widening age gap between the successive order of wives and because of the decreasing exposure to coitus, if all coitus occurs in marriage. Disease Studies show there are two mechanisms that could lead to higher prevalence rates of HIV in men and women who are in polygynous unions: partners in polygynous unions have more extra-marital relationships and thus increase each other's exposure to HIV; women who are recruited into a polygynous union are more likely to be HIV positive than those who marry a monogamous husband. In addition to these two mechanisms, variation in HIV prevalence rates by union type is possibly due to individuals in polygynous unions are typically part of a sexual network with concurrent partnerships. The ecological association between polygyny and HIV prevalence is shown to be negative at the sub-national level. HIV prevalence tends to be lower in countries where the practice of polygyny is common, and within countries it is lower in areas with higher levels of polygyny. Proposed explanations for the protective effect of polygyny include the distinctive structure of sexual networks produced by polygyny, the disproportionate recruitment of HIV positive women into marriages with a polygynous husband, and the lower coital frequency in conjugal dyads of polygynous marriages. For example, studies in Malawi have shown that for men and women in polygynous marriages, the rate of HIV is between 10–15%. About 14% of Malawi's population is infected with HIV, which causes AIDS, according to official figures. There are approximately 78,000 AIDS-related deaths and 100,000 new infections every year in the country. Criticism Criticism against polygyny focuses on the wellbeing of the wives in such marriages, including coercion, buying and selling of wives (such as through bride price, which is common in societies that practice polygyny), concerns about inequality and the fate of the young men left without wives (such as the lost boys in the FLDS Church), as well as the relation between polygyny practiced on a large scale in a society and war. A notable critic of polygyny was Thomas Aquinas nearly eight centuries ago. He contended that polygyny is unjust to wives and children. He also argues that it creates rival stepchildren and forces them to compete for attention, food, and shelter. According to Aquinas, polygyny violates the requirements of fidelity between husband and wife. Polygyny has been criticized by feminists such as Professor John O. Ifediora, who believes that women