Four questions about the middle of the curve
Slide to your guess, then reveal. Most people underestimate how crowded the middle is and overestimate their own place in it.
Draw a random group: its average is rarely exactly 100
Even a perfectly random group drifts from 100 by chance. The typical drift is 15 ÷ √n: about 3 points for a class of 25, under half a point for a thousand. Claims about one school, team or office averaging 108 often mean nothing more than this.
The average is 100 at 8, 30 and 80, while raw ability is not
Modern IQs compare you with your own age group, so every age band averages 100. Underneath, raw performance moves: reasoning with new problems and processing speed tend to peak in early adulthood and decline gradually, while vocabulary and general knowledge keep growing into later life. Psychologists call these fluid and crystallised abilities.
So an 80-year-old with IQ 100 solves fewer novel puzzles than a 25-year-old with IQ 100, and usually knows more words.
Your grandparents' average, scored on today's norms
Through the 20th century raw scores rose about 3 IQ points a decade in many countries (the Flynn effect), so each generation averaged 100 only against its own norms. Recent data from several countries show the rise slowing or reversing.
A typical adult tested in the 1930s, scored against norms from about 2000, would average roughly 79, on a classic 3-points-a-decade rate. On their own era's norms they averaged 100. Gains were largest on abstract reasoning tests and smallest on vocabulary.
Four questions to ask about any group average
- Wechsler, D. (2008). WAIS-IV Technical and Interpretive Manual. Pearson. Norming procedure and age bands.
- Flynn, J. R. (1987). Massive IQ gains in 14 nations: What IQ tests really measure. Psychological Bulletin, 101(2), 171–191.
- Bratsberg, B., & Rogeberg, O. (2018). Flynn effect and its reversal are both environmentally caused. PNAS, 115(26), 6674–6678.
- Horn, J. L., & Cattell, R. B. (1967). Age differences in fluid and crystallized intelligence. Acta Psychologica, 26, 107–129.
- Salthouse, T. A. (2009). When does age-related cognitive decline begin? Neurobiology of Aging, 30(4), 507–514.
- Furnham, A. (2001). Self-estimates of intelligence: Culture and gender difference in self and other estimates of both general (g) and multiple intelligences. Personality and Individual Differences, 31(8), 1381–1405.