It has been hypothesized that homosexual behavior may itself be an adaptation for same-sex affiliation or alliance formation, though this disposition would vary genetically among individuals and occur more often when competition for female partners is especially severe. Evolutionary psychologist David Buss criticized this hypothesis, stating that there is no evidence that most young men in most cultures use homoerotic behavior to establish alliances; instead, the norm is for same-sex alliances to not be accompanied by any sexual activity. Additionally, he states that there is no evidence that men who engage in homoerotic behavior do better than other men at forming alliances or ascending in status. Other researchers have also criticized it, commenting that the cross-cultural data on sexual practices are sketchy and uneven; that there is no need to assume that homosexual behavior, more than any other sexual behavior, is under direct selection rather than being a neutral byproduct; that the hypothesis ignores the existence of sexual orientation; that it contradicts findings that behaviorally homosexual or bisexual men have much lower rates of fatherhood; that primate homosexual behavior is not a uniform phenomenon and varies within and across species; and that since same-sex sexual partners are chosen on the basis of sexual emotion (in contrast to bonobos, for example), alliances of this kind would only occur as often as mutual sexual attraction, and such variability would seem to indicate a lack of design by natural selection.