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From Galen onwards, psychologists and physiologists have endeavoured to distinguish types or temperaments, with the help of which to classify the bewildering variety of mental and bodily characteristics in human
beings. Their attempts, however, have very seldom resulted in anything more than arbitrary (if convenient) divisions of mankind, for most measurable or definable characteristics present an almost infinite series of gradations. In one matter only is there a perfectly clear division of mankind into two easily distinguishable groups with an almost negligible number of 'intermediate' cases—the division into those who possess reproductive organs of a male or female kind. This primary sexual differentiation is, moreover, highly correlated with certain 'secondary' sexual characters of an anatomical or physiological kind. The question whether there exist any corresponding mental differences is, therefore, one of supreme interest to the psychologist. We might well expect that, here if anywhere, it would be possible to find evidence of clearly distinguishable mental 'types'.
And yet, so far as our present evidence goes, this is not the case. Here too, there is no sharp dichotomy, but at most a tendency for one sex to show a little more or less of any given character than does the other, so that there is a wide overlap in the curves of distribution for the two sexes, the differences between the extreme cases of either sex being enormously greater than the differences in the averages of the two sexes. Moreover, the mental differences seem to be a good deal smaller than the physical ones, and in the mental sphere itself the lower levels tested—the sensory and motor functions together with the more mechanical aspects of retention—reveal greater differences than the higher ones, there being apparently no significant difference at all so far as 'g' or pure intelligence is concerned. Hitherto, however, most experimental investigations have been carried out on the co