Case Study 2: What IQ Actually Measures (and What It Doesn't)
The Construct
IQ tests (WAIS for adults, WISC for children, Stanford-Binet) measure multiple cognitive abilities: - Verbal comprehension: vocabulary, verbal reasoning - Perceptual reasoning: pattern recognition, spatial processing - Working memory: holding and manipulating information - Processing speed: how quickly you perform simple cognitive tasks
The scores on these subtests are positively correlated — someone who scores high on vocabulary tends to score high on pattern recognition and working memory too. This common factor is g (general cognitive ability).
What IQ Predicts Well
| Outcome | Correlation with IQ |
|---|---|
| Academic grades | r ≈ 0.50–0.60 |
| Job performance (across occupations) | r ≈ 0.50 |
| Income | r ≈ 0.30–0.40 |
| Occupational attainment | r ≈ 0.50 |
| Health behaviors and outcomes | r ≈ 0.20–0.30 |
| Longevity | modest positive correlation |
These correlations are among the strongest predictive relationships in all of psychology. IQ genuinely captures something that matters for life outcomes.
What IQ Doesn't Predict Well
| Outcome | Relationship to IQ |
|---|---|
| Creativity | Weak above IQ ~120 (threshold theory) |
| Life satisfaction/happiness | Weak (r ≈ 0.10–0.15) |
| Relationship quality | Weak |
| Moral character | No consistent relationship |
| Wisdom | Different construct |
| Practical competence | Modest relationship |
IQ predicts cognitive and economic outcomes well. It predicts emotional, relational, and meaning-related outcomes poorly.
The Cultural and Environmental Dimension
The Flynn Effect: IQ scores have risen approximately 3 points per decade across the 20th century in developed countries. Since genetic changes don't occur this quickly, the Flynn Effect demonstrates substantial environmental influence on IQ — through improved nutrition, education, healthcare, and cognitive stimulation.
SES and IQ: Children raised in higher-SES environments tend to score higher on IQ tests — partly due to genetics (smart parents tend to earn more AND pass on genes for cognitive ability) and partly due to environment (better nutrition, more cognitive stimulation, better schools).
Heritability increases with age: In childhood, environment explains more IQ variance (~40–50% heritable). In adulthood, heritability rises to 60–80% as people select environments that match their genetic predispositions. This is a counterintuitive finding: genes matter more, not less, as you age.
The Honest Summary
IQ is one of the best-validated constructs in all of psychology. It measures something real, it predicts important outcomes, and it has been replicated across cultures and decades. It is also not what most people think it is — it doesn't measure creativity, wisdom, character, or the full scope of what we mean by "intelligence."
The most useful framing: IQ captures a real and important dimension of human cognitive variation. It is one of many factors that influence life outcomes. It is not destiny, not comprehensive, and not the measure of a person's worth.
Discussion Questions
- IQ is one of psychology's best-validated constructs. Why is it also one of the most controversial?
- The Flynn Effect shows IQ is partly environmental. Does this undermine or support the use of IQ tests?
- If IQ predicts job performance at r ≈ 0.50, should it be used in hiring? What are the fairness concerns?
- IQ doesn't predict happiness. What does this tell us about the relationship between cognitive ability and wellbeing?