AP Psychology · Lesson 1

Research methods and heredity

Know Know it

Psychologists test ideas with different research methods. An experiment is the only method that can show cause and effect. The researcher changes the independent variable and measures the dependent variable.

Random assignment puts each participant into a group by chance, so the groups start out alike. An operational definition states exactly how a variable is measured, such as hours of sleep recorded by a wrist tracker.

Other methods include correlational studies, case studies, naturalistic observation and surveys. A correlation coefficient runs from -1 to +1. Its sign gives the direction and its size gives the strength. A correlation never proves cause, because a third variable may explain both.

Nature and nurture work together. Twin studies compare identical twins, who share nearly all their genes, with fraternal twins, who share about half. Adoption studies compare children with their biological and adoptive parents. Heritability estimates how much of the variation in a trait within a group is linked to genetic differences.

Trap Heritability is about groups

A heritability of 0.6 does not mean that 60 percent of one person's trait comes from genes. It describes variation across a group in one environment, and it can change when the environment changes.

Worked example Easy

A researcher has one group of students study with music and another group study in silence, then compares their test scores. What is the independent variable?

  1. The students' test scores
  2. Whether students study with musicAnswer
  3. The number of students in each group
  4. The subject the students study

How to solve it

  1. Find what the researcher changes on purpose: music or silence.
  2. Find what is measured afterward: test scores.
  3. The factor that is changed on purpose is the independent variable.

Why each choice is right or wrong

  • A. Incorrect. The test scores are what the researcher measures, so they are the dependent variable.
  • B. Correct. The researcher changes the music condition on purpose, which makes it the independent variable.
  • C. Incorrect. The number of students in each group is the sample size, not the factor being changed.
  • D. Incorrect. The subject studied is never described as changing, so it is not the manipulated variable.

Explain Explain it

Task verbs: identify, explain

To explain a research idea, name it and then say what it does in this study. For example: random assignment matters here because it makes the two groups alike before the treatment starts.

Worked example Medium

Researchers find that identical twins raised apart are more alike in a trait than fraternal twins raised together. What does this finding most directly suggest?

  1. Genes contribute to differences in the traitAnswer
  2. Shared homes cause most of the trait
  3. The trait cannot be changed by experience
  4. Fraternal twins share more genes than identical twins

How to solve it

  1. Identical twins raised apart share genes but not homes.
  2. Fraternal twins raised together share homes but only about half their genes.
  3. Greater similarity in the first pair points to a role for genes.

Why each choice is right or wrong

  • A. Correct. Identical twins share nearly all their genes, so their greater similarity despite different homes points to a genetic contribution.
  • B. Incorrect. Fraternal twins shared a home yet were less alike, so shared homes cannot explain most of the trait.
  • C. Incorrect. A genetic influence does not mean experience cannot change the trait, because genes and environment interact.
  • D. Incorrect. Fraternal twins share about half their genes, while identical twins share nearly all of them.

Explain Your turn

A researcher wants to show that a new flashcard app causes higher vocabulary scores. Explain why she should randomly assign students to use the app or not.

Write it, then compare. Nothing you type here is saved or graded.

Compare with a model answer

A full answer does this

  • Says random assignment makes the groups alike at the start
  • Says this lets a score difference be credited to the app

Model answer

Random assignment uses chance to place students, so traits like prior ability and motivation end up spread about evenly across the two groups. If the app group then scores higher, the researcher can credit the app, the independent variable, rather than a difference that was there from the start.

Apply Apply it

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