In psychology, human intelligence is one of the most researched constructs, with its theoretical development spanning more than 100 years. Although there is no agreed upon verbal definition of intelligence, researchers and psychologists generally agree that it involves abstract reasoning and thinking and the ability to understand complex ideas and acquire knowledge. One question that arises is why do people differ in intelligence? Is it mostly due to one thing or is it mostly due to many things (e.g., memory, numerical ability, and verbal ability)? The g theory of intelligence describes differences in intelligence as mostly due to one thing. That “thing,” often referred to as the “g factor,” is commonly used to refer to general mental ability, general intelligence, or psychometric g. This entry ...