What do twin studies tell us about the importance of genetics on intelligence

Like most aspects of human behavior and cognition, intelligence is a complex trait that is influenced by both genetic and environmental factors.

Intelligence is challenging to study, in part because it can be defined and measured in different ways. Most definitions of intelligence include the ability to learn from experiences and adapt to changing environments. Elements of intelligence include the ability to reason, plan, solve problems, think abstractly, and understand complex ideas. Many studies rely on a measure of intelligence called the intelligence quotient (IQ).

Researchers have conducted many studies to look for genes that influence intelligence. Many of these studies have focused on similarities and differences in IQ within families, particularly looking at adopted children and twins. Other studies have examined variations across the entire genomes of many people (an approach called genome-wide association studies or GWAS) to determine whether any specific areas of the genome are associated with IQ. Studies have not conclusively identified any genes that have major roles in differences in intelligence. It is likely that a large number of genes are involved, each of which makes only a small contribution to a person’s intelligence. Other areas that contribute to intelligence, such as memory and verbal ability, involve additional genetic factors.

Intelligence is also strongly influenced by the environment. During a child's development, factors that contribute to intelligence include their home environment and parenting, education and availability of learning resources, and healthcare and nutrition. A person’s environment and genes influence each other, and it can be challenging to tease apart the effects of the environment from those of genetics. For example, if a person's level of intelligence is similar to that of their parents, is that similarity due to genetic factors passed down from parent to child, to shared environmental factors, or (most likely) to a combination of both? It is clear that both environmental and genetic factors play a part in determining intelligence.

Scientific journal articles for further reading

Plomin R, Deary IJ. Genetics and intelligence differences: five special findings. Mol Psychiatry. 2015 Feb;20(1):98-108. doi: 10.1038/mp.2014.105. Epub 2014 Sep 16. Review. PubMed: 25224258. Free full-text available from PubMed Central: PMC4270739.

Plomin R, von Stumm S. The new genetics of intelligence. Nat Rev Genet. 2018 Mar;19(3):148-159. doi: 10.1038/nrg.2017.104. Epub 2018 Jan 8. PubMed: 29335645. Free full-text available from PubMed Central: PMC5985927.

Sniekers S, Stringer S, Watanabe K, Jansen PR, Coleman JRI, Krapohl E, Taskesen E, Hammerschlag AR, Okbay A, Zabaneh D, Amin N, Breen G, Cesarini D, Chabris CF, Iacono WG, Ikram MA, Johannesson M, Koellinger P, Lee JJ, Magnusson PKE, McGue M, Miller MB, Ollier WER, Payton A, Pendleton N, Plomin R, Rietveld CA, Tiemeier H, van Duijn CM, Posthuma D. Genome-wide association meta-analysis of 78,308 individuals identifies new loci and genes influencing human intelligence. Nat Genet. 2017 Jul;49(7):1107-1112. doi: 10.1038/ng.3869. Epub 2017 May 22. Erratum in: Nat Genet. 2017 Sep 27;49(10 ):1558. PubMed: 28530673. Free full-text available from PubMed Central: PMC5665562

Sternberg RJ. Intelligence. Dialogues Clin Neurosci. 2012 Mar;14(1):19-27. Review. PubMed: 22577301. Free full-text available from PubMed Central: PMC3341646

Why Twin Studies?

Twins provide a valuable source of information for health and psychological research, as their unique relationship allows researchers to pull apart and examine genetic and environmental influences.  Twin study findings have been influential in detecting and treating various diseases and psychological disorders. 

How are they able to do this?  Twin studies allow researchers to examine the overall role of genes in the development of a trait or disorder.  Comparisons between monozygotic (MZ or identical) twins and dizygotic (DZ or fraternal) twins are conducted to evaluate the degree of genetic and environmental influence on a specific trait.  MZ twins are the same sex and share 100% of their genes.  DZ twins can be the same- or opposite-sex and share, on average, 50% of their genes.

If MZ twins show more similarity on a given trait compared to DZ twins, this provides evidence that genes significantly influence that trait.  However, if MZ and DZ twins share a trait to an equal extent, it is likely that the environment influences the trait more than genetic factors.  For example, Figure 1 presents MZ and DZ twin correlations for several physical and psychological characteristics.  As shown in the figure, MZ twins, on average, are twice as similar as DZ twins for these characteristics, suggesting that genes influence the development of these traits.

In addition to traditional twin studies, children-of-twins studies are a particularly useful way to examine genetic and environmental influences on traits and disorders and their transmission across generations.  For example, since MZ (identical) twins share 100% of their genes, the children of MZ twins are as genetically related to their twin parent’s twin sister or twin brother as they are to their parent.  Given this, their cousins through that aunt or uncle are like genetic half-siblings.  As these cousins typically live in different households, this relationship allows for a unique perspective into possible interactions between genetic influences and environmental factors. 

Results from twin studies may suggest that a particular trait or disorder has a genetic component, but this does not provide information about the location of this gene or genes.  Nevertheless, twin designs and methods are extremely useful for understanding the extent to which psychological and medical disorders, as well as behaviors and traits, are influenced by genetic factors.  This information can then be used to develop better ways to prevent and treat disorders and maladaptive behaviors.  Indeed, some of the most effective treatments for medical disorders (e.g., early onset breast cancer) have been developed partly as a result of twin study research.

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