The old debate about nature versus nurture has evolved into a nuanced conversation about “hereditability.” Both genetic and environmental factors play a role in human behavior, and researchers have explored many facets of the “gene-environment correlation.”
To Smoke or Not to Smoke
Before we examine the “gene-environment” correlation, let’s start with an example. One area in which the effects of genes on behavior have been studied involves smoking cigarettes and drinking alcohol. Of course, smoking and drinking are influenced by many factors, but genes play a role.
While there isn’t a gene for smoking, research suggests that people with certain patterns of personality are more likely to smoke than people with other personalities.
For example, research has shown that people who are higher in extroversion are more likely to smoke, as are people who score higher in neuroticism. One of the strongest predictors of smoking is the degree to which people enjoy and seek out stimulating activities, such as loud music, big parties, and risky activities. Of course, nicotine is a stimulant, so having a desire for stimulation may make smoking—and thereby inhaling nicotine—more enjoyable.
This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on Wondrium.
The results for drinking alcohol are somewhat mixed and inconsistent, but looking at all of the studies that have been conducted as a whole, the heritability for drinking alcohol is about 0.4.
The heritability of alcoholism is even higher—somewhere above 0.5. In other words, about 50 percent of the variability in whether people develop alcohol dependencies has a genetic basis.
Love and Marriage
Whether people get married—and whether they stay married—also have genetic underpinnings. A study at the University of Minnesota involving over 2,000 pairs of twins found that the heritability of marriage is 0.68. That’s huge. While some people get married and some people don’t, 68 percent of that difference is due to their genetic makeup.
Researchers cannot say precisely which inherited characteristics promote getting married or deter people from doing so. Presumably, some of the characteristics involve people’s motives and preferences, such as the degree to which they desire intimacy.
Learn more about the three main “love systems” in the brain
Genes also play a role in how well people’s marriages go. For example, women who are optimistic, warm, and low in aggressiveness are happier with their marriages than women who are less optimistic, less warm, and more aggressive, and those characteristics are partly heritable.
People who are optimistic, warm, and easy-going see the world differently than people who are pessimistic, cold, and hostile. All other things being equal, positive people will have more positive reactions to things that happen in their marriages.
Furthermore, optimistic, warm, positive people create happier marriages than pessimistic, cold, and angry people do. Studies show that people with positive personality characteristics are not only happier with their marriages, but their spouses are happier too.
Given the effects of genes on marital satisfaction, perhaps it’s not surprising that the likelihood of getting divorced is heritable as well. The heritability of divorce is estimated to be between 0.3 and 0.4. Thirty to 40 percent of people’s decisions to stay together or get divorced can be traced to their genes.
Research suggests that much of this effect has to do with characteristics that involve negative emotionality. If your genetic makeup predisposes you to be moody, hostile, or pessimistic, you will have more challenges in your close relationships, and you’ll be more likely to divorce.
The Environment and Human Behavior
Overall, across the dozens of traits that have been studied, most personality characteristics have heritability coefficients between 0.2 and 0.5, between 20 and 50 percent of the variability researchers observe across people has something to do with genes.
What is being inherited? Strictly speaking, it’s not actually your personality. Personality is just a description of tendencies and behavior patterns. If you are outgoing and enjoy going to parties, we might say you have an extroverted personality, but that’s just a label for patterns of behavior.
What’s being inherited is not a trait but rather the genes that influence your nervous system to respond in particular ways. Just as you may have inherited good or bad eyesight, a sensitive stomach, or flat feet, you may have inherited a brain that’s structured in ways that make certain emotions, thoughts, and behaviors more likely than others. This inheritance leads you to behave in the consistent patterns that we call personality.
Things get a lot more complicated, because not only do genes have direct effects on people’s behavior and create a brain that tends to respond in particular ways, but genes can also have indirect effects on personality by creating changes in people’s environments.
Each of us plays a role in creating our environment and life situation, and genes are partly responsible for the choices that we make in life. Our genes help to create our environment, and then that environment can influence our personality.
Scientists call effects in which genes affect people’s environments “gene-environment correlations.”
There are two kinds of gene-environment correlations. Consider each, and think about the degree to which your personality, which was partly determined by your genes and their effect on your brain, may have led you to behave in ways that then changed the environment around you, which then affected your personality.
Gene-Environment Correlation: Active
One effect genes have on people is what scientists call an active gene-environment correlation. It’s active because genes influence behavior in a way that leads people to seek out and construct certain situations. Then those situations that they create or choose for themselves influence their personalities.
For example, one highly heritable trait is activity level. Consider how some newborn babies are wiggling all over the place while others are much more sedate. The heritability of activity level is about 0.4. As these babies get older, the more active ones are going to be more interested in running all around, kicking balls, and playing games. When they’re the age that kids start playing sports, they’re going to be the first in line.
The children who are not as active grow up doing less active things, and they simply aren’t as interested in playing soccer or learning karate, for example. They spend more time reading or playing on their computer or hanging out with their friends.
Differences in their initial activity level have led these groups of kids into different pursuits. Those different pursuits then influence their personality differently. You learn different things and develop different skills from playing sports than from reading or playing on the computer. The activities that the children chose—activities that were influenced in part by their genes—changed their personality down the road.
Gene-Environment Correlation: Reactive
The second kind of gene-environment correlation is called a “reactive” or evocative gene-environment correlation. It’s called evocative because genes influence a person’s behavior in ways that evoke certain reactions from other people.
Let’s consider the trait of irritability. People differ in how easygoing versus irritable they are. Again, these differences are heritable. Consider the different reactions that irritable versus easygoing children consistently evoke from other people—from their parents, their teachers, and even from other kids. Irritable children will consistently evoke more negative responses from other people than easygoing kids will. Adults will be angry with them more often, other people will want to interact with them less because they are unpleasant. They’ll consistently create a less pleasant social environment around them than kids who are pleasant and easygoing. Over time, being exposed to less positive feedback from other people will influence their personality.
Another example of a reactive gene-environment correlation occurs when parents create environments that match their child’s personality. If a parent sees that a child enjoys music but doesn’t like sports, the parent will be more likely to buy the child things that involve music rather than sports, encourage music lessons rather than team sports, and so on. The child’s genes fostered behavior that led parents to create a different environment, which then has downstream consequences for the child’s personality.
These effects of genes on people’s social environments are cumulative. One initial genetic difference in a child’s activity level, irritability, or some other characteristic, can start a sequence of behaviors and events that cascade for the rest of the person’s life, building on each other over time. A relatively small genetic difference at birth can become compounded year after year.
This process continues throughout people’s entire lives. Today, no matter how old you are, the ways that you tend to react creates a different environment around you than if you had a different sort of personality. Of course, personality is affected less by these things as we age, but you are nonetheless creating your living situation through your personality even today.
Learn more about how our cognitive functions change over our lives
Part of the difficulty in understanding how personality develops is that the effects are complex, depending not only on many genes operating together and the many kinds of environments in which children grow up, but also on interactions between genes and the environment. In biology class, students may learn mostly about simple genetic effects, such as those that affect eye color, but when talking about personality, we’re talking about genetic effects happening on many parts of the brain simultaneously.
There’s not a single gene for extroversion, shyness, hostility, or getting divorced. These characteristics are due to patterns of genes interacting with complex environments throughout life.
We now know with certainty that genes have a pronounced effect on people’s personalities. Some personality traits and behaviors are directly caused by genetic influences on the brain. Others are caused indirectly, as a result of the complex ways that our genes shape and change our behavior and our environments.
By exploring these effects, science has taken us beyond the old nature-nurture debate and given us rich insights into human personality. As for the precise biological mechanisms, the many complex processes by which genes influence behavior remains a mystery.
Common Questions About Gene-Environment Correlation
There are three types of gene-environment correlations: active (preference for environment will be a reflection of genetic makeup), evocative (one person’s behavior induces a response from their environment such as between a husband and wife), and passive (a person’s environment as a child is influenced by the genetic makeup of the parents which influences their choices in life).
There are many ways in which the environment can change genes. Epigenetic factors are factors above the genetic sequence that flip genes on or off. There are also mutagens (pollutants), gene-gene interactions (one gene affects another which causes a change), and transcription factors (when environmental factors indirectly affect genes to cause an overall change in the organism).