Final answer:
Most societies discourage incest due to the potential for biological degeneration and to promote alliances through exogamous practices. Incest taboos are cultural norms that prohibit sexual relations between close kin to prevent inbreeding and its associated health risks. Marriage rules, including endogamy and exogamy, support this framework by defining appropriate marriage partners.
Step-by-step explanation:
Most societies discourage incest primarily due to social and biological reasons. Incest taboos serve as a cultural norm that prohibits sexual relations between close kin, particularly between parents and their offspring. This norm is underpinned by the potential for biological degeneration due to inbreeding, which can limit genetic diversity and increase the prevalence of heritable diseases. Societies also enforce these taboos as part of broader exogamous practices, which promote alliances between different groups and maintain social order through marriage rules. Therefore, the statement 'b: exogamy promotes alliances' is a key reason incest is discouraged across cultures.
In terms of marriage, societies often enforce endogamy and exogamy rules that dictate what is considered appropriate regarding marriage partners. These rules help individuals understand which groups they should marry into and which are considered off-limits. The incest taboo is universal, although the specific relatives deemed unacceptable for marriage can vary greatly from one culture to another.