ABSTRACT
The theory of mental measurement is based on the rather audacious idea that innumerable and multifarious differences among people can be compressed onto a single axis, and that people can be ordered along that axis by assigning each a number that has consistency and meaning, and that permits comparisons. This idea can be traced back to the late 19th century, to the laboratory of the Englishman Francis Galton (1869). Galton, a cousin of Charles Darwin, is credited with being the first to propose that intelligence is a unified mental ability, and with providing a methodological foundation for studying intelligence and other individual differences quantitatively (Jensen, 1982). Galton’s methodology of quantification spawned the mental testing movement by way of Galton’s American student, James McKeen Cattell.
