History - Reproductive Trade Offs

From The WEIRDest People in the World by Joseph Henrich.

Even more strikingly, recent analyses of genetic data from around the world suggest that rising levels of polygyny were so common in the last 10,000 years that they left a heel print on our Y chromosomes—that’s the DNA carried only by males. Using the rich information embedded in both Y chromosomes and mitochondrial DNA (which is inherited only from the mother), geneticists have estimated the number of mothers vs. the number of fathers going well back into our evolutionary history. In a purely monogamous world, we’d expect the ratio of the number of mothers to fathers to be something close to one. Before agriculture, the data show that there was a pretty constant ratio of roughly two to four mothers for every father. However, a few thousand years after agriculture began, while the number of mothers was rapidly increasing as populations expanded, the number of fathers plummeted. That is, the number of fathers dropped, while the overall population was rising! At the peak of this climb, there were over 16 mothers for each father.

 This genetic patterning is most consistent with a combination of high levels of polygyny and ferocious intergroup competition. Expanding agricultural clans, segmentary lineages, and chiefdoms either killed or enslaved all the men in the societies they conquered and took all the fertile females as wives, concubines, or sex slaves (Heyer et al., 2012; Karmin et al., 2015; Zeng, Aw, and Feldman, 2018). Consistent with this, dips in the number of fathers appear earliest in regions where agriculture first began, in the Middle East (e.g., Mesopotamia), as well as in both South and East Asia. Europe, where agriculture arrived later, reveals the deepest dip, hitting its nadir between 5,000 and 6,000 years ago. The Andean populations, where agriculture began relatively late, don’t show the dip until 2,000 to 3,000 years ago.

Interesting how the same data framed differently carries additional insight.  I was aware of historical differences in reproduction, i.e. (and I am making up the numbers) in the 1500s 80% of women had children but only 40% of men.  I digested that as hierarchies, disposability of males, etc.  Lots of plausible reasons.

Henrich is much more direct.  Agriculture yields surpluses which will sustain a larger population.  Fertile women become a means of production and are captured and absorbed by the tribe.  Simultaneously disposable males are, as it were, canon fodder and/or frozen out of the mating lottery.  

Nature has cruel ways.  I recall a couple of historians making the case that matrimony was a bedrock of civilization in part because of the domesticating aspect of males but also because unattached males are a menace.  You can have a repressive hierarchy which freezes them out of the mating process.  The cost of this is constant turmoil and risk of hierarchy overthrow from thwarted males. 

Alternatively, all males are allowed to participate in the mating lottery (i.e. marriage).  Takes care of the surplus, risk-taking, unattached, and destabilizing males.  

Even with that change in societal model, there is still a sex disparity in reproduction.  I think it is something like 85% of American women will ever have a child whereas only something like 80% of American men will have a child.  Not a huge difference but significant over generations.  

And not nearly as bad when agriculture was first getting launched.