Title: The Puzzle of Monogamous Marriage
Author: Joseph Henrich, Robert Boyd and Peter J Richerson
Scope: 3 stars
Readability: 2 stars
My personal rating: 4 stars
See more on my book rating system.

Topic of Book

Using a statistical analysis of data from African societies, the authors test a theory that monogamy developed in modern society because of strong benefits to society.

Key Take-aways

Monogamy has many strong benefits for society, including reducing conflict within families caused by:

• Violent competition between males.
• Rape, murder and other violent crimes
• Age difference between husbands and wife
• Gender inequality
• Domestic violence
• Sex trafficking
• Spouse and child abuse

Other books by the same authors

• Not by Genes Alone: How Culture Transformed Human Evolution
• The Origin and Evolution of Cultures
• Principles of Human Ecology
• Why Copy Others?
• The Secret of Our Success
• The WEIRDest People in the World

Important Quotes from Book

The anthropological record indicates that approximately 85 per cent of human societies have permitted men to have more than one wife (polygynous marriage)… Here, we develop and explore the hypothesis that the norms and institutions that compose the modern package of monogamous marriage have been favoured by cultural evolution because of their group-beneficial effects—promoting success in inter-group competition. 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. These predictions are tested using converging lines of evidence from across the human sciences.

The idea is that competition among communities—such as nations, polities or religious organizations—favours those norms, values, beliefs, practices and institutions that most effectively harness, reinforce and shape our motivations and behaviour in ways that generate success in inter-group competition. Over centuries, these processes can lead to the spread of social norms and institutions (formal and informal) that create societal level benefits and reduce aggregate societal costs, thereby giving an edge in inter-group competition.

The 15 per cent or so of societies in the anthropological record with monogamous marriage fall into two disparate categories: (i) small-scale societies inhabiting marginal environments with little status distinctions among males and (ii) some of history’s largest and most successful ancient societies.

[Longitudinal databases show that] Across all crimes, marriage reduces a man’s likelihood of committing a crime by 35 per cent. For property and violent crimes, being married cuts the probability of committing a crime by half. When men are divorced or widowed , their crime rates go up.

Controlling for all of these other factors, marriage reduces a man’s probability of committing a crime by roughly half. This effect is strongest for assault and weakest for property crimes, but is significant for both of these as well as drug crimes. The size of this marriage effect is similar to entering school and much stronger than being on parole or probation. Interestingly, unmarried cohabitation does not reduce crime rates. Having a job had mixed effects, none of which were particularly large. The positive effect on crime of living with a wife is even larger than the negative effect of heavy drinking.

[In Africa] The results across six different model specifications show that the greater the degree of polygyny across nations, the higher the percentage of unmarried men. Going from a negligible degree of polygyny (polygyny .  0 nationwide) to widespread polygyny (polygyny .  3 everywhere) increases the size of this excess pool by between 13 and 27 per cent.

Unmarried low-status men, often in bachelor-bands, engage in higher levels of aggressive, violent and anti-social activities.

Anthropological data provide an additional line of support for this view. In many non-industrialized societies, young unmarried men form groups of marauders who go on raids to steal wealth and wives, while raping and pillaging. Polygynous societies engage in more warfare [44], often with the goal of capturing women.

This results in suppressing women’s freedoms, increasing gender inequality and stimulating domestic violence. Women’s loss of influence on household decision-making and their lower age of marriage results in higher fertility. By contrast, normative monogamy diffuses the pressure to bring younger brides into the marriage market, and thereby reduces the spousal age gap, male efforts to control (‘protect’) women, gender inequality and total fertility.

Co-wife conflict is ubiquitous in polygynous households. From anthropology, a review of ethnographic data from 69 non-sororal polygynous societies from around the globe [66] reveals no case where co-wife relations could be described as harmonious, and no hint that women’s access to the means of production had any mitigating impact on conflict.

Stronger monogamous marriage norms are associated with less (i) domestic violence, (ii) maternal mortality, (iii) female genital mutilation, and (iv) sex trafficking, even after controlling for GDP.

Living in the same household with genetically unrelated adults is the single biggest risk factor for abuse, neglect and homicide of children. Stepmothers are 2.4 times more likely to kill their stepchildren than birth mothers, and children living with an unrelated parent are between 15 and 77 times more likely to die ‘accidentally’.