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.

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.

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