The meaning of the word "intelligence" has been hotly contested for many years. In today's psychological landscape, intelligence can be very generally defined as the capacity to learn from experiences and adapt to one's environment, but thanks to the many different theories of intelligence that have been developed over the last century or so, there are many different frames in which to discuss it.

"General" Intelligence

Francis Galton, influenced by his half-cousin Charles Darwin, was the first to propose a theory of intelligence. Galton believed intelligence was a real faculty with a biological basis that could be studied by measuring reaction times to certain cognitive tasks. Galton measured the head sizes of British scientists and ordinary citizens, but found no relationship between head size and his definition of intelligence.

A deeper search for understanding of human intelligence began in the early 1900s when Alfred Binet began administering intelligence tests to school-age children in France. His goal was to develop a measure that would help determine differences between normal and subnormal children. Binet's research assistant, Theodore Simon, helped him develop a test for measuring intelligence. It became known at the Binet-Simon Scale, the predecessor for the modern IQ test.

In 1904, Charles Spearman published an article in the American Journal of Psychology titled "General Intelligence." Based on the results of a series of studies collected in England, Spearman concluded that there was a common function across intellectual activities that he called g, or general intelligence. Since the article, research has found g to be highly correlated with many important social outcomes and the single best predictor of successful job performance. The current American Psychological Association definition of intelligence involves a three-level hierarchy of intelligence factors, with g at its apex.

In 1940, David Wechsler became a major critic of general intelligence and the Binet-Simon scale. He was a very influential advocate for the concept of non-intellective factors (variables that contribute to the overall score in intelligence, but are not made up of intelligence-related items, including lack of confidence, fear of failure, attitudes, etc.), and he felt that the Binet-Simon scale did not do a good job of incorporating these factors into intelligence. He suggested that these factors were necessary for predicting a person's capability to be successful in life. Wechsler further defined intelligence as the capacity of an individual to act purposefully, to think rationally, and to deal effectively with his or her surroundings or situation.

Multiple Intelligence

An early theory of multiple intelligence is attributed to Edward Thorndike, who in 1920 theorized three types of intelligence: social, mechanical, and abstract. Thorndike defined social intelligence as the ability to manage and understand people. He focused on behavior rather than consciousness in his research; as such, his studies constituted the beginning of investigations related to social intelligence.

In the mid-20th century, Raymond B. Cattell proposed two types of intelligence rather than a single general intelligence. Fluid intelligence (Gf) is the capacity to think logically and solve problems in novel situations, independent of acquired knowledge. Crystallized intelligence (Gc) is the ability to use skills, knowledge, and experience. It does not equate to memory, but it does rely on accessing information from long-term memory. Cattell hypothesized that fluid intelligence increased until adolescence and then began to gradually decline, while crystallized intelligence increased gradually but remained relatively stable across most of adulthood until declining in late adulthood.

In more recent decades, many new theories of multiple intelligence have been proposed. In 1983, Howard Gardner published a book on multiple intelligence that breaks intelligence down into at least eight different modalities: logical, linguistic, spatial, musical, kinesthetic, naturalist, interpersonal, and intrapersonal intelligences. A few years later, Robert Sternberg proposed the Triarchic Theory of Intelligence, which proposes three fundamental types of cognitive ability: analytic intelligence, creative intelligence, and practical intelligence.

Emotional Intelligence

In 1990, Peter Salovey and John Mayer coined the term "emotional intelligence" and defined it as "the ability to monitor one's own and others' feelings, to discriminate among them, and to use this information to guide one's thinking and actions. " Hendrie Weisinger also worked with theories of emotional intelligence. He emphasized the significance of learning and making emotions work to improve oneself and others. He documented and illustrated the positive effect emotions could have in personal settings and work environments. Both emotional intelligence and social intelligence have been positively associated with good leadership skills, good interpersonal skills, positive outcomes in classroom situations, and better functioning in the world.