To be honest, I never understood why people compared the herbivore men in japan with mgtow/redpill etc in the west. I think the causes are fundamentally different. Japanese, as a group, simply are shy and have high anxiety levels. How much of that is cultural and how much is genetic I will let you guess, but either way the result is the same. Extreme timidity prevents marriage and children.

In the west the similar effect, but with a completely different cause. Westerners as a group aren't nearly as timid as that. The are simply responding to the incentives rationally and maximizing their benefit and minimizing their risk. Because of the poor structuring of incentives, you have less marriage and less children.

In china they had a 1 child policy and a cultural bias against daughters, which led to infanticide. So now you have 40 mil more men than women. A shortage of women, not men, is what decreases a demographic.

The only thing common to all is mass education that includes females, but doesn't make them focus on skills and knowledge important for being mothers. Maybe this is the root cause of Japan's and the west's problems, but the factor's above contribute just as much or more in my opinion.