Yes, I think the changing class structure has something to do with the change. In a polygamous society some men have lots of wives and some men none, and the men who have lots are really, really rich. You need a class structure that is very different from what most modern societies have. I suppose someone may have sat down and figured out that the practice might lead to more unrest, but that seems a bit social-science-y for someone in China to do in say 1919 or more to the point too social science-y for a Chinese state to enforce/propagandize. And if someone did propagandize it where is the evidence? Have we just missed it? That’s why I think the Japan comparison works so well. All I know of as evidence for Japan is a handful of high-status men like Fukuzawa trying to convince other high-status men not to do it, but that might be all it would take. I would also not sell “The Westerners do/don’t do it” short. It’s not a sufficient explanation for anything, but it seems to fit in a lot of contexts.

A related recent discussion of polygamy is here:

http://blogs.psychologytoday.com/blog/the-scientific-fundamentalist/200802/the-paradox-polygamy-ii-why-most-women-benefit-polygamy-an

In a polygamous society, some men have several wives, and a lot of men have no wife. This ‘surplus’ of single men is useful in military contexts, but in times of peace could be a source of societal instability. I don’t have the text to hand, but I have read that this was part of the issue with women in China being at risk of abduction: aside from anything else, there were men who would never get a wife at all unless they stole one. Certainly the effect of a critical mass of restive young men who have no hope of ever marrying is something that is discussed in relation to Middle Eastern regions where polygamy is still widely practiced. Could desire for social order been part of the reason for abandoning polygamy in East Asia?

Yes, I was thinking about Fukuzawa when I put in that bit about Japan, although I was too lazy to go look up any of his stuff. I think Japan is different in that you get a lot more elite-class continuity between Bakumatsu and Meiji that you do with the May 4thers in China. On the other hand the two seem pretty similar in that in both places “Westerners don’t do it” was a strong argument without anything else needed to back it up and in that it seems to have been mostly changes in male attitudes that made (formal) polygamy end.

Actually, I personally know at least one family which could benefit from a multiple-marriage system: a family of four lesbians; Reality is always stranger than what we can imagine. Robert Heinlein, in Friday posited as one of the many societies in that book a system by which multiple men and women could become a collective family, with assets in common (and a fairly substantial buy-in for new members, if the family is well-off), sexual partner rotation and childrearing duties shared widely. Spider Robinson had something like that, too, in a small commune-style community.

The only way I could see polygamy becoming acceptable — and eventually legal — in the US would be family structures that were more balanced rather than overtly patriarchal. I’m not sure how you legislate it, though. (perhaps by requiring that multiple marriages be the result of combining married couples, instead of individuals?)