- Ian J Deary, professor of differential psychology
- Department of Psychology, University of Edinburgh, Edinburgh EH8 9JZ
People value their thinking skills and woe betide anyone who tries to measure them. Both the measurer and the yardstick are liable to be sacrificed on the altar of public ridicule. The professionalisation of this expert bashing may be seen in one of the bestselling books about measuring IQ (intelligence quotient)—Stephen Jay Gould's The Mismeasure of Man, now in its second edition.1 Never mind that it leaves the reader uninformed about the successes of research on differences in human intelligence or that it has been deemed misleading by the cognoscenti, 2 3 it satisfies our desire to tar and feather experts who dare to measure what we value about ourselves and wish to remain mysterious and complex.
To study differences in mental abilities is to share the predicament of meteorologists. These scientists deal with an aspect of our everyday lives about which most of us feel free to speak with authority unlike, for example, atomic theory or plate tectonics. Metereologists must simultaneously develop constructs and give practical predictions, which affect our lives. Their measurement tools and the mathematical framework behind them are formidable. They are playing a stochastic game, getting it more right than wrong over the entire season. However, we want them and IQ testers to be right every time or for every person: deterministic rather than stochastic.
Differences in mental abilities have a hierarchical structure, from narrow specific abilities to general ability
Environmental and genetic contributions to these differences are sizeable, the genetic contribution possibly increasing with age
Differences may change or remain stable during the adult lifespan, stability being especially high for verbal abilities
Differences in mental ability have some modest predictive validity for real life outcomes
Cognitive and biological bases of differences in mental ability are being explored but are not yet understood
Is there one type of intelligence?
If a …