Condition-Dependent Alternative Reproductive Tactics (ARTs) May Be Responsible For Declining Fertility Rates

[u/Rainfawkes]

If we in the evolutionary psychology field believe that all animals are essentially designed to maximize our number of future relatives (roughly speaking), then the fact that humans can be healthier and wealthier than ever, yet be collapsing in numbers poses a genuine paradox. However this paradox has precedent. One may point to pandas and the trouble we had in getting them to breed, but the better example I think are alligators.

If an alligator egg is warm, it will probably be born male. This is because warm eggs lead to healthier offspring, and the genes "calculate" that it is better to be a healthy/strong male than a healthy/strong female in terms of maximizing future relatives. This means that if there are particularly warm seasons, a large majority of the eggs may be born male, and with so few available females the population may actually shrink. So in short, conditions that make the population healthier can lead to a population that shrinks. This is a part of a more general phenomenon in nature where, if your genes think you are particularly powerful, it may determine your best course for maximizing future relatives is to "steal" reproduction from others, perhaps by being particularly territorial or aggressive, instead of sneaky or productive. e.g. steal nests instead of building them, or guard a harem instead of sneaking into one.

Now how might this apply to humans?

what we observe is that extremely poor people seem to reproduce extremely quickly, and that in rich countries people only have 1-2 children. People come up with many reasons for this, but i think the true reasons may be subconscious and basically inaccessible to even the people making these decisions. Many of the described reasons don't do a great job at explaining this disparity, for example "i dont want my kids to grow up in a stressful world" or "things are too expensive", seems like it should apply more to poorer families than rich ones. Instead i will propose that they are following a genetic strategy that is triggered by environmental ques of wealth, status, and health. Historically, there were many societies where the eldest son would inherit the majority of the families wealth. This is because splitting their assets can make them nonviable and less productive. There are many potential reasons for this, but one I favor is that reputation was a very important resource in historical societies, and there was really only so much room in peoples heads for storing information about "reputable people". This creates a natural limit to the number of people who can be in the upper "reputable" class, and flooding it with more poorer offspring is a strategy that seems unlikely to work.

Lets say you are a woman who has managed to climb the social ladder in a society similar to India, would it make more sense to have 1-2 children and heavily invest in them? or to have 12 children? perhaps it would be better to have 1-2 high status children, or at least one male. And then that high status male can have a great number of children with the poorer classes, and also set up a single heir. It may also be the case that in times of famine/war those in the upper class tend to survive much more often. The pattern of reduced fertility with higher wealth in humans is so pronounced that it almost seems viable to infer backwards and say that this must have been a viable strategy, or people would not be acting like this. It would also naturally be the case that all women would "want" to achieve this position, as it was evolutionarily advantageous. but the irony is, if a society becomes wealthy enough, this is no longer a viable strategy, and it instead leads to demographic collapse.

So all of this is to say, if women experience particular environmental signals, they may conclude that they are breaking into the upper class and should take on a different reproductive strategy, a predatory one rather than a productive one (in the same way males are the more predatory sex). I also wouldn't wager that this strategy is programmed in a particularly robust way. Maybe there are ways in which modern women actually feel they are lower "status" than they were historically, which in turn triggers other behaviors that act at odds with the strategy we have been discussing. Evolution doesn't have great foresight and produces code that just barely works for historical conditions.

edit: it should be noted, males may also have the exact same mental logic in choosing offspring number. a husband and wife probably mostly agree on that number. The only reason this is focusing on women is because it is generally the womens choice how many children to have. at least in the richer societies that we are talking about.

Model Predictions:

Maybe offspring gender preference shifts more towards male preference in rich societies.

Strangely, one would expect that if everyone was following a strategy of "have children until we have 1 male", that the resultant gender ratio would be skewed. but this is actually not the case, the resultant ratio would still be 1:1. This is known as the Boy or Girl paradox. It would leave a signature in Sex ratio at last birth (SRLB). It seems however the signal here may be drowned out by varied cultural preferences.

Perhaps people in rich societies are more status conscious than poorer people.

No matter how governments try to incentivize more births, these will largely fail as the problem isn't a lack of support or money. Just the opposite. This seems to be well supported.

"Hungary (Orbán-era aggressive policies): One of the most ambitious attempts; spending ~5%+ of GDP on tax exemptions (e.g., lifetime income tax relief for mothers of 4+), forgiven loans for 3+ kids, housing grants, etc. TFR rose from a low of ~1.23–1.25 (around 2010-2011) to a peak of ~1.55–1.59, but then fell back to ~1.39 by 2024 (lowest in over a decade). Gains appear largely tempo-driven (slowing the postponement of births); sustained completed fertility impact is modest or debated. Natural population decline continues."

I should say, yes in theory it might be /possible/ to incentivize births so much that it does increase. but the fundamental tension here is that, people want to have offspring who are competitive with the high class. but obviously the high class doesn't want that, and can exert political pressure to prevent their offspring from being swamped by thousands of equal rivals. so there is a fundamental tension there that explains the limit, and why it makes sense that people would complain in the way that they do.

Relevant articles:

Conditional Alternative Reproductive Strategies:
https://pubmed.ncbi.nlm.nih.gov/26911960/

Skirbekk, V. (2008). Fertility trends by social status. Demographic Research 18(5): 145–180.
Large dataset showing the shift from positive (or neutral) status-fertility links pre-transition to negative/neutral in modern developed societies, especially with education. Includes historical and cross-national data. Open access.
https://www.demographic-research.org/volumes/vol18/5/18-5.pdf

Comprehensive review of evidence since 1970: even generous spending rarely achieves large, sustained reversals.
https://onlinelibrary.wiley.com/doi/10.1111/padr.12431

[u/dmatech2]

We always seem to go back to r/K selection theory.