A major concern in 20th-century psychology has been to define and measure intelligence

Christopher Frith & Eve Johnstone, SCHIZOPHRENIA, A Very Short Introduction, 2003. "A major concern in 20th-century psychology has been to define and measure intelligence. The concept of intelligence arises from the observation that intellectual abilities tend to be correlated. Someone who is good at mental arithmetic is, on balance,  also likely to have a good vocabulary and to be good at problem solving. The best way of measuring this kind of general intelligence is to record performance on a wide range of different kinds of tasks. In many cases speed as well as accuracy is taken as a sign of good performance. Many intelligence (or IQ) tests of this kind have been developed, and information about performance on such tests is available from the large samples of the general population. The most widely used is the Wechsler Adult Intelligence Scale (known as the WAIS)." ... "As already noted, the concept of intelligence is based on the observation that people who are good at one kind of test tend also to be good at others. However at the same time that tests were being developed to measure IQ, neuropsychologists were demonstrating the opposite phenomenon. Circumscribed brain lesions can cause impairment in the performance of one kind of test, while performance on all others remains intact. The French neurologist Paul Broca was the first to demonstrate this in 1861 with his patient known as Tan who could not speak (except to say, 'tan tan tan'), but could understand speech".