There was a great article in yesterday’s NYTimes by Gretchen Reynolds on the genes underlying motivation to exercise. Strong evidence from both animal and human studies suggests that there is a genetic proclivity to exercise, making some of us excited, eager and willing to engage in physical activity while others decidedly not so. I could write a lot about this topic, but that’s not the point of this post. Rather, I want to focus on something else Ms. Reynolds touched on: twin studies. In her article, she notes that the genetic predisposition to be motivated to exercise is supported by studies in twins who tend to engage in similar amounts of physical activity regardless of disparate current environments or upbringings.
This point illustrates perfectly the practice of studying twins to try and elucidate the difference between nature and nurture in biological and behavioral variables such as aging, personality, and lifestyle. Both monozygotic (MZ) and dizygotic (DZ) twins are typically enrolled in research trials to distinguish between the contribution of genes and environment (see picture on right for a good illustration of the utility of this design). Indeed, there are numerous ongoing twin registries throughout the world (e.g., Finnish Twin Study, Italian Twin Study, Georgia Cardiovascular Twin Study) that have provided datasets to study outcomes ranging from cognitive function to grip strength.
With such an extensive twin research network, it seems that it would be easy to differentiate between traits and characteristics with a genetic basis versus those influenced by the environment. But the reality is much more complicated. Results from twin studies are often highly variable, typically because very few human behaviors or outcomes are attributable to genes OR environment alone. Rather, most are a mix, with a certain percentage of the trait genetically predetermined and another percentage malleable to external influences.
More recently, scientists have focused on epigenetics, the modification of the genome by chemical reactions that switch genes on and off throughout the lifespan. These epigenetic tags, or changes, can be altered by behavior and then passed down to subsequent generations. In other words, our experiences and lifestyles influence not only our DNA but that of our offspring. Genetic destiny should be considered a rough draft rather than a final plan, and the debate becomes not nature versus nurture but how best to nurture our nature.