Are genetics destiny? Simplistic thinkers may say so, but what scientists are learning is that, though many traits are heritable (e.g., height, extraversion, IQ), it is difficult to find a “gene” for the vast majority of traits. At the completion of the human genome project, there were high hopes that single genes could be located that are associated with human behavior, and in particular disease. With 70 percent of genes being expressed in the brain, genes are a good place to look to find potential causes, and more likely, risk factors, for psychiatric disorders. Psychologists and psychiatrists have long known that mental health is complicated, it changes across time, and in response to experience. Thus, a 1:1 ratio for gene to disorder is extremely unlikely.
There are numerous issues with gene association studies in which a single genetic polymorphism is compared to a behavior.
- Behavior varies, even within individuals based on factors like age, time of day, and environmental factors;
- Assessments of behavior are imperfect, and in particular, measures of abstract traits like emotion regulation or oppositional behavior are biased by the reporter of such behavior;
- The effects of single genes are often quite small, and require a very large sample size to accurately demonstrate the gene’s true effect;
- Grouping individuals together based on disorder status (e.g., people with and without ADHD) is problematic given both the dimension nature of most psychiatric disorders and that there are multiple etiological pathways to an ADHD diagnosis (including abuse and neglect during childhood); and, most importantly
- Genes are not expressed at the level of behavior. DNA makes RNA, which makes proteins, which on down the line affect neural circuits, and later on further, behavior.
Within all of this, we experience the world. We learn to avoid things associated with negative outcomes and are attracted to experiences with a positive reward. We have parents (or not), siblings (or not), friends (or not), school (or not), a home (or not), a neighborhood (or not), and each of these, and many many other external factors, shape our brain as well. So, let’s not be so simplistic to think genes are who we are. I like to think of DNA as the blueprint, and very many contractors impact the resulting outcome, which we have to remember, is a construction project that lasts an entire lifetime!
Viding E, Williamson DE, Hariri AR (2006) Developmental imaging genetics: challenges and promises for translational research. Dev Psychopathol. 18(3):877-92.