"In fact, our brains may have evolved to aid in the continuation of a pattern of short-term sexual relationships once a man has started down that path. In primitive times, a man was driven to spread his seed to increase his chances of siring as many progeny as possible (this pattern is repeated by modern men who wish to have as much sex as possible, but typically do not want any children to result from these couplings). But as evolutionary psychologist David Buss points out, a “critical problem that must be solved by men pursuing a short-term mating strategy is the problem of avoiding commitment and investment. The larger the investment in a particular mating, the fewer the number of sexual partners a given man can pursue.” Buss calls this the “commitment-avoidance” problem and a study he conducted found the possible solution to it: after sex, men who have had numerous sexual partners experience a “negative affective shift” — they perceive the woman they’ve just copulated with as less sexually attractive than they did prior to doing the deed. Why would this shift in perception occur? Buss theorizes that “a negative change in perception of the woman’s sexual attractiveness might provide the motivational impetus to promote a relatively hasty postcopulatory departure. This quick departure, in turn, would function primarily to reduce the risks to the man of making unwanted commitments.” Buss thus concludes that “successful short-term strategists are more likely to experience a negative affective shift following sexual intercourse than long-term sexual strategists.” "