Reading a road map upside-down and generating synonyms for the word "brilliant" are two very different skills. But each is a measurable indicator of general intelligence, a construct that includes problem solving abilities, spatial manipulation and language acquisition. Scientists generally agree that intelligence can be captured by psychometric tests. But the study of intelligence is dogged by questions of just how much IQ contributes to an individual's success and well-being, how genes and environment interact to generate smarts and why the average IQ score rose throughout the world during the twentieth century.