That social influences shape every person's practices, judgments and beliefs is a truism to which anyone will readily assent. A child masters his" native" dialect down to the finest nuances; a member of a tribe of canni bals accepts cannibalism as altogether fitting and proper. All the social sciences take their departure from the observa tion of the profound effects that groups exert on their members. For psycholo gists, group pressure upon the minds of individuals raises a host of questions they would like to investigate in detail. How, and to what extent, do social forces constrain people's opinions and attitudes? This question is especially pertinent in our day. The same epoch that has witnessed the unprecedented technical extension of communication by Solomon E. Aseh has also brought into existence the de liberate manipulation of opinion and the" engineering of consent." There are many good reasons why, as citizens and as scientists, we should be concerned with studying the ways in which human beings form their opinions and the role that social conditions play. Studies of these questions began with the interest in hypnosis aroused by the French physician Jean Martin Charcot (a teacher of Sigmund Freud) toward the end of the 19th century. Charcot believed that only hysterical patients could be fully hypnotized, but this view was soon challenged by two other physi cians, Hyppolyte Bernheim and A. A. Li6bault, who demonstrated that they could put most people under the hyp notic spell. Bernheim proposed that hypnosis was but an extreme form of a normal psychological process which be came known as" suggestibility." It was shown that monotonous reiteration of in structions could induce in normal per sons in the waking state involuntary bodily changes such as swaying or rigid ity of the arms, and sensations such as warmth and odor.

It was not long before social thinkers seized upon these discoveries as a basis for explaining numerous social phe nomena, from the spread of opinion to the formation of crowds and the follow ing of leaders. The sociologist Gabriel Tarde summed it all up in the aphorism:" Social man is a somnambulist." When the new discipline of social psy chology was born at the beginning of this century, its first experiments were