On the other hand historically there's been circumstances far worse than we are having now in terms of war, famine, diseases, living standards and it didn't stop people from reproducing at all. I really doubt it's that. I think it's something else.
Just because those are the most interesting events to study doesn't mean that in the past countries were constantly experiencing war, famine and epidemics. There were plenty of calm, relatively peaceful periods in between.
As far as living standard are concerned, while being a peasant was hard work during certain periods such as harvest and sowing, outside of those periods farmers actually worked less hours than we do today. That leaves lots of time to help out with chores in your village and maintain relationships with the people around you.
Historically, people had less children during wars and famines. There were usually baby booms after. People always managed amount of kids to the extend technology allowed that.
Disease and famine does not "remove excess men". A war removes them when men go fight to other places. If the war is in your country, there are a lot more victims among non fighters - typically 3:1. In WWII it was a lot more. And there, the victims are whoever is physically weaker.
I am skeptical of the claim that more women die in war than men.
Even so, any event that removes people from society largely indiscriminately of sex, removes excess men for the purpose of this argument. The pool of competition for men is greater than the pool of competition for women.
In other words, the difficulty of male competition increases with population density at a greater rate than the same of female competition. So you expect areas with high population density to have low fertility due to the male disadvantage. Calhoun's rat utopia is one such extreme example.
- There isn't any direct empirical research that supports the idea of male competition for a partner gets harder with population density. On the contrary, it seems competition for a guy finding a partner gets worse the lower the population density becomes. This would explain stories I've heard about dating in parts of Alaska.
- The sex ratio balance of a population seems to be the highest predictor of the level of competition for a partner. This makes intuitive sense to me: The less common gender will always have more options than the more common gender in an area.
What I don't know and would be interested to hear about: Is there a strong link between population density and gender ratio? In addition to this, I'm sure there's also all sorts of interesting facets you could examine like cohorts by age, sexuality, or partner preferences like height, build, appearance, etc. and how that factors into the perception vs reality of competition for a desirable cohort of partners vs total available partners.
