Lesson 1.5 · Unit 1 · CED topic 1.1

Heredity, environment, and the evolutionary perspective

Why are you the way you are? A person's behavior and mental processes come from an inherited biological blueprint and from everything that happens to that person after birth. The old argument over whether "nature" or "nurture" is more important has been replaced by a better question: how do the two work together? This lesson sets out the terms, shows how researchers try to pull the two influences apart, and introduces the evolutionary perspective, one of the major frameworks you will use to explain behavior throughout the course.

Nature, nurture, and their interaction

Heredity, or "nature," refers to the genetic or predisposed characteristics a person inherits from biological parents, which influence physical, behavioral, and mental traits and processes. Eye color is almost entirely hereditary, but so are tendencies toward certain temperaments and some risk for conditions such as depression. Environmental factors, or "nurture," refers to the external influences a person experiences: family interactions, education, nutrition, culture, friends, and chance events. The exam treats the two as interacting. Genes give a predisposition, and the environment shapes how, and whether, that predisposition shows up.

A simple example is height. Genes strongly influence how tall a child can become, but a child who is poorly fed during growth will not reach the potential that good nutrition would allow. Another is a child with a genetic tendency toward anxiety: in a calm, supportive home the tendency may never become a problem, and in a chaotic home it may. The genes and the environment are not adding up separately; they are interacting. You will see this idea again in the diathesis-stress model in Unit 5.

You do not need detailed genetics for the AP Exam. Terms such as genotype, phenotype, DNA, chromosomes, and dominant and recessive genes are beyond the scope of the exam, so concentrate on the broad idea of nature interacting with nurture and on how researchers study it.

The evolutionary perspective

The evolutionary perspective explains behavior and mental processes by asking how they helped our ancestors survive and reproduce. It rests on natural selection, the principle (from Charles Darwin) that traits that increase an organism's chances of surviving and reproducing are more likely to be passed on, so that over many generations such traits become more common in a population. Behaviors can be inherited tendencies too, so evolutionary psychologists ask what problem a behavior solved for early humans. Examples include fear of snakes and heights (which would help someone avoid danger), the bond between infant and caregiver (which keeps a helpless infant alive), and the preference for sugary and fatty foods (which were scarce and valuable sources of energy).

The perspective also has limits. Explanations can be built after the fact ("it must have helped our ancestors"), which makes them hard to test, so a hypothesis from this perspective should still be falsifiable. Natural selection acts on populations over many generations, so it does not explain why one individual acts a certain way on a given day.

The exam also asks you to know that evolutionary ideas have been misused. Some theorists have used the logic of inheritance and "fitness" to argue that some groups of people are superior to others, and have promoted discrimination against groups they judged inferior. The best-known example is eugenics, a movement that sought to improve the human population by encouraging reproduction among people thought to have desirable traits and preventing it among others. In the United States in the early twentieth century, eugenics laws led to the forced sterilization of tens of thousands of people, many of them poor, disabled, or from minority groups. The movement rested on bad science and on prejudice, and it was used to justify horrific policies in Nazi Germany. Psychology rejects it. The lesson to remember is that a scientific theory can be applied in unethical and discriminatory ways, and that you can be asked to recognize and evaluate such an application.

Studying heredity: twin, family, and adoption studies

Researchers cannot randomly assign people to have certain parents or genes, so research on the effects of genes on behavior relies on non-experimental designs that use natural relationships. Three are required.

• Twin studies compare identical (monozygotic) twins, who share essentially 100% of their genes, with fraternal (dizygotic) twins, who share on average about 50%, as ordinary siblings do. If a trait is influenced by heredity, identical twins should be more alike on it than fraternal twins, because their genes are more alike while their shared environment (same family, same age) is similar. Studies of identical twins who were separated early in life and raised in different homes are especially informative, because any similarity cannot be due to a shared upbringing.
• Family studies look at how common a trait is among biological relatives. If a trait has a genetic basis, closer relatives (parents, siblings) should be more likely to share it than distant relatives. A weakness is that relatives usually share an environment as well as genes.
• Adoption studies compare adopted children with their biological parents (who gave genes but not a home) and with their adoptive parents (who gave a home but not genes). If adopted children resemble their biological parents on a trait, heredity is implicated; if they resemble their adoptive parents, environment is implicated.

All three are correlational: each compares naturally occurring groups and measures similarity, and none can show cause and effect the way an experiment can. Researchers often summarize their results with a heritability estimate, which is a statistic about how much of the differences among people in a group can be linked to genetic differences. It is not a measure of how much of one person's trait is "genetic," and it can change when the environment changes.