Thus it can easily be understood that any races which tended to indulge to any large extent the impulses which prompt to incestuous unions between parents and children would be at a disadvantage as compared with those races in which these unions did not occur or occurred less frequently; the latter races tending therefore to supplant the former. Given then the existence of a strong impulse towards parent-children unions, we can see how biological factors may very well have favoured the growth of strong counteracting factors, such as manifest themselves in the repression of the tendencies towards this form of incest. These considerations would of course apply not only to relations between parents and children but to all other unions in which the age difference between the partners is considerable. They would not however apply to unions between brother and sister or between cousins who are of approximately equal age. The influences which lead to the aversions to these latter unions must be sought elsewhere[261]. [Sidenote: Fear of the consequences of parental jealousy] (6) A potent set of influences calculated to inhibit tendencies to incest are those connected with sexual jealousy. A boy's love towards his mother, as we have seen, almost necessarily brings him to some extent into conflict with his father, while a girl's affection to her father is similarly calculated to bring about the jealousy of her mother. This arousal of jealousy on the part of the parents may produce repression of the incest tendency in the child in a variety of ways; of which perhaps the most frequent and important are:-- _a._ fear of punishment at the hands of the jealous parent, and _b._ unwillingness to cause injury or sorrow to this parent because of genuine affection being felt towards him (or her)--affection which of course may quite well co-exist with very considerable jealousy and rivalry. Both these motives appear prominently in psycho-analytic investigations of the conditions underlying the repression of the Œdipus complex in normal and neurotic persons of the present day, and both have been operative for long ages in the past wherever the family has existed in a monogamous form in which the parents lived together for a considerable period after the birth of their children. Very similar motives may also co-operate in the repression of incest tendencies as between brothers and sisters, the jealousies in this case being for the most part those between brothers or between sisters respectively. [Sidenote: Strong family ties conflict with social development]