By Mark Leary, Ph.D., Duke University
Nature versus nurture—genetics versus the environment—is an old debate in psychology. When it comes to understanding human behavior, what’s most intriguing is the relationship that nature and nurture share. Learn about the interplay between the two factors and the resulting expressions of personality.
The Relationship between Genes and the Environment
Beginning with a concrete example, the attribute of height is well known to be partly genetically determined. Taller parents tend to have taller children. What, then, is the heritability for height? What proportion of the observed variability in people’s heights is due to genetics?
People are often surprised to learn that the variability depends on the group of people being studied. In the United States, the heritability for height is about 0.87. That is, among adults in the United States, 87 percent of the variability in how tall they are is due to their genes.
This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on Wondrium.
In Nigeria, however, the heritability of height is only 0.62—only 62 percent of the variability in height observed in Nigeria is due to genetic differences. That proportion is much lower than in the United States. Does that mean that genes for height work differently in Nigerians than in Americans?
Not at all. The above statistic means that environmental influences play a larger role in determining people’s height in Nigeria than in the United State. Or to say it differently, people in the United States don’t vary as much in the environmental factors that affect height as people in Nigeria.
Most people in the United States get enough to eat and most babies are not malnourished. In Nigeria, some people get enough food, but many people do not. Environmental factors affect differences in height more in Nigeria than in the United States.
Because many people in the United States have adequate food, nutrition doesn’t exert much of an effect on how tall people are, so most of the observed differences in height—87 percent to be precise—are due to genetics. In Nigeria, the environment plays a bigger role, so only 62 percent of differences in height are genetic.
Learn more about the identification of DNA as the carrier of the genetic code
Human Behavior Uncovered
The above heritabilities are only estimates. When a characteristic has a heritability above zero, researchers know that genetic influences are operating, but they cannot always specify exactly how large that influence is because it depends on the group they are studying. Of course, if we get about the same heritability estimate no matter what group we test, we can be more confident of the precise value.
How do researchers figure this out—how do they determine the heritability of a personality characteristic? They have several methods, and many of them are based on comparisons of the personalities of twins.
(Image: Aldona Griskeviciene/Shutterstock)
Twins come in two varieties—identical or monozygotic twins, and fraternal or dizygotic twins. Of course, identical twins are perfectly genetically identical because they came from the same fertilized egg.
In contrast, fraternal twins share only 50 percent of their genes, just like ordinary brothers and sisters do. By comparing the personalities of pairs of monozygotic and dizygotic twins, researchers can estimate the heritability of personality characteristics.
Researchers are particularly interested in comparing twins who were raised together by the same parents, to twins who were raised separately by different families. By comparing twins who were raised together to twins who were raised apart, researchers can more cleanly separate genetic influences on personality from environmental ones.
If identical twins who were raised by different families since birth have similar personalities, then those similarities are probably because they have the same genes—since the environments in which they were raised were different.
Learn more about the complex processes by which nature and nurture interact to create differences in siblings
Hundreds of twin studies have been conducted around the world, many of them with several thousand pairs of twins. Researchers who study twins often maintain registries of pairs of twins who are willing to be studied again and again. So there is a great deal of data available that allows scientists to separate genetic from environmental influences on twins’ personalities.
What Have We Found about Human Behavior?
(Image: Sangoirii/Shutterstock)
Let’s start with the two most important personality traits—extroversion and neuroticism. These traits are the most important because as we look at differences among people, how extroverted and neurotic people are, relates more strongly to their behaviors than do any other personality traits.
Extroversion involves the degree to which people are talkative and sociable. People who score high in extroversion are outgoing, enjoy interacting with other people, and seek out exciting, stimulating activities. People who are low in extroversion—we often call them introverts—are quiet, less outgoing, and sometimes shy.
Research on tens of thousands of twins shows that the heritability of extroversion is somewhere in the vicinity of 0.5 to 0.6, depending on the sample of people studied. That means that about 50 percent of the variability that we observe in how extroverted versus introverted people are is due to genetic factors.
Neuroticism involves how much people experience negative emotions. People who are high in neuroticism tend to be more moody, emotional, anxious, and more prone to stress than people who are low in neuroticism. People low in neuroticism show greater emotional stability.
Learn more about the extent that genes explain qualities such as intelligence, aggression, or introversion/extroversion
Research shows that the heritability of neuroticism is somewhere in the range of 0.3 to 0.5, depending on the sample. In other words, when we look out and see differences in how emotionally stable versus emotionally volatile people are, a little less than half of those differences in neuroticism—30 to 50 percent—can be traced to genetic factors.
Attitudes and Values in Our DNA?
One of the biggest surprises in behavioral genetics was the discovery that attitudes and values also have genetic underpinnings. Traditionally, psychologists have assumed that values and character traits—such as integrity and compassion—are instilled by parents, teachers, and religions. They are in part but genetic influences are also at play.
For example, researchers estimate that traditional, conservative values and attitudes—for example, conservative attitudes toward the death penalty, gay marriage, and censorship—have a heritability of 0.59. In other words, about 60 percent of the variability that we see in basic political attitudes has a genetic component.
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That may help to explain why, no matter where you fall on the political spectrum from conservative to liberal, you often can’t understand how people could hold the other viewpoint.
We don’t have much conclusive evidence about how people’s political attitudes are influenced by their genes. The biological mechanism or process by which genes contribute to the making of a person with conservative or liberal views—is one of the mysteries of behavior we haven’t solved.
But we can imagine certain possibilities. We know liberals and conservatives differ on many personality characteristics that we know have a genetic basis. Perhaps those personality characteristics lead people to resonate more with certain political attitudes. For example, research shows that liberals and conservatives differ in the degree to which they are tolerant, practical, orderly and organized, stubborn, open to new experiences, and easily threatened.
Learn more about why people who live in different environments may develop different personalities
Perhaps people with a certain configuration of personality characteristics will naturally find certain political perspectives more appealing than other perspectives. These heritable differences in personality may be responsible for the fact that political attitudes are heritable. Genes can, therefore, influence not only specific personality characteristics but also complex patterns of attitudes and behavior.
Common Questions About Nature Versus Nurture
When discussing whether one’s behavior or attributes are a result of nature or nurture, we are asking whether the person’s genetics (nature) or the person’s environment and upbringing (nurture) have more of an impact on the quality being discussed. For instance, psychologists have often explored the question of whether depression is more closely linked to a family history of depression or external events such as childhood abuse and/or neglect.
Physical traits such as skin color and eye color are examples of nature—that is, they are genetically passed down. Other things, such as life expectancy and cancer, are more complicated in that while they are influenced by genetics, one’s life choices also play a role.
IQ is a combination of nature and nurture, but the influence of nurture (environment) is strongest for children under the age of 12. This is because a person’s brain has a high degree of plasticity during these early years, and therefore parents need to provide a stimulating, intellectually-engaging environment for their children.
Aggression is generally believed to be a learned behavior. However, new studies have found a defective gene that can be linked to aggressive behavior in some males.
