There exist several experiments on mice. The scientist wanna observe what will happen in an over-population environment.
Then they observe homosexuality between males, lose of maternal ability among females, committing suicide and so on. All these lead to abolishing of reproduction function between mice and the population of mice plummets eventually. This is a physical occurrence, and economists have invented "the law of diminishing marginal return" (LDMR) to explain in.

LDMR states that when there is a growth of single factor of production, under ceteris paribus (i.e. all other factors remain unchanged in quantity), the marginal return will eventually vanish (i.e. convexity of absolute return will be less than zero).

Now we observe human population today. The population (and thus the price level) keeps rising, while all other factors, such as salary and technology level, remain unchanged. Then human productivity (and thus morality) as absolute return will eventually drop. PTGF/PTD phenomenon mirrors slumping morality between human. Our mankind is regressing!