The study of intelligence has a rich history spanning over four millennia, dating back to 2200 BC in Ancient China. The Imperial Court designed a series of tests to assess candidates’ abilities in agriculture, military, law, finance, and geography. These assessments covered tasks similar to those found in modern tests and included verbal and nonverbal reasoning, as well as speaking and writing skills. The Ancient Greeks, universities of the Middle Ages, and Socrates’ disciples also developed methods for evaluating intellectual abilities.

In the 1800s, researchers such as Jean Esquirol and Edouard Seguin began studying the intelligence of mentally disabled individuals, laying the groundwork for performance and nonverbal intelligence tests. Francis Galton asserted the innate nature of intelligence and developed the first individual intelligence test. He also pioneered psychometric and statistical methods. James McKeen Cattell, a colleague of Galton's, coined the term “mental test” and emphasized the importance of standardizing test administration.

Alfred Binet, with the help of Theophile Simon and Victor Henri, established a series of tests for cognitive skills such as memory and imagination, launching the intelligence testing and clinical testing movements. David Wechsler created the first test to assess adult intelligence, which later became known as the Wechsler Adult Intelligence Scale (WAIS). The group intelligence test evolved from Binet's individually administered intelligence test, with the United States developing group intelligence tests during World War I and II to screen recruits.

William Stern introduced the concept of intelligence quotient (IQ), while Lewis M. Terman designed the American version of the Binet test, known as the Stanford-Binet test. Terman updated the test to make it easier to administer to adults. Wechsler further developed the concept of assessing adult intelligence using written tests, and the WAIS has been modified several times to extend the age range of the test.

The Cattell-Horn theory of fluid and crystallized intelligence asserts that intelligence comprises a combination of abilities. According to this theory, there are two general types of intelligence: fluid intelligence, which pertains to the ability to think and reason flexibly, and crystallized intelligence, which involves knowledge, facts, and skills accumulated over one's life.