A Japanese phenomenon, young men who don't work and don't play with others, instead withdrawing from social contact. Hat tip: Marginal Revolution.
I've some sympathy with them--I wasn't that extreme but I also used to withdraw, at least outside work.
It strikes me that it's something which can exist only in a wealthy society, where people have enough to support unproductive members of the society. As such it's a bit related to the US decline in labor force participation by men in the prime working ages. Whether it's society's safety nets or the informal workforce, it's a measure of how far we've changed from a rural agrarian society.