But there are factors to consider. Migration itself imposes costs to political stability. The rise of euro-skeptic nationalism across Europe owes much to panic about migration, especially the kind of crisis-fueled and uncontrolled migration of the past two summers. It is worries about immigration that are driving up the polls for Brexit in the U.K., and for Marine Le Pen in France. Breaking apart the European Union will impose tremendous costs on Europe’s economy.

Of course, the “bodies” themselves present other problems, mostly because they are attached to minds with ambitions of their own. Why should recent immigrants and their descendants, most of them poorer than the average European, abide by the welfare promises made by others decades or even a century before their arrival? Why should the resources produced from an immigrant’s work go to maintain the relatively high quality of life of older, whiter, richer Europeans who obtained this level of living by over-consuming their resources and refusing to invest in their future by creating a posterity for themselves?

The same is true in America. For years we’ve conceived of the ongoing retirement of the Boomers and their enrollment into Social Security and Medicare as a generational conflict. But the demographic transformation of America means that this generational conflict will also be a racial one. And it may begin to look like an old, white America is exploiting those non-white “warm bodies” that have crossed into the country over the past 20 years. The welfare states of Europe and America exist on a social bond between generations, a series of obligations and burdens that may not easily transfer to another nation’s descendants, especially if they are being asked to carry the heaviest parts of it.