Nihonjinron literally means "discussions of things Japanese", referring to ideas, practices, or other miscellanea which is unique to the Japanese people. Because of Japan's long history of isolation from other cultures, the Japanese people developed idiosyncrasies and habits which are very different from the rest of the world. Nihonjinron is an attempt to codify what it is to be Japanese.
For example, all languages have polite and informal protocols- the difference between talking to the President and talking to a friend who's known you for ten years; yet, the Japanese have so many levels and distinctions within keigo (polite language) that subtle hierarchical levels cannot be easily interpreted or learned by outsiders. Not knowing or practicing this could lead to someone's unfortunate death; to bow slightly too high when addressing someone of rank, to use the wrong form of 'please', whatever the infraction may be once meant that a samurai (the feudal police system) could behead a citizen with impunity for being so inconsiderate and rude. So sensitive to these seemingly little things inherent to their system of showing politeness were the "old Japanese" of a bygone era, a foreigner would have been comparatively obscene, even when genuinely attempting to follow proper cultural demands. Of course, it's a different world today! The standards have been very relaxed in recent years. However, remnants of these habits remain. Japanese language is definitely unique to Japan, despite borrowed influences from China, Portugal, and America/the West.
There are hundreds of categories similar to that one under the heading of 'nihonjinron'. Even though the category of literature dates back centuries, the genre really took off with the defeat of Japan at the time of the atomic bombings in the mid-1940s. With a country in ruins socially and economically, it was important to rebuild a national identity. Itemizing and detailing theories of how Japan was unique (and somehow better) than the rest of the world was one way to cope.
At its best, nihonjinron is an exploration of what precisely Japanese cultural identity is about; what makes them special, different from anyone else in the world. Indeed, a worthy venture. A person from Saudi is not the same as a person from Afghanistan, or from Oman, or from Iran. All are individually unique, despite being bound together with some common ties. In much the same way, Japan has common influences from many countries which they had contact with (even if that contact was relatively limited), and yet, they are completely distinct from anywhere and anyone else. A positive affirmation of nationality and accomplishment, if you will.
At its worst, some theories of nihonjinron are outright delusional and racist, often based on pseudo-scientific tripe- such as the notion that Japanese think with different parts of their brains than everyone else does, or that they have special senses that no other people possess. Having special seasons or climates that do not appear anywhere else, or being unable to grasp the basics of how Japanese language works because it is just so radically, completely different from anything else heard on Earth. Yet, there are thousands, probably millions of fluent speakers of Japanese who were not born in Japan to two Japanese parents, who also had two Japanese parents each. The dark side of nihonjinron is a form of nationalism taken to rigid, xenophobic extremes.
Good or bad, to be aware of nihonjinron theories is to be able to decode some part of what you hear and see in the culture. Then you can decide for yourself what to challenge or reject, and what to accept as generally true.
My opinion is that Japan is like any other country, despite the alien differences we may have between us. People are alike world over, as Rod Serling would say. You've got your social problems, your great technological advances, your unique places to visit. You've got your sensible people, your paranoid people, your nationalist people, and people that embrace another human being for the faulty, fallible characters that we are. Nihonjinron, any of it, is useful for gaining one type of insight into a foreign place if taken with a kilo of salt. That's all.
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