AP Psychology

Nature and nurture

Every behavior and mental process comes from a body that has both an inherited starting point and a history of experience. Heredity, often called nature, means the genetic and predisposed characteristics that influence physical, behavioral and mental traits. Environment, often called nurture, means the external factors a person experiences, such as family interactions, education, diet, culture and peers. Neither works alone. A child may inherit a tendency toward a quick temper, for example, but whether that tendency shows up as aggression depends on how the people around the child respond to it, so the two sets of factors interact and shape the outcome together.

Psychologists therefore rarely ask whether a trait is "genetic or learned". They ask how much of the variation between people in a trait is linked to differences in heredity and how much to differences in environment, and how the two combine.

Studying the effects of genes

Genes cannot be manipulated in people, so researchers rely on natural comparisons between relatives. These are non-experimental designs, which means they can show that a trait runs in families but cannot by themselves prove what causes it.

• Twin studies compare identical twins (who come from one fertilized egg and share essentially all of their genes) with fraternal twins (who come from two eggs and share, on average, about half of the genes that differ between people). If a trait is influenced by heredity, identical twins should resemble each other more than fraternal twins do, even when both kinds of pair are raised in the same home.
• Adoption studies compare adopted children with their biological relatives and with their adoptive relatives. Resemblance to the biological parents, who supplied the genes but not the home, points to heredity; resemblance to the adoptive parents, who supplied the home but not the genes, points to environment.
• Family studies look at how often a trait appears in people who are more or less closely related. A trait that becomes more common as the relationship gets closer is a candidate for genetic influence, although relatives also share environments, so this design is the weakest of the three at separating the two.

The strongest evidence comes from twins raised apart, because they share genes but not homes. Even then, twins often share a prenatal environment and may be placed in similar adoptive families, so the comparison is never perfectly clean.

The evolutionary perspective

The evolutionary perspective asks how natural selection has shaped behavior and mental processes. Natural selection is the principle that traits that help an organism survive and reproduce in its environment become more common in later generations. An evolutionary psychologist might argue that the strong human tendency to fear snakes and heights exists because ancestors who avoided them were more likely to survive and have children. Critics point out that such explanations are hard to test and can be made up after the fact to fit almost any behavior.

Some thinkers have also misused evolutionary ideas to justify discrimination against groups of people. The most notable example is eugenics, a movement that argued that some groups were biologically inferior and that reproduction should be controlled to improve the population. Eugenics rested on bad science and on prejudice, and it led to forced sterilization and other serious harm. It is a reminder that a psychological theory can be applied in ways that discriminate, and that researchers have an ethical duty to consider how their work will be used.

Divisions of the nervous system

The nervous system is the body's fast communication network. It is organized into the central nervous system and the peripheral nervous system, with the peripheral system divided further.

The central nervous system (CNS) is made up of the brain and the spinal cord. The brain interprets information and decides what to do with it, and the spinal cord carries messages between the brain and the rest of the body. The CNS interacts with every process in the body.

The peripheral nervous system (PNS) is made up of all the nerves outside the brain and spinal cord. It carries information from the senses and the body into the CNS and relays commands from the CNS out to the muscles, organs and glands. It has two branches.

The somatic nervous system

The somatic nervous system governs voluntary processes. It carries sensory information to the CNS and sends messages from the CNS to the skeletal muscles, so it is the pathway used when a person decides to pick up a cup or kick a ball.

The autonomic nervous system

The autonomic nervous system governs involuntary processes such as heart rate, digestion, breathing rate and the widening or narrowing of the pupils. It works largely without conscious control, and it has two parts that act in opposite directions.

The two divisions are not simply on and off. They work together all the time, and the balance between them shifts with circumstances. A person who has just narrowly avoided a car accident shows a strong sympathetic response, with a pounding heart and shaky hands, and then, as the danger passes, the parasympathetic division gradually restores a calmer state.