A number of motivational factors have been investigated in attempts to explain at least some of the Negro-white difference in intelligence test performance in terms of differences in motivation. The evidence to date does not support the differential motivation hypothesis. This should not be too surprising, since experimental studies of the effects of motivational factors on intelligence testing have generally shown either very small or non-significant effects, and when differences have been found they tend to show that conditions most typical of those in which intelligence tests are normally given yield the best scores. Burt and Williams (1962), for example, found that children obtained slightly higher scores when taking tests for school promotion rather than for experimental purposes. Intelligence tests are quite insensitive to external motivational manipulations. Tiber and Kennedy (1964) tested middle-and lower-class white children and lower-class Negro children with and without several different incentives, such as praise after each test item, verbal reproof, and candy reward. These various testing conditions had no significant effects on StanfordBinet IQs and showed no interaction with social class or race. Tiber and Kennedy concluded that the IQ differences usually found between such social class and racial groups cannot be attributed to motivational differences. This conclusion is too sweeping, of course, since other motivational factors not under the experimenter's control could affect test performance. But the fact remains that scores on IQ tests have proved highly resistant to experimental manipulations of incentives and motivational sets.