Behavior and Genetics: Learn It 1—The Nature-Nurture Question

  • Explain the nature vs. nurture debate
  • Understand the basics of genetic variation
  • Describe how the environment can influence or interact with genetics

nature vs. nurture

Are you the way you are because you were born that way, or because of the way you were raised? Do your genetics and biology dictate your personality and behavior, or is it your environment and how you were raised? These questions are central to the age-old nature-nurture debate. In the history of psychology, no other question has caused so much controversy and offense: we are so concerned with knowing the answer because we want to be able to easily pinpoint the success (or failures) of a person to their genetic gifts or to their hard work and dedication. The problem is, most human characteristics aren’t usually clear-cut so we cannot affirm our nature–nurture expectations strongly one way or the other.

Two similar-looking puppies.
Figure 1. Researchers have learned a great deal about the nature-nurture dynamic by working with animals. But of course many of the techniques used to study animals cannot be applied to people. Separating these two influences in human subjects is a greater research challenge. [Photo: mharrsch]
One major problem with answering nature-nurture questions about people is, how do you set up an experiment?

In nonhuman animals, there are relatively straightforward experiments for tackling nature–nurture questions. Say, for example, you are interested in aggressiveness in dogs. You want to test for the more important determinant of aggression: being born to aggressive dogs or being raised by them. You could mate two aggressive dogs—angry Chihuahuas—together, and mate two nonaggressive dogs—happy beagles—together, then switch half the puppies from each litter between the different sets of parents to raise. You would then have puppies born to aggressive parents (the Chihuahuas) but being raised by nonaggressive parents (the Beagles), and vice versa, in litters that mirror each other in puppy distribution.

The big questions are: would the Chihuahua parents raise aggressive beagle puppies? Would the beagle parents raise nonaggressive Chihuahua puppies? Would the puppies’ nature win out, regardless of who raised them? Or… would the result be a combination of nature and nurture? Much of the most significant nature–nurture research has been done in this way (Scott & Fuller, 1998), and animal breeders have been doing it successfully for thousands of years. In fact, it is fairly easy to breed animals for behavioral traits.

With people, however, we can’t assign babies to parents at random, or select parents with certain behavioral characteristics to mate, merely in the interest of science (though history does include horrific examples of such practices, in misguided attempts at “eugenics,” the shaping of human characteristics through intentional breeding). In typical human families, children’s biological parents raise them, so it is very difficult to know whether children act like their parents due to genetic (nature) or environmental (nurture) reasons. Nevertheless, despite our restrictions on setting up human-based experiments, we do see real-world examples of nature-nurture at work in the human sphere—though they only provide partial answers to our many questions.

Adoption Studies

The science of how genes and environments work together to influence behavior is called behavioral genetics. The easiest opportunity we have to observe this is through an adoption study. When children are put up for adoption, the parents who give birth to them are no longer the parents who raise them. This setup isn’t quite the same as the experiments with dogs (children aren’t assigned to random adoptive parents in order to suit the particular interests of a scientist) but adoption still tells us some interesting things, or at least confirms some basic expectations. For instance, if the biological child of tall parents were adopted into a family of short people, do you suppose the child’s growth would be affected? And what might this outcome tell you about the difference between height in terms of nature-nurture?

Twin Studies

Identical twin boys look at each other, one with a straight face and the other with an open-mouth laugh.
Figure 2. Studies focused on twins have led to important insights about the biological origins of many personality characteristics. [Photo: ethermoon]

Another option for observing nature-nurture in humans involves twin studies. There are two types of twins: monozygotic (MZ) and dizygotic (DZ). Monozygotic twins, also called “identical” twins, result from a single zygote (fertilized egg) and have the same DNA. They are essentially clones. Dizygotic twins, also known as “fraternal” twins, develop from two zygotes and share 50% of their DNA—they are ordinary siblings who happen to have been born at the same time. To analyze nature–nurture using twins, we compare the similarity of MZ and DZ pairs. Sticking with the features of height and spoken language, let’s take a look at how nature and nurture apply: Identical twins, unsurprisingly, are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height.

Twin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called quantitative genetics, the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, with twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990; such twins are very rare and play a smaller role than is commonly believed in the science of nature–nurture), or with entire extended families (see Plomin, DeFries, Knopik, & Neiderhiser, 2012).

Mother splashing with daughter in a fountain.
Figure 4. Research over the last half century has revealed how central genetics are to behavior. The more genetically related people are the more similar they are not just physically but also in terms of personality and behavior. [Photo: 藍川芥 aikawake]

It may seem surprising to learn that genetic influence on behavior is a relatively recent discovery. In the middle of the 20th century, psychology was dominated by the doctrine of behaviorism, which held that behavior could only be explained in terms of environmental factors. That was disproved through research finding that the best predictors of an adopted child’s personality or mental health are found in the biological parents they have never met, rather than in the adoptive parents who raised them.

One important thing to keep in mind when drawing conclusions about the nature-nurture debate is that when subjects are biologically related, no matter how clearly a situation may seem to point to environmental influence, it is never safe to interpret behavior as wholly the result of nurture without further evidence. For example, if data shows that children whose mothers read to them often are likely to have better reading scores in third grade, it is tempting to conclude that reading to your kids out loud is important to success in school; this may well be true, but the study as described is inconclusive because there are genetic as well as environmental pathways between the parenting practices of mothers and the abilities of their children. This is a case where “correlation does not imply causation.” To establish that reading aloud causes success, a scientist can either study the problem in adoptive families (in which the genetic pathway is absent) or by finding a way to randomly assign children to oral reading conditions.