# Measuring Intelligence

Psychology I · Individual Differences · https://tryals.app/learn/psychology-i/measuring-intelligence

## From Ratios to a Bell Curve

Modern testing uses the **Wechsler scales**: the **WAIS** for adults and **WISC** for children. These replaced old ratio formulas with **deviation IQ**: a raw score is compared to a peer group, set to a **mean of 100** and **standard deviation of 15** ($\mu = 100, \sigma = 15$).

This solves the ratio formula's adult breakdown, making scores comparable across ages. Scores form a **normal distribution** (bell curve), where 68% fall within 1 SD and 95% within 2 SDs.

| IQ Range | Population Share |
| --- | --- |
| 85-115 (±1 SD) | About 68% |
| 70-130 (±2 SD) | About 95% |

> **Pitfall:** Assuming an isolated score lacks context. Always check the **norm group**, as tests are periodically re-normed.

## The Flynn Effect

The **Flynn effect** is the discovery that raw test scores rose across 20th-century generations by roughly 3 points per decade. Genes change too slowly to cause this.

Instead, the rise is driven by better nutrition, more schooling, and abstract test familiarity. Because tests are re-normed to keep the mean at 100, raw scores from past decades cannot be compared to today without adjusting for this generational shift.

$$\mu = 100, \sigma = 15$$

## Practice questions

4 of this lesson's 10 practice questions, with answers. The full set is in the app.

### 1. Test publishers restandardise IQ tests every decade or so, at considerable expense. Why do the norms **expire**?

A. Because test items become known and are shared between candidates
B. Because statistical techniques improve, requiring recalculation
C. Because a score is defined against a reference population
D. Because the underlying construct of intelligence changes over time

**Answer:** C. Because a score is defined against a reference population

**Why:** The score is a comparison, so it decays as the comparison group ages out. Scoring a modern child against 1990 norms adds several free points, which matters enormously at thresholds used for special-education placement or, in some jurisdictions, capital cases.

Page: https://tryals.app/practice/psychology-i/measuring-intelligence/test-publishers-restandardise-iq-tests-every-decade-or-so-at

### 2. Select every statement that is TRUE about measuring intelligence.

A. Deviation IQ compares people to same-age peers.
B. Deviation IQ has a mean of 100 and an SD of 15.
C. The Flynn effect is a decline in raw scores across generations.
D. Genes change too slowly to explain the Flynn effect.
E. Roughly two-thirds of the population falls between 85 and 115.

**Answer:** A. Deviation IQ compares people to same-age peers.; B. Deviation IQ has a mean of 100 and an SD of 15.; D. Genes change too slowly to explain the Flynn effect.; E. Roughly two-thirds of the population falls between 85 and 115.

**Why:** The mean-100/SD-15 scale, the 68% within one SD, the too-slow genes point, and peer comparison are all true. The Flynn effect is a RISE in scores, not a decline - the one false option.

Page: https://tryals.app/practice/psychology-i/measuring-intelligence/select-every-statement-that-is-true-about-measuring-intelligence

### 3. Raw IQ performance has risen substantially across the twentieth century, yet the average IQ score has stayed at exactly 100 the whole time. Why did nobody notice the Flynn effect for decades?

A. Because the rise was too small to detect until many decades of data had accumulated.
B. Because IQ tests measure innate ability, which cannot change, so no rise was expected or looked for.
C. Because the rise occurred only in a few countries, so international averages concealed it.
D. Because the tests are periodically restandardised to a mean of 100.

**Answer:** D. Because the tests are periodically restandardised to a mean of 100.

**Why:** The measure is designed to hide it, 100 is defined as the current population's average, so if everyone improves nothing moves, and restandardisation absorbs the gain each time. Flynn looked where it could not hide: scored against 1940s norms, a 1990s sample averaged well above 100. Three points per decade is far too fast for the gene pool to have changed.

Page: https://tryals.app/practice/psychology-i/measuring-intelligence/raw-iq-performance-has-risen-substantially-across-the-twentieth

### 4. Sort each proposed cause by whether it is a plausible explanation of the Flynn effect or NOT.

**Answer:**

- Plausible cause of the Flynn effect: Improved nutrition, More years of schooling, Greater familiarity with abstract test-taking
- NOT a plausible cause: Rapid genetic change across a few generations, Tests getting secretly easier each year

**Why:** Nutrition, schooling, and test-taking familiarity are plausible environmental drivers; rapid genetic change is ruled out (genes change too slowly), and the tests are re-normed, not secretly eased. The Flynn effect points to environment, not genes.

Page: https://tryals.app/practice/psychology-i/measuring-intelligence/sort-each-proposed-cause-by-whether-it-is-a-plausible-explanation-of