Lecture 13: Why Are People Different - Personality, Intelligence and Heredity

What this lecture covers

After several lectures on universal features of human psychology, this lecture turns to individual differences: why you and the person next to you are not psychologically identical. Paul Bloom focuses on the two traits psychologists study most, personality and intelligence, covering how each is measured and what counts as a good measurement, before turning to the more contested question of where these differences come from.

After watching, you can distinguish reliability from validity in a psychological test, describe the Big Five personality model and the concept of general intelligence ("G"), and explain how behavioral genetics uses twin and adoption studies to separate the roles of genes, shared environment, and non-shared environment in producing psychological differences.

Key ideas

• Reliability vs. validity: a reliable test gives consistent results over repeated testing; a valid test actually measures the trait it claims to measure. A test can have one property without the other.
• The Big Five ("OCEAN"): openness, conscientiousness, extroversion, agreeableness, and neuroticism are the trait dimensions that personality researchers converge on as capturing most meaningful variation between people.
• General intelligence ("G"): performance across different cognitive subtests correlates, suggesting a general factor above and beyond each specific ("S") skill; IQ tests are scored so the population average is always 100.
• Heritability is about differences, not origins: heritability describes how much of the variation between people in a population is explained by genetic differences, not how much of any one person's trait is "caused" by genes.
• Twin and adoption studies: comparing monozygotic twins, dizygotic twins, and adopted (genetically unrelated) siblings lets researchers estimate the relative contributions of genes and shared environment.
• Shared vs. non-shared environment: shared environment is what siblings raised together have in common (such as parenting style); non-shared environment is everything else, including unique experiences.
• The Flynn effect: average IQ scores have risen substantially across generations, showing that large changes in measured intelligence can occur without any change in genetics.
• Judith Harris's "nurture assumption" argument: behavioral genetics data suggest that shared environment (including parenting style) explains little of the variance in adult personality and intelligence, a claim Bloom presents as genuinely controversial and worth scrutinizing.

Walkthrough

Personality, intelligence and determining difference (0:00)

Bloom introduces the shift from universals to individual differences and frames personality as a stable trait across situations and time, distinct from a one-off emotional reaction. He explains reliability and validity using examples like a bathroom scale and a joke "intelligence test" based on birth date, then illustrates unreliable, invalid measures with online personality quizzes and the Rorschach inkblot test, which he describes as widely used in clinical and legal settings despite having essentially no scientific validity.

Measuring personality with "The Big Five" (13:53)

The lecture traces attempts to reduce personality to a manageable number of dimensions, from Gordon Allport's list of eighteen thousand trait words to Eysenck's two- and three-factor models and Cattell's sixteen factors, arriving at the modern consensus around the Big Five (openness, conscientiousness, extroversion, agreeableness, neuroticism). Bloom presents evidence for the model's validity: it is stable after about age thirty, multiple observers (roommates, professors, parents) tend to agree on someone's ratings, and scores predict real-world outcomes such as fidelity, job changes, and number of sexual partners.

Defining and measuring intelligence (19:47)

Bloom surveys expert definitions of intelligence (abstract reasoning, problem solving, ability to acquire knowledge) and introduces Spearman's distinction between specific abilities ("S") and general intelligence ("G"), the factor that explains why performance across different cognitive tests correlates. He explains how modern IQ tests like the Wechsler are scored around a population average of 100, with a normal distribution where about 68% score between 85 and 115. He then discusses the controversy sparked by The Bell Curve, noting the broad expert consensus that IQ predicts real-world outcomes like school and military success, while also flagging that its social importance is partly a self-fulfilling prophecy created by institutions that use IQ-like tests (the SAT, GRE) as gatekeepers.

Question and answer on personality and intelligence (30:29)

Students ask how test designers prevent people from gaming personality tests (through catch questions and repeated items), how emotional intelligence relates to standard IQ, and how stability over time is established despite personality being able to change somewhat. Bloom emphasizes that a good test's value ultimately rests on its ability to predict real-world outcomes, not just internal consistency.

The roles of genes and environment in explaining human differences (36:32)

Bloom clarifies that heritability concerns explaining differences between people, not the origin of any individual's trait, using the contrast between height (highly heritable) and number of legs (not heritable, since leg loss is usually accidental) as an illustration. He introduces the logic of behavioral genetics: comparing monozygotic twins (100% shared genes), dizygotic twins (50%), and adopted siblings (0% beyond chance), all of whom, if raised together, share 100% of their environment. He highlights the "gold standard" of identical twins reared apart, noting striking similarities researchers have found in their attitudes, behaviors, and even brain scans, and explains why within-group heritability does not imply that between-group differences (such as IQ differences among racial groups) have a genetic cause, using Richard Lewontin's fertilized-plots analogy.

Genes, environment and intelligence (51:16)

The lecture presents two findings from behavioral genetics: high heritability for nearly every psychological trait, and the more surprising claim that shared environment (including parenting) contributes very little to the non-genetic variance in adult personality and intelligence, with most of the remainder attributed to non-shared environment. Bloom discusses the Flynn effect as evidence that large IQ changes can happen without genetic change, and presents Judith Harris's argument from The Nurture Assumption that correlations between parenting and child outcomes are often confounded by shared genes or by children shaping parental behavior, using a widely publicized "family meals" study as a case study in mistaking correlation for causation.

Before you watch

• This lecture assumes familiarity with basic natural selection and genetics concepts from earlier in the course.
• It follows directly from the two-part lecture on emotions and marks a shift in the course toward individual differences, which continues in later lectures on sex differences.
• No prior statistics background is required; the lecture explains correlation-based reasoning as it goes.

Check your understanding

1. What is the difference between reliability and validity in a psychological test, and why can a test have one without the other?
2. What evidence supports the Big Five as a valid model of personality, beyond just internal consistency?
3. Why doesn't high heritability of a trait within a group imply that differences between groups are genetically caused?
4. How does the Flynn effect challenge a purely genetic explanation of intelligence differences over time?
5. What are two alternative explanations, besides "good parenting causes good outcomes," for the correlation between parenting behavior and child outcomes?