Most of the characteristics of interest to psychologists such as personality are influenced by

By Mark Leary, Ph.D., Duke University

Human behavior is a fascinating field of study. Anyone who has ever taken a psychology class is familiar with the nature-nurture debate. Explore our latest understanding of this dichotomy and what we know about human behavior and personality.

The Debate About Personality

How does the nature-nurture debate apply to human behavior and personality? Think for a moment about all of the people that you know. In particular, think about how the people you know differ from each other.

We all know certain people who are very extroverted: people who are outgoing, talkative, and sociable. We also know people who are just the opposite: quiet, shy, and reserved.

This is a transcript from the video series Understanding the Mysteries of Human Behavior. Watch it now, on Wondrium.

You probably know people who are warm and agreeable, and other people who are often unfriendly or even hostile. Some people are dependable and conscientious, and while others can hardly be counted on for anything. We all know ethical people and others who do all sorts of shady things.

No matter what personality characteristic we can think of, the people we know vary from one another.

Where does all of this variability in personality come from? Why are people so different, and how did they end up the way they are? For that matter, why did you turn out the way that you did? Think about the central personality characteristics that you have. Where did they come from?

These questions are complicated and have complex answers, but for the past hundred years or so, researchers in the field of personality psychology have been trying to figure it all out. Behavioral scientists have made great strides in understanding where individual differences in people’s personalities come from, but the processes are so complex that we still have many unanswered questions.

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Nature-Nurture and Personality

For many years, the question was framed simplistically as the so-called nature-nurture debate. Is personality due mostly to nature—that is, the genetic disposition with which a person is born—or is it due mostly to nurture—the circumstances in which a person is raised? Researchers fought about the answer to this question as if the resolution were binary and simple, but it is not.

Science has swung back and forth on the answer to the nature-nurture debate over the years. In the first half of the 20th century, the widely accepted answer among psychologists was that most differences among people are learned, and therefore due to nurture.

In fact, John Watson, the founder of behaviorism, said that if you gave him a dozen healthy babies, he could take any one of them at random and train him or her to become any type of person—that he could mold people’s interests, motivations, emotions, abilities, and traits into whatever he chose. The idea that people are born more or less as a blank slate, and gain their personalities through experience and learning, dominated American psychology for many years.

As researchers dug more deeply into the nature of personality, they concluded that Watson was wrong. Personality is certainly influenced by what happens to people—how they are raised, what they learn, and their personal experiences. Personality is also undoubtedly influenced by a person’s inborn biological makeup.

The nature-nurture debate is dead. Both nature and nurture—both genetic and environmental influences—play a role in the development of personality. The scientific field that studies the question of where differences in personality come from is known as behavioral genetics.

Researchers in behavioral genetics are interested in the non-genetic determinants of personality, as well. Genetic influences can’t be studied without considering non-genetic factors; they’re all connected.

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It’s a little like asking what factors are important in baking a cake—the ingredients (nature) or how the cake mix is prepared and baked (nurture). You can’t have a cake without both ingredients and a high temperature, and you can’t have a personality without both genetic factors and personal experiences that occur within certain environments.

The Differences in Our DNA

Researchers in behavioral genetics are interested in both genetic and environmental influences on personality, as well as the interplay between the two.

Significantly, more than 99 percent of every person’s genes are identical to those of every other person. You share 99 percent of your genes with all other human beings. You share about 98 percent of your genes with chimpanzees, our closest relative in the animal kingdom.

The genetic similarities among all people and between humans and other animals are important and interesting, but that other 1 percent of our genes—less than 1 percent actually—are the genes that make each of us look and act differently from other people. Those genes underlie differences in people’s personalities.

This one-percent genetic difference is important because if one says you share 50 percent of your genes with your mother, it actually means you share 50 percent of the genes that differ among people. You share over 99.5 percent of your genes with your mother.

There is no way a scientist can come to your house, study you intensely, and then tell you the degree to which your specific personality has been influenced by various genetic and environmental factors.

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What researchers can do, however, is examine the extent to which certain personality characteristics—such as extroversion, stubbornness, or gullibility—are affected by genetics in general, across all people. Looking at the way people vary in those characteristics, researchers can determine how much of that variability across people is due to genetic versus environmental factors.

Digging into the Genes

Behavioral geneticists express the degree to which a personality trait is influenced by genes using a statistic called “heritability.” Heritability refers to the proportion of the observed variability in a group of individuals that can be accounted for by genetic factors. That is, what proportion of all the variability that exists in a personality trait—such as extroversion or greediness—is due to people’s genes?

When you think about how the people that you know personally differ from each other, you see lots of variation. Some people have shy personalities, some people have sociable personalities, and so on. Heritability shows, rather precisely, how much of the variability that exists in people’s personalities is due to genetic factors.

Geneticists define heritability more technically; they say that heritability is the proportion of phenotypic variance that is attributable to genotypic variance. Phenotypic variance is the variability that researchers observe in a trait or characteristic, and genotypic variance is variability in people’s genes.

When we say that heritability is the proportion of phenotypic variance that is attributable to genotypic variance, we’re saying that it’s the proportion of the total variability that we observe in a characteristic that’s due to variability in genes.

For instance, out of all of the variability that exists across people in their degrees of stubbornness, what proportion of that variability is due to genetic influences?

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If the heritability of a characteristic, such as stubbornness, is zero, then none of the variability in that trait is due to genetic factors. On the other hand, if the heritability is 1, then all of the variability we see in stubbornness is genetic. If heritability is 0.2–0, then 20 percent of all the variability among people in stubbornness is genetic.

Heritability is not a constant value across all groups, situations, and times. Remember that heritability is the proportion of observed variability that is due to genetic variability. If everybody grew up in exactly the same environment, heritability will always be very high because none of the variability in their personality would be due to differences in their environment.

Common Questions About Nature-Nurture and Personality

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