Polygyny can increase, decrease, or have no effect on fertility. Understanding how this can occur requires consideration of both the proximate determinants of fertility and the ultimate effects of polygyny as a female reproductive strategy. Several factors reduced the fertility of polygynous women in 19th century Utah, including marrying at an older age, marrying older men, and conflict between co-wives. Sterility did not explain the reduced number of children in polygynous women, nor is there evidence of a “dilution effect” from sharing a husband. If women could anticipate a reduction in their own fertility, why would they still choose polygyny? Evidence suggests that they chose it because the children of polygynous men had increased fertility, high enough to offset the low fertility of polygynous women themselves. Am. J. Hum. Biol. 14:222–232, 2002. © 2002 Wiley-Liss, Inc.