Nature vs. Nurture vs. Uh, Nanurture?

Nothing illustrates a more clear human bias than the importance placed on genes and inheritable traits for explaining our current behavior. We are a product of our parent’s and thus it is reasonable to fixate on their potential genetic influence playing out through our physical traits and even our behavior, however, as Robert Sapolsky explains in his book, Behave, the importance placed on genes in shaping our behavior, by researchers and thus society as a whole, is often misguided. To break a general misconception, genes are not autonomous in any sense of the word. The expression of genes and the subsequent production of proteins is regulated closely by transcription factors, which attach to a small chain of DNA called a promoter at the head of every gene to effectively turn it “on”. To complicate things, transcription factors are produced from genes, as they are proteins, genes can be influenced by multiple TFs and TFs themselves can activate more than one gene. Complexity, complexity, complexity. In addition to transcription factors, a basic “on/off” mechanism, there are also major portions of DNA that are noncoding called “introns”; the coding portions are called “exons”. Splicing enzymes, also proteins, regulate the combination of introns and exons that are coded for the desired protein and this is dependent on the transcription factors presently activating the promoter in the region of interest1. Wow. This is just scratching the surface, but my point and Sapolsky’s point is this; genes don’t determine that much and more importantly, the regulatory systems in place impacting gene expression respond directly to information from the environment. This includes both the environment in the cell and all that constitutes our surrounding environment.         

Concerning the nature vs. nurture debate, for those who remain in the far-naturist school, you have now seen how fundamentally complex genes and their regulation is. You have also seen how this regulation is primarily at the mercy of the environment. For those who are undetermined on their position in this debate, stick with me. My purpose in writing this is to stress that each conversation about the power of genetic influence should be framed by the fact that any effect on behavior is context-dependent. As Sapolsky writes, “…genes aren’t about inevitability. Instead they’re about context-dependent tendencies, propensities, potentials and vulnerabilities”1.

People have made inferences about genetic inheritance for millenia, far before any technology available could shed light on the inner mechanisms. This bias is rooted in far more than just science. This picture illustrates a pretty applicable example of this type of inference from the show “Game of Thrones”.

         You might be wondering, “Ok Sam, I get it, the environment is super important in determining our gene expression, what are we supposed to do with that?”  Enter gene/environment interactions. These are situations in which the gene and environment both have moderating effect on one another in eliciting a particular behavior. Basically, you ask “What does the environment have to do with a particular behavior?” the answer, “Depends on the genetic makeup of the person,” and vice-versa. Transcending the nature vs. nurture debate, these examples represent cases in which it is unhelpful to determine which has the greater influence, as these factors are related and reliant on each other to lead to a particular behavior.

         One particularly striking examples of a gene/environment interaction is illustrated in a few studies, one of which was conducted by Tucker-Drob et al. In this study, the heritability of genes that influence IQ was investigated in subjects at 10 months and 2 years of age. Heritability refers specifically to how strongly genes influence the variability of a trait passed on. In this case, researchers are interested in heritability because it gives insight into how much genes are determining why humans have higher mental ability than other humans and not why humans have higher mental ability than dogs. The researchers in this study found that at 10 months genes accounted for little to no variation in mental ability across all subjects. However, at 2 years, genes accounted for nearly 50% of the variation in mental ability of children raised in high-SES homes. However, at 2 years old, genes still accounted for little to no variation in mental ability of those subjects raised in low-SES homes2. Socio-economic status, a human construct and truly environmental influence can determine whether or not a person’s cognition can be affected by genetic influence. A low-SES environment can result in a lesser influence and therefore less ability to reap any heritable benefits.

         Another example concerns depression. This mental illness involves serotonin abnormalities, specifically in the clearing of excess serotonin from the synapse. The serotonin transporter molecule has this responsibility, and having a particular variant of the gene that codes for the production of this molecule can increase the risk of developing depression. However, this was only true for those subjects, with the genetic variant, that had experienced childhood maltreatment3.

         A final question, “How do we move forward with this information?” One review by Reiss, Leve and Neiderhiser poses an answer to this question, specifically related to gene/environment interactions and how we might best move forward in exploring these and applying this knowledge to prevention and intervention strategies in the future. In this review, the researchers break-down gene/environment interactions into four basic patterns of influence. From here, the major points made are that research of gene/environment interactions must; develop proper assessments of whatever environmental moderating factor that they are investigating, increase precision in lab settings so that the true nature of the interaction can be explored in a controlled setting, search for the initial biological mechanism that predisposed an individual to this gene/environment interaction, and develop processes to better understand the timing of these interactions4. This paper provides some valuable insights into the importance of gene/environment interactions as well as future directions that could really improve their existing impact on the development, prevention and intervention of many illnesses. The environment is so so so important in shaping who we are as humans and how we experience our lives.

References

[1] Sapolsky, R, Back to when you were just a fertilized egg, Behave, New York (2017) Penguin Press.

[2] Tucker-Drob, EM, Rhemtulla, M, Harden, KP, Turkheimer, E & Fask D, Emergence of a gene x socioeconomic status interaction on infant mental ability between 10 months and 2 years, Psych. Sci. (2011) 125-133.

[3] Caspi, A, Sugden, K, Moffitt, TE, Taylor, A, Craig, IW, Harrington, H, McClay, J, Mill, J, Martin, J, Braithwaite, A & Poulton, R, Influence of life stress on depression: Moderation by a polymorphism in the 5-HTT gene.

[4] Reiss, D, Leve, L & Neiderhiser, JM, How genes and the social environment moderate each other, Am. J. Public Health, (2013) 111-121.

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