Short answers, pulled from the story.
As of 2019, Burkina Faso (36%), Mali (34%), and Gambia (30%) have the highest recorded polygyny prevalence. These countries are part of a region researchers call the "polygamy belt" of West Africa and Central Africa. Outside Africa, the highest rates are found in Afghanistan, Yemen, and Iraq.
Under Islamic marital jurisprudence, Muslim men may have up to four wives at a time, based on verse 4:3 of the Quran, provided they can treat all wives justly and support them equally financially. In most Muslim-majority countries polygyny is legal, with Kuwait imposing no restrictions; Turkey, Tunisia, Albania, Kosovo, and Central Asian countries prohibit it.
Studies of the Ngwa group in eastern Nigeria found that women in polygynous unions are 22-26 percent less fertile on average than women in monogamous ones, with the fertility deficit growing for each successive wife. Women in these marriages face elevated risks of sexually transmitted infections, including HIV, as well as higher rates of anxiety, depression, and other mental health disorders compared to women in monogamous marriages.
Economist Michèle Tertilt found that polygynous countries typically have higher fertility rates, fewer savings reserves, and lower GDP than monogamous ones. A 2014 study estimated that banning polygyny could reduce fertility by 40 percent, increase savings by 70 percent, and increase GDP by 170 percent, because monogamous men tend to invest more in their families and broader institutions.
Polygyny is illegal in the United States. It is practiced by some Mormon fundamentalist sects, such as the Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS Church), despite its illegality. The FLDS Church is estimated to have around 10,000 members residing in communities across Utah, Arizona, Texas, Colorado, South Dakota, Montana, and British Columbia.
Writing nearly eight centuries ago, Thomas Aquinas argued that polygyny is unjust to both wives and children. He contended that it creates rival stepchildren forced to compete for attention, food, and shelter, and that it violates the requirements of fidelity between husband and wife.