In 2014, soaring inequality in advanced nations finally received the attention it deserved, as Thomas Piketty’s “Capital in the Twenty-First Century” became a surprise (and deserving) best seller. The usual suspects are still in well-paid denial, but, to everyone else, it is now obvious that income and wealth are more concentrated at the very top than they have been since the Gilded Age . and the trend shows no sign of letting up.

But that’s a story about developments within nations, and, therefore, incomplete. You really want to supplement Piketty-style analysis with a global view, and when you do, I’d argue, you get a better sense of the good, the bad and the potentially very ugly of the world we live in.

So let me suggest that you look at a remarkable chart of income gains around the world produced by Branko Milanovic of the City University of New York Graduate Center (which I will be joining this summer). What Mr. Milanovic shows is that income growth since the fall of the Berlin Wall has been a “twin peaks” story. Incomes have, of course, soared at the top, as the world’s elite becomes ever richer. But there have also been huge gains for what we might call the global middle . largely consisting of the rising middle classes of China and India.

And let’s be clear: Income growth in emerging nations has produced huge gains in human welfare, lifting hundreds of millions of people out of desperate poverty and giving them a chance for a better life.

Now for the bad news: Between these twin peaks . the ever-richer global elite and the rising Chinese middle class . lies what we might call the valley of despond: Incomes have grown slowly, if at all, for people around the 20th percentile of the world income distribution. Who are these people? Basically, the advanced-country working classes. And although Mr. Milanovic’s data only go up through 2008, we can be sure that this group has done even worse since then, wracked by the effects of high unemployment, stagnating wages, and austerity policies.

Furthermore, the travails of workers in rich countries are, in important ways, the flip side of the gains above and below them. Competition from emerging-economy exports has surely been a factor depressing wages in wealthier nations, although probably not the dominant force. More important, soaring incomes at the top were achieved, in large part, by squeezing those below: by cutting wages, slashing benefits, crushing unions, and diverting a rising share of national resources to financial wheeling and dealing.

Perhaps more important still, the wealthy exert a vastly disproportionate effect on policy. And elite priorities . obsessive concern with budget deficits, with the supposed need to slash social programs . have done a lot to deepen the valley of despond.

So who speaks for those left behind in this twin-peaked world? You might have expected conventional parties of the left to take a populist stance on behalf of their domestic working classes. But mostly what you get instead . from leaders ranging from Francois Hollande of France to Ed Milliband of Britain to, yes, President Obama . is awkward mumbling. (Mr. Obama has, in fact, done a lot to help working Americans, but he’s remarkably bad at making his own case.)

The problem with these conventional leaders, I’d argue, is that they’re afraid to challenge elite priorities, in particular the obsession with budget deficits, for fear of being considered irresponsible. And that leaves the field open for unconventional leaders . some of them seriously scary . who are willing to address the anger and despair of ordinary citizens.

The Greek leftists who may well come to power there later this month are arguably the least scary of the bunch, although their demands for debt relief and an end to austerity may provoke a tense standoff with Brussels. Elsewhere, however, we see the rise of nationalist, anti-immigrant parties like France’s National Front and the U.K. Independence Party, or UKIP, in Britain . and there are even worse people waiting in the wings.

All of this suggests some uncomfortable historical analogies. Remember, this is the second time we’ve had a global financial crisis followed by a prolonged worldwide slump. Then, as now, any effective response to the crisis was blocked by elite demands for balanced budgets and stable currencies. And the eventual result was to deliver power into the hands of people who were, shall we say, not very nice.

I’m not suggesting that we’re on the verge of fully replaying the 1930s. But I would argue that political and opinion leaders need to face up to the reality that our current global setup isn’t working for everyone. It’s great for the elite and has done a lot of good for emerging nations, but that valley of despond is very real. And bad things will happen if we don’t do something about it.