The relative contributions of mental speed and accuracy measures to Primary Mental Ability (PMA) IQ prediction were studied in 52 high ability students (mean age 16, mean IQ 120). Both speed and accuracy independently predicted IQ, but not speed over and above accuracy. Accuracy was demonstrated to be universally advantageous in IQ performance, but speed to vary according to the test. Superior performance might best be achieved by attention to accuracy at whatever speed the subject is comfortable working.For many years theorists have struggled to define intelligence. One issue that remains controversial when describing intelligent performance is how mental speed contributes to IQ test results. There is no doubt that a speed-accuracy tradeoff is involved in timed intelligence tests (Furneaux, 1960;Heim, 1970;Pachella, 1974; Pew, 1969). However, the nature of mental speed as it relates to high ability performance still needs to be described.In learning new material or in recalling that previously learned, individuals who respond more quickly are often viewed as more intelligent (Spiegal & Bryant, 1978). When laypersons were asked to generate a list of behaviors that characterize intelligence they provided answers such as, learns rapidly, acts, talks and makes judgments quickly (Sternberg, Conway, Ketron & Bernstein, 1981 ). This assumption that "smart is fast" has fascinated psychologists as well.