Who owns America’s ever-swelling pile of government debt? This is a question that has provoked considerable angst among US politicians recently; or at least it has in relation to national identity.

Little wonder. Half a century ago, the share of US public debt held by foreigners was less than 5 per cent; but in 2008 that ratio breached 50 per cent. And while it has since fallen back slightly (because the Federal Reserve has been gobbling up bonds) the shift in ownership is nevertheless stark – along with the new power of creditors such as China.

But there is a second important point about America’s debt that has hitherto received surprisingly little attention: the shifting nature of bond investors who hail from inside the US. In past decades, it has often been assumed that Treasury bonds were widely held by the public.