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)