The author discusses the inadequacies of all the theories and illustrates his remarks from the theories of Spencer and Bergson. Spencer stated that laughter affords an outlet for surplus nervous energy, escaping by the motor nerves in most frequent use, those supplying the muscles of speech and respiration. This theory is inadequate, because laughter occurs independently of this escape of energy. Bergson states that the essential function of laughter is disciplinary. Mc Dougall points out that this theory no more covers the essential facts of laughter than Herbert Spencer's. They both fail to answer the question: For what end did the human species acquire this capacity for laughter? Laughter is a highly complex co-ordinated series of movements, maintained by an impulse so strong and definite that it often defies the control of the will. The author proceeds to consider the conditions which excite laughter and the condition of laughter itself. Laughter interrupts the train of mental activity; it diverts or
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rather relaxes the attention, and so prevents the further play of the mind upon the ludicrous object. So powerful is laughter to interrupt conative process, that its more intense degrees arrest well-practised and habitual bodily actions; and the hearty laugher collapses, temporarily incapacitated for all mental or bodily activity. Secondly, the bodily movements of laughter hasten the circulation and respiration, and raise the blood pressure; and so bring about a condition of euphoria which gives a pleasurable tone to consciousness.
McDougall does not think that laughter always expresses pleasure: he thinks that laughter has been wrongly regarded as the normal expression of pleasure or the more intense degree of the feeling which is expressed by the smile. This he states is unquestionably the normal expression of pleasure. Although admitting that we are often pleased when we laugh he contends that the things we laugh at are essentially displeasing and that they would, in point of fact, displease us if we did not respond with laughter, inasmuch as they consist in the minor defects, mishaps and misfortunes of our fellows. Laughter is primarily and fundamentally the antidote of sympathetic pain. The capacity of laughter has been acquired by the individual as a protective reaction against all the minor pains of his fellows.
In summing up, McDougall states that laughter is an instinctive reaction of aberrant type. The objects which primarily excite this instinct are such actions, situations and aspects of human beings as would excite in us some sympathetic pain or distress, if we did not laugh. The biological function of laughter is defence of the organism against the many minor pains to which man is exposed by reason of the high sensitivity of his primitive sympathetic tendencies. This defence is achieved in two ways; first, the arrest of the train of thought; secondly, the bodily stimulation resulting from laughter.