The average individual often knows very little. He has a sketchy knowledge about most topics, and knowledge in depth only about a very few. The factors that condition what and how much he knows are myriad. Of importance are time, education, income, sex, age, race, occupation, and where a man lives. Controlled, education tends to nullify the other factors, although they may re-emerge as predictors of what, and how much, a man is likely to know. Status in life is a noteworthy medium. Thus income, occupation, age, and race count. So does sex: women know more about health, and men know more about science and public affairs. In public affairs, the best predictors of knowledge are education, exposure to television, and interest in political campaigns. In health, the best predictors are education, print, and sex. In science, they are education, status in life, and print. If controlled, education can be used to stimulate a desire to know. The conclusions result from re-analysis of data collated between 1940 and 1967, in 35 national surveys. (GO)