Charles Spearman (British psychologist, 1863–1945) described a concept he referred to as general intelligence or the "g factor". He utilized the method named 'factor analysis' to investigate a few mental ability tests; his conclusion was that the results and scores on these tests were very similar:

People who did well on one cognitive test usually performed well on other tests, while those who performed badly on one test usually scored badly on others. Spearman concluded that intelligence is a general cognitive ability that can be measured and numerically expressed.

Louis L. Thurstone (1887-1955) didn't approach intelligence as a single, general ability; his theory focused on seven different primary mental abilities:

• Associative memory: The capacity to memorize and recollect.
• Numerical ability: The capacity to solve arithmetic problems.
• Perceptual speed: The capacity to see differences and similarities among objects.
• Reasoning: The capacity to find rules.
• Spatial visualization: The capacity to visualize relationships.
• Verbal comprehension: The capacity to define and understand words
• Word fluency: The capacity to produce words rapidly.

• Bodily-kinesthetic intelligence: the capacity to control your body and handle objects skillfully.
• Interpersonal intelligence: the capacity to find and respond in the right way to the moods, motivations, and desires of other people.
• Intrapersonal intelligence: the capacity to be self-aware and aligned with your inner feelings, values, beliefs, and thinking processes.
• Logical-mathematical intelligence: the capacity think conceptually and abstractly, and the capacity to discern logically or numerical patterns.
• Musical intelligence: the capacity to produce and appreciate rhythm, pitch, and timbre.
• Naturalistic intelligence: the capacity to know and categorize animals, plants, and other objects in nature.
• Verbal-linguistic intelligence: the capaity to have developed verbal skills and sensitivity to the sounds, meanings, and rhythms of words.
• Visual-spatial intelligence: the capacity to think in images and pictures, to visualize accurately and abstractly.

Robert Sternberg (American psychologist) proposed the concept "successful intelligence; this concept involves three different factors:

• Analytical intelligence:the ability to evaluate information and solve problems.
• Creative intelligence: the ability to come up with new ideas.
• Practical intelligence: the ability to adapt to a changing environment.