Sex Differences in Variability in General Intelligence: A New Look at the Old Question

Area of Science:

• Psychology
• Human Genetics
• Educational Psychology

Background:

• Historically, general intelligence is hypothesized to be more variable in males than females.
• This hypothesis suggests males are overrepresented at the extreme ends of the intelligence distribution.
• Empirical analysis of large, representative samples is needed to clarify these sex differences in intelligence variability.

Purpose of the Study:

• To investigate sex differences in the distribution of general intelligence.
• To examine variability in general intelligence across different ranges of the distribution.
• To test a mixture model explaining population intelligence distribution.

Main Methods:

• Analysis of two population-wide general intelligence surveys of 11-year-olds in Scotland.
• Examination of the normality and variability of intelligence score distributions.
• Comparison of male and female intelligence variability, especially at higher score ranges.

Main Results:

• Intelligence distributions showed significant departures from normality, with lower variability at the higher end.
• Modal IQ scores were approximately 105, despite a mean IQ of 100.
• Males demonstrated greater variability than females, even above the modal score level.

Conclusions:

• The population distribution of general intelligence may be a mixture of two normal distributions.
• One distribution represents typical intelligence variation, the other reflects conditions associated with mental retardation.
• Observed sex differences in variability at the high end did not explain differences in high-level achievement.