Psychology › Personality & intelligence › Intelligence and the g factor
IQ tests
1905 (Binet–Simon)· Concept
Well replicatedStrong, repeated evidence.
Standardized tests that estimate intelligence and report it on a scale where the population average is set at 100.
See it on the Psychology map →What it says
In 1905 Alfred Binet and Théodore Simon built the first practical test in Paris to find children who needed extra help at school. William Stern coined 'intelligence quotient' in 1912; Lewis Terman's Stanford–Binet (1916) and US Army tests of about 1.7 million World War I recruits made testing mainstream in America. Modern tests like the Wechsler scales combine subtests such as vocabulary, puzzles and memory span. About two-thirds of people score between 85 and 115.
Why it matters
IQ tests became psychology's most widely used tool, shaping schooling, hiring, the military and courts. They were also misused to justify eugenics and immigration restriction.
The evidence
Scores are highly reliable, with retest correlations near 0.9, and predict grades and job performance. Binet warned scores were not fixed; the 1927 Supreme Court case Buck v. Bell, upholding forced sterilization, leaned on test labels.
Open questions
Critics argue tests miss creativity, practical know-how and rational judgment. Smart people can still decide poorly, a gap psychologist Keith Stanovich calls dysrationalia.
Connections (4)
▸Jean Piaget's stages of cognitive development grew out of work on IQ testsWell replicated
The connection
Young Piaget worked in Binet's former laboratory in Paris, standardizing reasoning tests for children. He became more interested in their wrong answers than their scores, noticing that children of the same age made the same kinds of errors. That observation grew into his theory that thinking develops in distinct stages.
Who showed it and when
Paris, 1919–1921, at a school laboratory under Théodore Simon, Binet's co-author, using reasoning tests devised by British psychologist Cyril Burt.
The evidence
Piaget's 1952 autobiography and his biographers describe the Paris work; his early papers on children's reasoning grew directly from it.
Open questions
Historians debate how much Binet's own, largely forgotten work on how children think anticipated Piaget's ideas.
▸IQ tests measure Intelligence and the g factorWell replicated
The connection
IQ tests are the main tool for estimating g. A battery like the Wechsler Adult Intelligence Scale combines vocabulary, matrix puzzles, memory span and speed tasks; because these scores correlate, their total gives a good estimate of general ability plus a profile of strengths and weaknesses.
Who showed it and when
From Binet's 1905 Paris test through David Wechsler's adult scales, first published in 1939 and revised regularly, most recently in 2024.
The evidence
Different IQ tests agree closely, and a 2004 study by Wendy Johnson and colleagues found g from three separate batteries correlated almost perfectly.
Open questions
Psychologist Edwin Boring quipped in 1923 that intelligence is 'what the tests test.' Whether tests capture creativity, wisdom or practical skill is still argued.
▸The Flynn effect: rising IQ scores forced renorming of IQ testsWell replicated
The connection
Because average raw performance rose about three points a decade, tests scored against old norms gave inflated IQs, so publishers re-standardize every 10 to 20 years. It matters in court: an outdated norm can push a defendant's score above the cutoff for intellectual disability.
Who showed it and when
Flynn compared Stanford–Binet and Wechsler standardization samples in 1984; US courts have debated adjustments since the Supreme Court's 2002 Atkins v. Virginia ruling.
The evidence
People who take an old and a new version of the same test reliably score higher on the old one, matching Flynn's estimates.
Open questions
Whether courts should adjust individual scores for norm age is legally contested, and recent reversals complicate future norming.
▸Stereotype threat was claimed to lower scores on IQ testsMixed evidence
The connection
The 1995 study used hard GRE verbal questions, a cousin of intelligence tests. Media and some textbooks then said stereotype threat explained racial gaps in test scores. But the study had statistically equalized students by prior SAT scores, so a gap existed even without threat. Steele and Aronson themselves did not claim it explained the whole gap.
Who showed it and when
Steele and Aronson at Stanford, 1995; Paul Sackett, Chaitra Hardison and Michael Cullen's critique in American Psychologist, 2004.
How solid is it?
Lab studies support some score drop under threat, but evidence that it affects real, high-stakes tests, where decisions are actually made, is weak.
Open questions
How much of any real-world score gap it explains; a 2004 study of actual AP Calculus exams found asking gender before the test made negligible difference, a conclusion a 2008 reanalysis disputed.