Honing Fertility Fall Theories

Richard Hanania has done me the honor of carefully thinking through my & other fertility theories, finding consequences and data tests I didn't think of. If you have time, go read him first, then come back here. 

He calls my status theory “Kings and Queens” (KAQ), and interprets it as suggesting a wealth threshold effects. Which he tests, finding favorably:

Overall, I think this is very strong evidence for an effect where the relationship between wealth and fertility starts out weak and negative, becomes strong and negative as more and more people feel that they and their children are potential kings and queens up to the $5K or $10K range, and then disappears as practically everyone is subject to this effect.

But he notices that the theory needs another assumption, which he finds somewhat ad hoc:

We have to imagine something in our evolutionary programming that says “limit fertility when you are close to becoming a king, but be fruitful when you actually become a king.” … One could argue that we use absolute living standards to decide we’re aristocrats, but then rely on signals of relative status to decide we’re kings, like if you hear trumpets playing before you walk into a room. When I pointed this out to Robin recently, he acknowledged that this was indeed an implication of KAQ in a postscript to his article. But this makes the theory somewhat ad hoc, relying on signals of absolute wealth in one situation and then conveniently switching to relative status in another.

I see two different possibilities here. First, the concept of royalty (kings and queens) just might equal that of the highest status folks around. In which case royals would need to carefully ensure no one else in their society showed anything like their levels of wealth and status signs. And foreign royalty would just not be allowed to show off their status signs to locals, for fear locals would accept them as their new royalty. In this case, the evolved heuristic would be to go wild on fertility when you joined this highest status group. Seems like this could work.

The second possibility is that the concept of royalty is not just equal to that of highest status, in which case we’d have to have had different markers we track to see who are our local royals. As this would be an important thing to get right, we’d be pretty good at it. In which case our evolved heuristic could be to go wild on fertility when those markers apply to us personally. Seems like this could also work. 

Hanania also considered my more recent theory that we just copy high status fertility behavior, ignoring a key selection effect:

Recently, Hanson came up with another theory, which says that we evolved to imitate successful people, and due to selection effects, among elites the most successful around them tend to be those who had the fewest kids. …

As for why this limited fertility norm has only spread to the general population now, one has to “again postulate that humans encoded this behavior as something that mainly makes sense for parents who are relatively rich and high status, but not otherwise.” This strikes me as not very convincing, and perhaps not even internally consistent. The theory holds that evolution equips us with a feeling of where we are in a hierarchy based on absolute wealth, but that mechanism is at the same time maladaptive, because it causes elites to fall prey to selection effects. …

A simpler version of this imitation theory, one that doesn’t rely on threshold effects, might say that the most successful members of society were always those who came from families that limited their fertility. But most of the population usually did not have enough exposure to elites to notice. Under this theory, elites have always been behaving maladaptively and falling prey to selection effects, but not enough people have been elites for this to change our evolutionary trajectory. Now, with greater wealth and communications technology, more and more people are exposed to the ultra-successful. This would explain both decreasing fertility as wealth increases, and also decreasing fertility as time moves forward, given the spread of radio, TV, and the internet. …

We can call this “elite exposure theory.” I think that for it to work, one must in addition assume that at the level of a village or whatever, the most successful members of society didn’t actually limit their fertility that much, but once people expand their horizons, the true outliers in status and prestige disproportionately come from small families.

I agree that Hanania’s elite exposure version of the theory looks better; we have seen big historical effects of social communication on local fertility changes. So yes, fertility should fall when people notice that their local elites tend to have lower fertility. But I suspect that this selection effect should itself vary with context, and was previously overwhelmed by contrary effects. Which would then explain why everyone hasn’t always known about the correlation; it wasn’t always true or as strong.

For foragers, there was a big effect where higher status men attracted more and higher status women to have more kids with them. And higher status women attracted higher status men to have kids with them. Creating a positive correlation between status and fertility. Yes, it was also true than when choosing how much time to spend with each kid, parents with more kids could spend less time with each kid. But time spent on each kid was then only a modest fraction of the influence of parents on kid status. 

In contrast, wealth and property passed on to each kid was a much larger fraction of the influence of farmer world elites on kid status. So in that world we should expect a larger selection effect, which would then be more quickly noticed and emulated. So as a society became larger and more unequal, and with better communication, we’d expect a transition from elites having higher fertility, to a selection effect of elites having lower fertility, which then gets amplified as people learn about this fact and start to copy elite fertility habits. 

Thus in forager worlds, and in simple relatively-egalitarian farming worlds, the correlation between fertility and status would be positive or near zero; it would only get substantially negative when there were enough elites for whom inherited property formed a large enough fraction of how parents effected kid status. 

I offered two fertility theories, Hanania refined and critiqued them, and now I’ve try to refined them further. Who will take the next step?

Added Nov 18: Ruxandra Teslo criticizes the Kings and Queens story here.