4.1: Nature v nurture research

In the section on "Issues in Development" we talked about how nature v. nurture is one of the basic, age-old questions or issues that come up in developmental science. People often ask, “Are we the way we are because of our genes or our environment?” We could make this question slightly less ridiculous and more manageable by narrowing down the question to something like “Are we as tall as we are because of our genes or our environment?” or “Are we as musical as we are because of our genes or our environment?” It becomes immediately clear that even if we are inclined to answer questions such as these, we would answer each of these last two questions differently. Perhaps, most of us would respond by saying that the answer to the first question is genes and the second, environment.

However, most psychologists today agree that this question is highly archaic, and unreasonable. They argue that asking this question is like asking whether a piece of music that you hear is coming from the musician or the instrument she is using. They would argue that those two are completely impossible to disentangle.

Yet many people persist in asking the question, and certainly it is clear that our abilities, traits, disorders, body parts and the way they function are all a result of our genes to some extent and the environmental influences that have acted upon us to some extent. The question that most people ask today is not “Is it genes OR environment that is responsible for…?” but rather “To what extent is the variability in … influenced by genes versus the environment?” So first we acknowledge that both play a role in whatever ability or trait we are looking to discuss. Then we want to get more specific and ask which plays a greater role and to what degree.

In order to find out whether genes play an important role in particular characteristics, researchers perform several different types of research:

Twin research

In this type of research, psychologists would study pairs of identical/monozygotic twins, and pairs of fraternal/dizygotic twins. The idea is that firstly, identical twins share a hundred percent of their genes, while fraternal twins share on average approximately fifty percent. So if identical twin pairs are more similar in whatever trait is of interest to the psychologist than fraternal twin pairs are, then that means that genes play a more important role in influencing that trait. For example, if you were to take ten pairs of identical and fraternal twin pairs each and found that the identical pairs’ musical abilities were more similar to each other, and that the fraternal twin pair’s musical abilities were not as similar as those of the identical pairs’, we would assume that difference between identical and fraternal twins is attributable to genetic differences. There are studies for example looking at “concordance rates” of psychological disorders like autism. In these studies the question that researchers are asking is “Given that one sibling is diagnosed on the Autism Spectrum (ASD), how likely is it that the other sibling will also be diagnosed with ASD?” For example, in a study with 277 twin pairs, ASD concordance was 31% for DZ and 88% for MZ twins (Rosenberg et al., 2009).

Adoption studies

Clearly when children are adopted into non-biologically related families, it is a clear opportunity to measure the similarity of their traits to those of their biological parents and compare those to the similarity with their adoptive parents. Early studies showed fairly large differences between the low IQ scores of biological parents and the much higher scores of their children adopted away. But subsequent research argued that much of those differences might be accounted for by mis-measurement of the biological parents’ IQ, the Flynn effect and the fact that when children were re-measured at a later point, their IQ was found to be lower. So, in this context, it was suggested that it might be better to compare the IQ of siblings, one who had stayed with the biological parent and one who had been adopted away. It was found that there was a significant difference between those comparison groups on average (Kendler et al., 2015). These adoption studies are considered a powerful tool for analyzing the effects of genetic versus environmental influence on traits.

Genomic examination

Human cells contain approximately 20,000 genes. The function of the Human Genome Project was to decode the DNA and identify the genes as well as continue to try and figure out which genes are responsible for what traits. Several genes have known or expected effects, and genomic studies examine the interplay of genetic and environmental factors in psychological outcomes. For example, Kaufman et al (2006) examined the interplay of two genes - a neurotrophic (neuronal food) gene variant and a serotonin protein expresser gene variant – and child abuse on depressive symptoms in children. In this study a particular version of the BDNF and 5HTTLPR gene predicted highest depression, but only in children who had suffered abuse, and this outcome was moderated in those children who had social support.

Population comparisons

Behavioral geneticists have also looked at the differences between particular populations on different psychological outcomes to see whether a particular genetic variant might be more common among a particular ethnic or cultural group. For example, Gregersen, Kowalsky, Kohn and Marvin (2001) studied the gene for “absolute pitch” that appears to more commonly appear in individuals of Chinese, Japanese and Korean ethnicities. They examined whether this could be accounted for by early childhood music education or the exposure to tonal languages (which Mandarin is, but Japanese and Korean are not).

Conclusions

But the underlying assumption in many of these approaches is that the effects of genes and environment are additive. When we say that the concordance rates of schizophrenia are 48%, the assumption is that the other 52% variability is accounted for by the environment.

However, it is important to realize that the effects of genes and environment are escalative in nature. It is not a simple additive sum of the two like for example if you were to add oil to a cup and then add some water, the level of liquid in the cup would increase of course, but the two would remain dissociable always. Clearly that is not the case with human traits and characteristics. We are an indissociable blend of our genetic and environmental influences, as many of the studies already cited above have already begun to indicate.

References:

Gregersen, P. K., Kowalsky, E., Kohn, N., & Marvin, E. W. (2001). Early childhood music education and predisposition to absolute pitch: teasing apart genes and environment. American journal of medical genetics, 98(3), 280–282. https://doi.org/10.1002/1096-8628

Kaufman, J., Yang, B., Douglas-Palumberi, H., Grasso, D., Lipschitz, D., Houshyar, S., Krystal, J. H., & Gelernter, J. (2006). Brain-Derived Neurotrophic Factor–5-HTTLPR Gene interactions and environmental modifiers of depression in children. Biological Psychiatry, 59(8), 673-680. https://doi.org/10.1016/j.biopsych.2005.10.026.

Kendler, K.S., Turkheimer, E., Ohlsson, H., Sundquist, J., & Sundquist, K. (2015). Family environment and cognitive ability. Proceedings of the National Academy of Sciences, 112 (15), 4612-4617. doi: 10.1073/pnas.1417106112

Rosenberg, R.E., Law, J.K., Yenokyan, G., McGready, J., Kaufmann, W.E., Law, P.A. (2009), Characteristics and concordance of autism spectrum disorders among 277 twin pairs. Arch Pediatr Adolesc Med., 163(10),907-14. doi: 10.1001/archpediatrics

Sahu M, Prasuna JG. Twin Studies: A Unique Epidemiological Tool. Indian J Community Med. 2016 Jul-Sep;41(3):177-82. doi: 10.4103/0970-0218.183593. PMID: 27385869; PMCID: PMC4919929.

Attributions:

Child Growth and Development by Jennifer Paris, Antoinette Ricardo, and Dawn Rymond, 2019, is licensed under CC BY 4.0

Lifespan Development: A Psychological Perspective by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0 (modified by Jennifer Paris)