Data ability to solve a laboratory problem are reported for samples of middle-and working-class families in Bombay, India; Minneapolis, Minnesota; and San Juan, Puerto Rico. In all three samples, working-class family groups were less successful in solving the problem than middle-class families. In Bombay the differences were so great they could not be encompassed within the same measurement procedures. Tests of three factors which could account for the lower problem-solving ability of working-class samples are also presented. A "differential motivation" theory was tested by means of an index of the effort expended in the task. The resulsts show no social class difference. Tests of a "communication block" and a "cognitive style" theory revealed large social class differences in volume of intrafamily communication and in creativity.It is concluded that the differences in problem-solving ability,as well as restricted working-classs communication and creativity (which appear partly to explain the differences in problem-solving ability) are similar in all three societies,despite vast differences in culture. However, the more urbanized and industrialized the society, the smaller the social class differences.