related economic shocks had a significant association with rates of polygamy in subsaharan Africa, rather than female agricultural contributions (which are typically relatively small in the West African savanna and sahel, where polygyny rates are higher), finding that polygyny rates decrease significantly with child mortality rates. Types of polygyny Polygynous marriages fall into two types: sororal polygyny, in which the co-wives are sisters, and non-sororal, where the co-wives are not related. Polygyny offers husbands the benefit of allowing them to have more children, may provide them with a larger number of productive workers (where workers are family), and allows them to establish politically useful ties with a greater number of kin groups. Senior wives can benefit as well when the addition of junior wives to the family lightens their workload. Wives', especially senior wives', status in a community can increase through the addition of other wives, who add to the family's prosperity or symbolize conspicuous consumption (much as a large house, domestic help, or expensive vacations operate in a western country). For such reasons, senior wives sometimes work hard or contribute from their own resources to enable their husbands to accumulate the bride price for an extra wife. Polygyny may also result from the practice of levirate marriage. In such cases, the deceased man's heir may inherit his assets and wife; or, more usually, his brothers may marry the widow. This provides support for the widow and her children (usually also members of the brothers' kin group) and maintains the tie between the husbands' and wives' kin groups. The sororate resembles the levirate, in that a widower must marry the sister of his dead wife. The family of the late wife, in other words, must provide a replacement for her, thus maintaining the marriage alliance. Both levirate and sororate may result in a man having multiple wives. In monogamous societies, wealthy and powerful men established enduring relationships with, and established separate household for, multiple female partners, aside from their legitimate wives; a practice accepted in Imperial China up until the Qing Dynasty of 1636–1912. This constitutes a form of de facto polygyny referred to as concubinage. Household organization Marriage is the moment at which a new household is formed, but different arrangements may occur depending upon the type of marriage and some polygamous marriages do not result in the formation of a single household. In many polygynous marriages the husband's