In Nigeria, polygyny has been implicated as one of the determinants of fertility levels. To understand the effects of the practice on fertility levels in the state, this study was undertaken. The specific objective was to examine the effect of polygyny on fertility levels in Benue State, Nigeria. Theory of Reasoned Action (TRA) was adopted in the study. The study used cross-sectional research design, with Benue state as the study area. The population of the study were all adults. The formula by Cochran was used by the researcher to determine the sample size of 1,174 respondents. Multistage sampling techniques were used in the study. Questionnaire and Key Informant Interview (KII) guide were used. Face and content validity and test-retest type of reliability was used. The data were analysed with descriptive statistics and inferential statistical tools as well as manual content analysis.

The findings showed that for majority of the respondents, polygyny provided many wives and thus reduced the frequency of sexual intercourse between the husband and the wives. Furthermore, polygyny encouraged child birth and enhanced early marriage. The practice encouraged sexual abstinence and gave room for exclusive breastfeeding. Generally, it enhanced birth spacing and limiting and gave prestige to husbands and also enhanced child survival. The study concluded that polygyny has influenced the level of fertility in Benue State. The study recommended among others the choice of polygyny is determined by individual, family and socio-cultural factors. To checkmate its influence on fertility levels, those factors must be addressed, including demand for children, child preference, social pressure and self-discipline. This could be achieved through sustained awareness creation through formal and informal mechanisms.