Kind of a vicious circle, active population decreasing, so no unemployment, lots of extra hours, which lead to people not having time to marry/have kids... In a country in which most people are employable and have high work ethic, it's getting a problem. That being said, Japanese are unreasonable when it comes to work, most of their office hours are entirely unproductive, and like everywhere in Asia, men are expected to socialize with co-workers after working hours (yuck!), so that's increasing the problem. I think they're starting to be aware of that shit, and the government is trying to force employees to have some free-time. If you look at this chart, it could mean that most mean in their 20's work more than 50 hours a week, mean that a lot of men in their prime have not so great income, and absolutely no time for their family.
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Also, for most job, and most people, there is a point after which, working more hours is just not productive, and even a point after which working is not only not productive, but counter-productive, so they're probably just shooting themselves in the feet