Although the prevalence of polygyny has decreased in the world in recent years, polygyny still remains as a frequent family structure in West Africa. Using 2006 Demographic and Health Survey (DHS) from Mali, we investigate the effects of polygyny on child health. Using the gender of the firstborn as an instrumental variable for marrying an additional wife, we find that a child’s nutritional status in terms of height-for-age and weight-for-age z-scores is worse in polygynous households compared to monogamous households. Second, we analyze the effect of mother’s rank among co-wives on her own child’s health and find that mother’s rank has a statistically significant effect on a child’s nutritional status. However, the effect of mother’s rank changes across different institutions of polygyny: depending on the ethnic group, the co-wife relationship could be more collaborative than competitive. The findings suggest a more nuanced view of polygyny than currently found in the literature: the effect of the institution of polygyny depend upon the rules governing behavior within the institution.