Malthus' Theory. In a nutshell, when a population overgrows its ability to maintain itself, the consequences as a result of these shortages in resources will normally restore the balance, or sometimes even create a surplus.
Malthusian catastrophe - Wikipedia, the free encyclopedia
In Malthus' case, he was mostly referring to the ratio of food production versus population growth. If it comes to a point where food production isn't able to keep up with the rate in which the populace is growing, people will start dying to the point where food supply meets the popular demand again. The point where a population isn't able to sustain itself anymore, is called the Malthusian Catastrophe. In addition, according to Malthus, hunger is often the cause of conflicts and disease outbreaks that might even diminish the populace to such an extent where there is now an abundance in resources (Far less mouths to feed).
Relating to this case. With a decrease in population, it follows the same way. Only this time, aside from food of course, there's also
wealth. People may die, but their earthly possessions won't just simply dissappear, they will get redistributed amongst the current living populace (Inheritance and whatnot). So basically, the more a population diminishes, the more wealth becomes available to the people that are still alive. Now lets say that Japan's wealth constitutes over €5.000.000 spread over a population of 10 people (just to make it easy). That means that rougly every person has an average net wealth of €500.000 right? Now lets say that the population for whatever reason now decreases to 5. The €2.500.000 of the 5 who just died will now be redistributed amongst the surviving 5, meaning that the wealth of the surviving 5 now doubles to €1.000.000.
The point in case being that population decrease is good
over the longterm for living conditions of the surviving populace.
Some historians and intellectuals even believe that this was the reason why China introduced the One-child policy, not only to avert a possible civilian unrest amongst an evergrowing populace who could, at some point, start ''cannibalizing'' itself, but also because it could help improve standard living conditions for the people over the longterm.
Of course this theory isn't without criticism and/or its flaws, mind you.