The principal ground of the low order of intelligence displayed by simple crowds is that the ideas and reasonings which can be collectively understood and accepted must be such as can be appreciated by the lower order of minds among the crowd. These least intelligent minds bring down the intelligence of the whole to their own level. This is true in some degree even of crowds composed of highly educated persons; for, as in the case of the emotions and sentiments, the higher faculties are always more or less specialised and differentiated in various ways through differences of nurture and training; whereas the simpler intellectual faculties and tendencies are common to all men.
A second condition, which co-operates with the foregoing to keep the intellectual processes of crowds at a low level, is the increased suggestibility of its members. Here is one of the most striking facts of collective mental life. A crowd impresses each of its members with a sense of its power, its unknown capacities, its unlimited and mysterious possibilities; and these, as I have shown in Chapter III of my _Social Psychology_, are the attributes that excite in us the instinct of subjection and so throw us into the receptive suggestible attitude towards the object that displays them. Mere numbers are capable of exerting this effect upon most of us; but the effect of numbers is greatly increased if all display a common emotion and speak with one voice; the crowd has then, if we are in its presence, a well nigh irresistible prestige. Hence even the highly intelligent and self-reliant member of a crowd is apt to find his critical reserve broken down; and, when an orator makes some proposition which the mass of the crowd applauds but which each more intelligent member would as an individual reject with scorn, it is apt to be uncritically accepted by all alike; because it comes to each, not as the proposition of the orator alone, but as a proposition which voices the mind of the crowd, which comes from the mass of men he sees around him and so comes with the power of a mass-suggestion.
A further ground of the suggestibility of the crowd is that prevalence of emotional excitement which was discussed in the foregoing pages. It is well recognised that almost any emotional excitement increases the suggestibility of the individual, though the explanation of the fact remains obscure. I have suggested that the explanation is to be found in the principle of the vicarious usage of nervous energy, the principle that nervous energy, liberated in any one part of the nervous system, may overflow the channels of the system in which it is liberated and re-enforce processes initiated in other systems. If this be true, we can see how any condition of excitement will favour suggestibility; for it will re-enforce whatever idea or impulse may have been awakened and made dominant by 'suggestion.' The principle requires perhaps the following limitation. Emotion which is finding outlet in well-directed action is probably unfavourable to all such 'suggestions' as are not congruent with its tendencies. It is vague emotion, or such as finds no appropriate expression in action, that favours suggestibility. The most striking illustrations of the greatly increased suggestibility of crowds are afforded by well-authenticated instances of collective hallucination, instances which, so long as we fail to take into account the abnormal suggestibility of the members of crowds, seem utterly mysterious, incredible, and super-normal.