That's a very bold and unsubstantiated claim.
Why was that true for Japanese and not every country that lost a war?
Japan was the only nation that lost a war and surrendered after nuclear bombings unique in the history of warfare.
Japan also held that their Hirohito Emperor was god and Hirohito was forced by the Americans to admit by radio broadcasts that he was just a mortal like everyone else which shocked, panicked and demoralized many Japanese.
Until Shinzo Abe, Japan had a long series of unstable governments that rarely lasted a year. Abe’s protege is now PM and seems to be enjoying wide spread popularity. Currently, a revisionist history , championed by Abe, is taught in Japanese schools which doesn’t mention much about Japan from 1895-1945.
Much of what I have mentioned, including about Japanese women not wanting to procreate nearly enough with Japanese men was learned from Japanese women that I have encountered in the GTA.