Approximately 85 per cent of societies in the anthropological record permit men to marry multiple wives. Taking wives is always positively associated with status, wealth or nobility, even among highly egalitarian foraging societies. After the origins of agriculture, as human societies grew in size, complexity and inequality, levels of polygynous marriage intensified, reaching extremes in the earliest empires whose rulers assembled immense harems. Today, however, with absolute wealth gaps greater than any seen in human history, monogamous marriage is both normative and legally enforced in most of the world’s highly developed countries. While the roots of the package of norms and institutions that constitute modern marriage can be traced back to classical Greece and Rome, the global spread of this peculiar marriage system has occurred only in recent centuries, as other societies sought to emulate the West, with laws prohibiting polygyny arriving in 1880 in Japan, 1953 in China, 1955 in India and 1963 in Nepal.So normative monogamy was a relatively recent Western invention. And what were the benefits? The paper concludes:
In suppressing intrasexual competition and reducing the size of the pool of unmarried men, normative monogamy reduces crime rates, including rape, murder, assault, robbery and fraud, as well as decreasing personal abuses. By assuaging the competition for younger brides, normative monogamy decreases (i)the spousal age gap, (ii) fertility, and (iii) gender inequality. By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity. By increasing the relatedness within households, normative monogamy reduces intra-household conflict, leading to lower rates of child neglect, abuse, accidental death and homicide.Note that it is not just looking at monogamy (one man one woman marriage), but "normative" monogramy, meaning that it is enforced by law, religion, or other cultural norms.
Biologist Razib Khan comments:
So how did monogamy come to be so common? If you follow Henrich’s work you will not be surprised that he posits “cultural group selection.” That is, the advantage of monogamy can not be reduced just to the success of monogamous individuals within a society. On the contrary, males who enter into polygamous relationships likely have a higher fitness than monogamous males within a given culture.Thus changes to marriage law, such as no fault divorce, are apt to have consequences to society that cannot be analyzed from pure libertarian individualism. Government might be simpler by getting out of the marriage business, but it would be abandoning one of the pillars of modern civilization.