1.1 Interaction of Heredity and Environment

Heredity, or nature, means the traits and tendencies you inherit from your biological parents through your genes. Height, eye color and a tendency toward being shy or outgoing all have a hereditary part. Environment, or nurture, means everything outside you that you experience: your family, your friends, your school, your culture, what you eat and even conditions before birth.

Psychologists today don't ask 'nature or nurture?' They ask how the two interact. Genes set a range of possibilities, and experience decides where in that range you end up. A child may inherit the potential to be tall, but poor nutrition can keep that potential from showing. Genes can also shape your environment: a naturally outgoing child seeks out more social situations, which builds social skills further.

Because the two are so tangled, the best answer on almost any 'is this caused by genes or experience?' question is 'both, and they affect each other.'

The evolutionary perspective explains behavior through natural selection. Natural selection is the process by which inherited traits that help an organism survive and reproduce become more common over many generations, because the organisms with those traits leave more offspring.

Evolutionary psychologists ask what problem a behavior might have solved for our ancestors. For example, a strong liking for sweet and fatty foods would have helped people survive when calories were scarce. A quick fear of snakes, heights or angry faces would have helped people avoid danger. Today those same tendencies can cause trouble, such as overeating when food is everywhere.

A limit to remember: evolutionary explanations are hard to test directly, because we can't run experiments on our ancestors. A good explanation still needs evidence from present-day research.

These studies are correlational. Researchers can't randomly assign children to families, so they measure how strongly relatives' traits are related. They can show how much genes and environment are associated with a trait, but they can't fully prove cause and effect.

• Twin studies compare identical twins, who share essentially all their genes, with fraternal twins, who share about half on average, like any brothers or sisters. If identical twins are much more alike on a trait than fraternal twins, genes probably play a larger role in that trait.
• Twins raised apart are especially useful. They share genes but not a home, so similarities between them point toward heredity.
• Family studies check whether a trait runs in families, and whether closer relatives are more alike than distant ones. The weakness is that families share homes as well as genes, so the two causes are mixed together (confounded).
• Adoption studies compare adopted children with their biological parents (shared genes) and their adoptive parents (shared environment). Resemblance to biological parents suggests genetic influence; resemblance to adoptive parents suggests environmental influence.

Eugenics was a movement to 'improve' the human population by encouraging some people to have children and preventing others from doing so. Francis Galton coined the term in 1883. In the early 1900s, eugenic ideas were used to justify forced sterilization laws in the United States, immigration limits aimed at particular groups, and later the horrors of Nazi Germany. These policies targeted people based on race, ethnicity, poverty and disability.

Eugenics is in the course as a warning. Claims about heredity were stretched far beyond the evidence and used to discriminate. You'll see the same pattern again in 2.8, where intelligence test scores were used to limit people's opportunities.

You won't be tested on the details of genetics, such as DNA, chromosomes, genotype versus phenotype, or dominant and recessive genes. Focus on how heredity and environment interact and how researchers study them.