On the correlation between intelligence and the size of the head

O n account of the considerable intrinsic interest of the question as to whether or not there is a sensible degree of correlation between intellectual capacity and the size of the brain or of the head it seems desirable to have all the exact and direct evidence on the matter which can be obtained. It has been alternately maintained that such a correlation does exist, and that it does not exist, but until comparatively recently there have been no attempts to obtain by adequate methods any definite quantitative statement regarding this question. It is, I think, perfectly obvious that the only way in which a definite answer may be obtained to the problem is to apply proper methods of reduction to a sufficiently large body of homogeneous statistics which record for each individual some measure of the intellectual ability on the one hand and of the size of the brain or head on the other hand. It is idle to attempt to reach any answer by an examination of individual cases, because, to name one point only, our general experience teaches beyond question that if such a correlation exists at all it is of a low order, and in consequence even its existence, to say nothing of its amount, cannot be determined from individual instances. T h e important thing to determine first of all is whether, if we take a large sample out of a homogeneous population, the individuals of more than average intellectual ability possess brains or heads above the average in size. It is to get evidence on this direct question of fact that, so far as I understand it, the biometrical work on the problem has been undertaken.