According to current popular ideas about the science of happiness, 10% of the variance in happiness in a given population is due to life circumstances (age, nationality, sex, race/ethnicity, and income); 50% is due to genetics; and 40% remains under our direct control. These numbers have been repeated in countless journal articles and books, and discussed as fact in college courses around the world.
These perfectly round numbers raise more questions than answers, and as of today, I still don’t trust them.
When you are given a graph that is this clean, it seems reasonable to be skeptical. For instance, where do we fit in information showing that for certain people, exposure to environmental factors such as toxins or nutriments can switch genes on or off? This is a point that my colleagues and I discussed in prior work (link is external):
Although traits may be substantially influenced by genetic factors, individuals select, utilize, and adapt to their environments, affecting gene expression. Behavior geneticists distinguish between gene–environment transactions and gene–environment interactions.
In most cases, the interface between genetics and environment consists of transactions necessary for growth, reproduction, or development. An example of a transaction in the case of character strengths might be the necessity of extensive social experiences to enable the full development of the genetic disposition for social intelligence or leadership.
The term gene–environment interaction refers to a specific process in which the same stimulus affects individuals with differing genes in different ways. Research has revealed several examples of gene–environment interactions that match the diathesis-stress model of psychopathology (e.g., adolescent conduct disorder; Cadoret, Yates, Troughton, Woodworth, & Stewart, 1995). Future investigations may uncover a diathesis-nutriment interaction for character strengths like creativity, in which those with a genetic predisposition respond to enriched, stimulating environments with augmented creativity, whereas those without the predisposition respond less robustly.
Future research should endeavor to delineate how the interplay of genes and environments creates character strengths. In addition, future research might attempt to investigate the presence of non-additive effects. Such effects are difficult to detect unless the samples are extremely large.
What about research showing that as we get older, the genetic influence on our thoughts, feeling, and behavior changes? Consider these meta-analytic findings (link is external) on various forms of intellect and cognitive functioning, from 12,721 sibling pairs; and on personality, from 21,057 sibling pairs (this is not a sample of 100 college students!):
In early life, shared environmental effects are the major stabilizing force for cognition. Over the first decade of life, the proportional contribution of genetic effects increases from approximately 10% to approximately 70%, and largely remains at that level for the remainder of the lifespan. The proportional contribution of the nonshared environmental effect gradually increases from nearly 0% to about 20% by late life.
In sharp contrast, genetic effects explain nearly 100% of the stability of personality in early life, but this proportion is slowly shaved away as the nonshared environmental contribution increases across the lifespan. By late adulthood, just over 50% of the stability of personality is attributable to genetic factors, with the remaining proportion attributable to nonshared environmental factors. Whereas genetically linked processes guide the stabilization of cognition, unique life experiences guide the stabilization of personality across the lifespan.
When I read this work, I am confused as to how scientists arrive at a single number to account for the genetic contribution to happiness. Genetic influences are often interwoven with environmental factors—check out this killer study (link is external) of how genomic expression in West African infants depends on whether they are born in the rainy or dry season—and they change depending on when you study someone on the timeline, from toddler to elder. Instead of taking a single equation as fact, ask questions about exactly where the numbers came from: How many studies? What was the quality of those studies? Challenge authority with questions, such as: Why should we expect the numbers to be similar across time in the same person, much less different people from different cultures?
Let me end with tips on how to critically think about behavioral genetics, from one of the leading thinkers in this field, Eric Turkheimer (link is external). He arrived at three laws of behavioral genetics—and as of last week, a 4th law has been added (link is external). Keep these laws in mind anytime someone tries to sell you on the importance of genetics.
Remember, these are not theories, they are laws:
1. All human behavioral traits are heritable. [That is, they are affected to some degree by genetic variation.]
What this means is that if you read about the god gene, the divorce gene, or research suggesting that the length and expression of a single gene explains your preference for The Spin Doctors (link is external), think again: Everything has a genetic contribution, so we should never get excited about the discovery of what is obvious. Complex human behavior is unlikely to be governed by a single gene. If one gene is too valuable, humans become too vulnerable. (See Law #4.)
2. The effect of being raised in the same family is smaller than the effect of genes.
3. A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.
Much of what influences our personality has to do with the presence of life events, both positive and negative, and our response to choice points: Do I approach or avoid my co-worker who regularly demeans me? Do I wake up early and work out or sleep in? Do I ask out the girl I’ve had a crush on for months or do I keep my feelings to myself? No single decision matters, but the patterns do. The decisions we make, the people we surround ourselves with, and the behaviors we engage in are the building blocks for the quality of our lives. Small changes accumulate over time leading to large changes in who we become.
4. A typical human behavioral trait is associated with very many genetic variants, each of which accounts for a very small percentage of the behavioral variability.
Instead of a single “divorce gene,” it makes more sense to consider a series of temperamental qualities that influence the probability of divorce, including self-control, impulsivity, behavioral inhibition, distractibility, adaptability, persistence, and mood. For instance, low self-control, high impulsivity, low behavioral inhibition, high distractibility, low adaptability, low persistence, and highly frequent and enduring bad moods is a suboptimal profile for a long, healthy romantic relationship. Each of these qualities has a strong genetic basis; together, they help explain why there is a genetic contribution to whether or not someone gets divorced.
Knowing the origin of what matters most to human beings is invaluable. Behavioral genetics is part of this origin story. Let’s take a moment to understand what a genetic contribution means and be skeptical of overly simplistic ideas about what humans do in the wild.