Societies often have to fend off attacks, both from without and from within. But there are some cases in which the structure of the society itself is self-defeating and contradictory. One example is antagonistic relations between the sexes, where men and women fear contamination from each other. In such cases, pollution ideas are used to keep men and women in their assigned roles. In some societies, male dominance is strongly enforced and female contact is feared and shunned. In particular, menstrual blood is often regarded as a source of danger and contamination to men. Women's sexual purity is often closely guarded. At the same time, “softening legal fictions” (180) often intervene to protect the freedom of the sexes. But where male dominance is countervailed by principles protecting women, contradiction and paradox often result and the society becomes unstable.