Observing that rates of polygynous marriage tend to increase with wealth difference, the authors of this paper attempt to understand why monogamous marriage has flourished in Europe and other parts of the world even as wealth differences have expanded. The authors theorize that monogamous marriage promotes the success of the groups that employ the practice by suppressing intra-group competition and that this is what has happened in Europe. Subsequently, the authors test hypotheses that are implicated in this theory.