Conventional measures of intelligence are obtained using standard tests, called intelligence quotient tests or, more commonly, IQ tests. These tests have been shown to be reliable and valid. Reliability means that they measure the same thing from person to person, whereas validity means that they measure what they claim to measure. IQ tests measure a person's ability to reason and to solve problems. These abilities are frequently called general cognitive ability, or "g."
Almost all genetic studies of the heritability of intelligence (how much is due to genetics and how much is due to the environment) have been obtained from IQ tests. To understand the studies, therefore, it is important to understand what IQ tests measure, and how their use and interpretation have changed over time.
The standard IQ-measurement approach to intelligence is among the oldest of approaches and probably began in 1876, when Francis Galton investigated how much the similarity between twins changed as they developed over time. Galton's study was concerned with measuring psychophysical abilities, such as strength of handgrip or visual acuity. The concept of general cognitive ability was first described by Charles Spearman in 1904. Later, Alfred Binet and Theophile Simon (1916) evaluated intelligence based on judgment, involving adaptation to the environment, direction of one's efforts, and self-criticism.
Most standard test results now include three scores: VIQ, PIQ, and FSIQ. The VIQ score measures verbal ability (verbal IQ), PIQ measures performance ability (performance IQ), and FSIQ provides an overall measurement (full scale IQ). Commonly used IQ tests include the Stanford-Binet Intelligence Scale, the Wechsler Intelligence Scale for Children (WISC), and the Wechsler Adult Intelligence Scales. The results achieved by individual testtakers on one of these IQ tests are likely to be similar to the results they achieve on the others, and they all aim to measure general cognitive ability (among other things). Measures of scholastic achievement, such as the SAT and the ACT correlate highly with "g."