Worth mentioning: the US and Japan, as well as most first-world countries, have "totalization agreements" to take care of this. If you've paid into US Social Security for your entire working career and then retire to Japan, Japan pays you (fairly) equitably, and this is net out against e.g. me working for my entire career in Japan and then drawing US Social Security. (This includes two major question marks, like whether I'll be in the US at retirement and whether US Social Security will be worth enough to justify a postage stamp to get it.)