Moreover, even supposing that the existence of such an institution as exogamy could be in itself satisfactorily accounted for on some such grounds as those advanced by theories of this kind, it is at once evident that we have here no adequate explanation of the strictness with which the system is enforced, the severe penalties that are exacted for infringement of its rules (which is very often punishable by death), the intense nature of the incest horror generally, and the fact that this horror persists even where, as in civilised countries, there is no organised system of exogamy in the technical sense. The psychological researches of Freud and his followers would seem to have shown conclusively that this intense aversion to incest (like all repugnances and taboos of a similar kind) is the negative expression of a correspondingly intense desire for the forbidden thing, and therefore no explanation which neglects to take into consideration this desire can be regarded as even approximately satisfactory. If the aversion to incest had arisen merely as a consequence of the age-long practice of exogamy--which itself is due to other causes--there would be no reason why this aversion should be intimately connected with the positive tendency to incest or of sufficient strength to overcome this tendency, with the powerfulness of which we are now well acquainted. In view of the existence of this strong tendency to incest (which was not appreciated before the work of Freud), it seems no longer possible to maintain, either that exogamy can have arisen independently of the counter-impulse to repress this tendency, or that this counter-impulse, which finds its psychic expression in that incest horror so generally observable both in primitive and cultured man, can be satisfactorily explained as the result of any institution or custom itself unconnected with the tendency to incest. [Sidenote: The biological absurdity of parent-child incest]