Many countries have agreements where if you pay into a national pension in one, it will count as having contributed to the other should you ever return. For ex

Many countries have agreements where if you pay into a national pension in one, it will count as having contributed to the other should you ever return. For example in Japan where I pay into the national pension plan, most of that could be counted for Social Security if I moved back to the States (or, I could opt for a single payment immediately).