What percentage of nineteenth-century Mormons practiced polygyny? Estimates of the answer have evolved as have the methods of posing the question.1 In 1885, Church leaders John Taylor and George Q. Cannon wrote that “the male members of our Church who practice plural marriage are estimated as not exceeding but little, if any, two per cent, of the entire membership of the Church.”2 Expressing the number of practicing males as a fraction of the entire Church population, including members outside of Mormon Country, was no doubt intended to generate a low-sounding figure.

Leonard Arrington and Davis Bitton estimated general or overall polygyny prevalence to be 5 percent of husbands and 12 percent of wives.3 These estimates resulted from an effort to express the earlier claim of 2 percent in a more readily interpretable form. They were not based on actual marriage data. Subsequent data-based studies, some of which are cited in appendix A, suggested that Mormon polygyny prevalence was considerably higher than had been supposed. The fraction of Mormon males with more than one wife was estimated to fall between 13 and 33 percent, depending on the time and place. Estimates of the fraction of Mormon females in polygynous relationships ranged between 25 and 56 percent.

Polygyny is not unique to nineteenth-century Mormons. Of the 1,170 societies recorded in Murdock’s Ethnographic Analysis, polygyny is present in 850, or about 73 percent of them.4 Estimates of prevalence for the examples listed in appendix A range as high as 76 percent of husbands (in Ijebu, Nigeria in 1952) and 72 percent of wives (in Mosogat and Igueben, Nigeria, in 1977–78).5

In what follows, we use a simple demographic model to derive mathematical limits on polygyny prevalence. These limits provide benchmarks from which to assess whether polygyny prevalence in a given context is high or low compared to what is sustainable. If prevalence is high, the model may suggest where to look for reasons. Furthermore, where data are of low quality, the benchmarks provide a check on the reliability of the reported prevalence.6

Several theories of polygyny are available.7 The analysis here is in the spirit of a statement by Eugene Hillman, a Catholic missionary who spent years among the Masai tribe in North Tanzania: “Polygyny is generally practiced only where there is a surplus of marriageable-age women in relation to marriageable-age men. . . . The major reason for a surplus of marriageable-age women, however, is the notable discrepancy in the chronological ages of men and women when they actually get married. Women marry relatively early in life, while men marry relatively late.”8

Women’s tendency to marry at younger ages than men means that, even if each age cohort exhibits the same number of males as females, there will be more females of marriageable age.9

The argument is illustrated in figures 1a and 1b. Figure 1a shows a hypothetical, perfectly symmetric population pyramid. The number of males in each age group is given by the length of the bar to the left of the center point (labeled zero), and the number of females of each age cohort is measured to the right. Each five-year cohort is about 15 percent larger than the prior cohort, reflecting population growth rates of about 3 percent per year. If people marry only within their cohorts, the ratio of marriageable men to marriageable women is one, and thus there is little room for polygyny.

If men delay marriage relative to women, however, the effect is similar to shifting the pyramid as in Figure 1b, where each male cohort is compared to the younger female cohort. The result is a ratio of about 115 marriageable-age females to 100 marriageable-age males, even though the overall number of females and males is the same.

The difference in marriageable ages has not been universally accepted as a proximate cause of polygyny. For example, Chojnacka argued that such reasoning reverses cause and effect.10 However, the tendency of women to marry older men is observed in monogamous cultures as well, suggesting that polygyny does not cause (though it may exacerbate) the difference in marriageable ages.11

Given the difference in marriageable ages, population growth adds to the imbalance by increasing the size of the youngest cohorts of marriageable females. In this sense, polygyny is self-reinforcing: allowing more women to marry increases the growth rate and exacerbates the imbalance.12

Mortality also has an effect. Typically more males than females are born, but infant mortality is significantly higher among males. Furthermore, males die disproportionately in wars and occupational accidents. In some contexts, these effects are counterbalanced by female infanticide and adult mortality due to childbirth complications.

Migration can be a factor as well.13 The analysis is complicated by temporary emigration, as when males of marriageable age leave for a period of years for employment, military service, or—as was common in nineteenth-century Mormondom—church assignment.

Our simple demographic model suggests that, given the parameters observed in nineteenth-century Utah, polygyny prevalence in excess of 15 to 20 percent of males and 25 to 30 percent of females is implausible. Of course, pockets of higher percentages, offset by lower percentages elsewhere, are possible. St. George was apparently one of those pockets.

The next section informally describes our definition of sustainable polygyny prevalence. We then explain the implications of sustainability for estimating demographic limits on polygyny prevalence. Finally, we apply the framework to nineteenth-century St. George, find that polygyny prevalence was above sustainable levels, and offer some conjectures on how the high prevalence might be explained.

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Summary and Remarks

Women’s tendency to marry older men, along with other factors, induces an imbalance in the marriage market. An understanding of this tendency can inform the analysis of marriage institutions, including those of monogamous cultures. Specifically, a substantial preponderance of marriageable women relative to marriageable men must result in at least one of the following three phenomena: (1) large numbers of never-married women, (2) large numbers of sometimes-married women, or (3) polygyny.

The practice in the modern industrialized world, where the third outcome is prohibited by law, is naturally a combination of the first and second outcomes. Tax incentives, changing mores, and broader opportunities for women have combined to create a significant number of marriageable women who remain single by choice. At the same time, high divorce rates allow high turnover in the marriage market, a practice sometimes referred to as serial polygamy. However, even when polygyny is both legally and socially acceptable, there are mathematical limits to its prevalence. Figures 2 and 3 suggest that, in the long run, polygyny by more than 20 percent of husbands and 30 percent of wives is on the high end of what is mathematically plausible, unless the difference in marriageable ages is very large.