Why is IQ normally distributed?

Intelligence is a complex trait influenced by many genes and environmental factors, producing a normal distribution in populations.

What percentage score between 85-115?

Approximately 68% of the population scores within one standard deviation (85-115), the average range.

The bell curve shows that extreme scores (very low or very high) are rare, with most people clustering around average intelligence.

Overall shape remains normal, but mean scores vary by education level, socioeconomic status, and other environmental factors (Flynn Effect).

Peter Kolomiets

|March 22, 2026|Updated Apr 13, 2026|6 min read

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Herrnstein, R. J. & Murray, C.. Herrnstein, R. J. & Murray, C. (1994). The Bell Curve
Deary, I. J.. Deary, I. J. (2001). Intelligence: A Very Short Introduction
Gould, S. J.. Gould, S. J. (1981). The Mismeasure of Man

Put what you've learned into practice with these free assessments:

An IQ of 100 represents average intelligence by design. Understand how average scores are defined and what they predict.

Genius intelligence typically starts at IQ 130-140, representing the top 2-3% of the population. Explore what these scores mean.