status interaction on infant mental ability between 10 months and 2 years. Psychological Science, 22(1), 125-133.
Tucker-Drob, E. M., & Bates, T. C. (2015). Large cross-national differences in gene× socioeconomic status interaction on intelligence. Psychological science, 0956797615612727.
Turkheimer, E., Haley, A., Waldron, M., D’Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological science, 14(6), 623-628.
Ksiazkiewicz, A., Ludeke, S., & Krueger, R. (2016). The role of cognitive style in the link between genes and political ideology. Political Psychology.
Dickens, W. T., & Flynn, J. R. (2001). Heritability estimates versus large environmental effects: the IQ paradox resolved. Psychological review, 108(2), 346.
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.
Duyme, M., Dumaret, A. C., & Tomkiewicz, S. (1999). How can we boost IQs of “dull children”?: A late adoption study. Proceedings of the National Academy of Sciences, 96(15), 8790-8794.
Locurto, C. (1990). The malleability of IQ as judged from adoption studies. Intelligence, 14(3), 275-292.
Tang, S. W., Chu, E., Hui, T., Helmeste, D., & Law, C. (2008). Influence of exercise on serum brain-derived neurotrophic factor concentrations in healthy human subjects. Neuroscience letters, 431(1), 62-65.
Friedman, N. P., Miyake, A., Young, S. E., DeFries, J. C., Corley, R. P., & Hewitt, J. K. (2008). Individual differences in executive functions are almost entirely genetic in origin. Journal of Experimental Psychology: General, 137(2), 201.
Kyllonen, P. C., & Christal, R. E. (1990). Reasoning ability is (little more than) working-memory capacity?!. Intelligence, 14(4), 389-433.
Oberauer, K., Süβ, H. M., Wilhelm, O., & Wittmann, W. W. (2008). Which working memory functions predict intelligence?. Intelligence, 36(6), 641-652.
Süß, H. M., Oberauer, K., Wittmann, W. W., Wilhelm, O., & Schulze, R. (2002). Working-memory capacity explains reasoning ability—and a little bit more. Intelligence, 30(3), 261-288.
Colom, R., Rebollo, I., Palacios, A., Juan-Espinosa, M., & Kyllonen, P. C. (2004). Working memory is (almost) perfectly predicted by g. Intelligence, 32(3), 277-296.