Friday, January 7, 2022

Notes on Reproductive Rates in Polygynous Unions in Ross Hassig, "Polygamy and the Rise and Demise of the Aztec Empire" (2016)

  

The reproductive advantages of polygyny are not always what they are assumed to be. Actual studies of polygynous unions suggest that fewer men than women will contribute genetically to succeeding generations. This outcome is illustrated by the exceptional case of Genghis Khan, who, through formal marriages and other unions, sired so many children in his lifetime that today sixteen million men can trace their ancestry to him. Whether or not those estimates are even roughly accurate, however, such cases are highly exceptional and focus on numbers and growth rather than on the impact of polygyny on the organization of that society considered more broadly. . . . . While it has been argued that is wifely status is determined by the number of offspring produced, it will not decline with additional marriages; if status is determined by marriage order, however, it will. But in fact, reproductive rates for polygynous wives vary not only by wife order but also by husband’s age. First wives typically produce more children than second wives in polygynous unions and so on per added wife. A barren first wife reduces the per capita reproduction rate of polygynous wives. But even if the first wife is fertile, the mere addition of a second wife nevertheless reduces the number of children subsequently wives. In some cases, the fertility of the first polygynously married woman is higher than that of monogamously married women, but fertility rates fall on a per-wife basis in polygynous families such that overall, monogamous wives produce more children per woman. For example, in one study of 3,355 wives of polygynists among nineteenth-century Mormons in the United States, women averaged 5.9 children each whereas an equal number of monogamists from the same group averaged 8 children. Among the Shipibo of Peru, polygynously married women had 1.3 fewer children per reproductive span than did monogamously married women, and the percentage of living children for the latter was higher, too: 70.9 versus 63.2 percent. African data indicate that the fertility rate of monogamous women is one-third higher than of polygynous women.

 

Different cultural practices and values often account for variations in family sizes. But most of the reason for the difference in fertility rates between polygynously married women and monogamous ones is biological and not simply cultural.

 

First, there is a pronounced difference in rates of sexual intercourse in monogamous marriages compared with polygynous ones. The frequency of coitus is higher for monogamous wives, who receive the exclusive sexual attention of their husbands, than for polygynous wives, who must share it with co-wives. The frequency of coitus is greater for polygynous men than for monogamous ones, but it is more frequent for monogamous women than for polygynous ones. However, divided attention only accounts for part of the lower reproduction among polygynously married women: age differences also play a role. Husbands in such unions tend to be significantly older than their wives, especially their later wives, and with advanced age comes reduced rates of coitus.

 

Second, sexual intercourse with multiple wives exacts a cost. Although the polygynous husband enjoys a greater frequency of coitus, this increased frequency lowers the sperm rate per emission and thus reduces the likelihood of impregnating his partner per attempt. Moreover, it slightly skews the normal 1:1 sex ratio of births. In polygynous unions, slightly more girls are born than boys, which arises from the fact that girls are likelier to be conceived if coitus occurs on or within a day of ovulation, and boys are likelier if coitus occurs at other times. This observed consequence of polygynous marriage is presumed to be the result of which partner chooses the time of coitus. When wives select the time, as often occurs in polygynous marriages with multiple wives competing for the sexual attention of one husband, coitus is likeliest to occur during ovulation, when the women experience their maximum libido. As a result, slightly more girls tend to be conceived. But when men choose the time, as is more often the case in monogamous relationships in which the man’s sexual attentions are focused exclusively on one woman, coitus is distributed throughout the reproductive cycle, and boys are likelier to be conceived. These biological facts suggest that monogamous societies should produce approximately equal numbers of boys and girls, skewed slightly in favor of the former, while polygynous societies should favor the latter.

 

Third, reproductive rates are also affected by birth intervals—the longer the time between births, the fewer children a given woman can have—and these durations tend to differ between polygynous and monogamous women. While birth intervals are shorter for first wives than for later ones in polygynous households, and average, monogamous wives have birth intervals four months shorter than polygynous ones. When one polygynous wife is already pregnant or nursing, the husband, in theory, has another wife available. But the idea that polygyny guarantees the availability of a sexually receptive wife is not invariably true, especially in societies with long postpartum taboos on sex or in which mothers nurse their children for years, as was the case in Mexico, at least after the Conquest. These circumstances remove women from the sexually receptive pool for extended periods and greatly increase the likelihood that none of a man’s polygynous wives will be sexually available to him at any given time. Indeed, postpartum taboos on sexual relations are more common, longer, and more ideally observed in polygynous societies than in monogamous ones. Whether the Aztecs observed postpartum abstinence in polygynous unions, however, is uncertain.

 

Fourth, rates of reproduction are also affected by the residence patterns of polygynous wives. Two basic residential patterns occur in polygynous marriages. In one, each wife lives separately, and in the other, all the wives live together (as if often the case in societies that practice sororal polygyny), though in the latter case, co-resident wives sometimes have independent rooms within their husband’s house or separate houses within their husband’s residential compound. (Ross Hassig, Polygamy and the Rise and Demise of the Aztec Empire [Albuquerque: University of New Mexico Press, 2016], 61, 65-67, notes silently removed)