Another instinct which is of especial import ance to the social psychologist is, obviously, the gregarious instinct. The different types of so cial groups formed by human beings are espe cially interesting to those who regard the mind of a group as an independent entity. We may distinguish groups linked by the sex and pa rental instincts, i.e., families; groups linked by economic necessity, by a common aim, such as philanthropy or religion, by likeness of external circumstances or situation, and so on. Each type of group has its peculiar features and the mind of an individual differs according to which type of group he is related to at a given time. It is interesting also to study the mental differences between groups, such as the races and the sexes, separated by congenital unlike nesses.
An innate tendency of the first significance to social psychology is suggestibility. It is really the tendency which we all have to carry out ideas into action, but the term usually indi cates this tendency in the case where the ideas are recognized as coming from some other mind. The strongest influence which opposes suggesti bility is the fighting instinct, inherited from the lower animals. Individuals vary greatly in the degree of their suggestibility; some persons balk at every suggestion coming from others, even though it be in the line of their own de sires, while others are at the mercy of the last person who advises them. Measurement of the suggestibility of a given individual is a task of much practical value in diagnosis. It has been attempted by the method of showing pictures and then questioning the individual about what he has seen. A certain number of the questions relate to details which were not really present in the picture, and the number of such sugges tions that the person accepts is regarded as indi cating the degree of his suggestibility. Binet, in his (La suggestibility,' developed a number of other simple methods of testing suggestibil ity, especially in children. Every normal per son tends to resist suggestions to a certain de gree. Thus Sidis, in his (Psychology of Sug gestion,' states as the law of normal suggesti bility that it varies inversely with the directness of the suggestion; direct suggestions tend to ex cite the fighting instinct. In certain cases, how ever, even a normal person will accept direct suggestions. The most important conditions of such acceptance are the prestige of the sug gestor and repetition of the suggestion. Indirect suggestions are suggestions given in such a way that they do not obviously come from another mind; they seem to proceed from the individ ual's own thoughts or from the surroundings. The whole problem of securing the acceptance of suggestions is probably the most practically important part of social psychology. It con fronts the advertiser, the salesman, the educa tor, the religious and the political leader. The
psychology of advertising, for instance, is now a distinct branch of research to which the busi ness world is lending strong financial support and from which it is getting valuable returns.
Under certain conditions suggestibility be comes abnormal in degree, that is, the fighting instinct ceases to oppose it. This usually oc curs when the field of consciousness is nar rowed, so that the criticism which a suggestion ordinarily encounters in the mind of the per son to whom it is made is lacking. The sug gested idea takes entire possession of the at tention, and no reasons for opposing it are con sidered. Such a narrowing of the field of con sciousness, called dissociation, occurs in hysteri cal states, in strong emotions and in the hypnotic trance. Persons thus abnormally suggestible may be dangerous to the community. The ex istence of abnormal suggestibility thus is a fact of which the criminal law must take cognizance.
It is probable, however, that no person other wise thoroughly normal can be hypnotized into the performance of an act of crime. Abnor mally suggestible persons are likely to be men tally defective; although Breuer claims that the hysterical temperament is found in persons of the highest intellect and character. Goddard, in The Criminal Imbecile,' records a case of murder apparently due to the abnormal sugges tibility of a feeble-minded person in the hands of another individual.
Abnormal suggestibility is interestingly manifested, even by normal persons, in a psy chological crowd. A mere aggregate of indi viduals, however closely crowded physically, does not constitute a psychological crowd. The individuals must have their attention fixed on a common object. As soon as a number of per sons concentrate their attention in the same di rection, they tend to become abnormally sug gestible. This is due largely to the loss of the normal sense of the criticism of others. A con siderable portion of the criticism with which a person in the normal state opposes a suggestion is derived from his thought of what persons other than the suggestor may think of his con duct. The feeling that others are with us, as in a psychological crowd, obliterates this criti cism; thus persons in a crowd do what they would never do as isolated individuals. Since the emotions are the experiences which all man kind has in common, while ideas vary greatly from one person to another, an aggregate of persons becomes unified into a psychological crowd more quickly under the influence of emo tions than under that of ideas. The crowd has the characteristics common to all humanity and often displays instincts which are in the normal individual suppressed; it is incapable of real thought. Since most persons enjoy emotions and dislike to think, most persons enjoy enter ing into psychological crowds. LeBon's The Crowd) is the classic discussion of the phe nomena of crowd psychology.