Recall: Great Britain, Denmark, and Switzerland did not adopt the euro. The stock markets that have been hardest hit by Brexit are Italy, Spain, and Greece: the center of the eurozone crisis. From Timothy Lee at Vox:

Brexit isn’t the most serious threat to the EU — the euro is

One surprising thing about Britain’s vote to leave the EU is that the British economy has been doing better than a lot of European countries. Unemployment in the United Kingdom has fallen to 5 percent, its lowest level in a decade. In contrast, the average unemployment rate among countries that (unlike Britain) have joined the EU’s common currency, the euro, is still above 10 percent.

And many economists argue that’s not a coincidence — that poor policies by the European Central Bank have systematically weakened growth in countries that have adopted the euro.
A new paper from economist David Beckworth makes the case that the economic woes of eurozone countries like Spain and Greece can ultimately be traced back to the euro itself. He argues that other problems in those countries, like their problems with high debt, were made worse by the ECB’s tight-money policies.

This argument has huge implications. It suggests that without reforms, eurozone countries could continue suffering from slow growth and abnormally severe recessions for decades to come. That, in turn, will fuel public resentment against the EU and increase the danger that other countries will follow the UK’s lead.