Inequality, authors of the left usually argue, is not inevitable. The gap between the rich and the poor was made by man and can be undone by man. But Walter Scheidel’s recent book on the subject, The Great Leveller, ends on a sobering note: “All of us who prize economic equality would do well to remember that with the rarest of exceptions, it was only ever brought forth in sorrow.”

In his 528-page study of inequality “from the Stone Age to the 21st century”, the Stanford historian finds that significant equalisation has only ever been achieved by what he calls the “Four Horsemen of Levelling”: mass warfare, violent revolution, state collapse and lethal pandemics. The two world wars, the Russian and Chinese Revolutions, the fall of the Roman empire and the Black Death are among the most notable examples. Outside of such tumultuous upheavals, governments have struggled to constrain inequality.

When we spoke recently by phone, I asked Scheidel what inspired his epic undertaking. “Thomas Piketty reached further back into the past than people usually do [in his book Capital] and, for him, that was a couple of centuries,” the 51-year-old Austrian told me. “Since I primarily work on ancient history, I thought I should look at the full run of history. I discovered that no one had tried to do it before, which isn’t very surprising because you have to be so eclectic in your approach.”

In 1999, when Scheidel moved to the United States from Vienna, inequality was rarely discussed in the media or in academia. After the financial crisis, however, “It really flared up. There has been a significant shift.”

Though he intuited the correlation between violence and equality, Scheidel was stunned by the remorselessness of the data. “I fully expected to come across reasonably strong counter-evidence,” he said.

What unites the Four Horsemen is that they “upend the established order, which tends to favour the rich and powerful… It’s really only severe epidemics and the collapse of states that made a big difference for thousands of years.” Owing to the labour shortage caused by the Black Death, many workers’ wages doubled or tripled in western Europe, while landlords’ fortunes declined. In The Great Leveller, Scheidel recalls how state collapse left Roman aristocrats dependent on handouts from the pope and Mayan nobility subsisting on the same diet as commoners.

The remaining two horsemen – mass warfare and violent revolution – acted to reduce inequality during the 20th century. In both cases, the state assumed a significantly expanded economic role and private property was confiscated or destroyed.

I asked Scheidel whether his findings proved the Marxist thesis that there is no parliamentary road to socialism. “It’s not black and white. Those two things [parliamentary and revolutionary politics] interacted during the 20th century,” he replied. “Political democracy, trade unions, social legislation, universal education – all kinds of wonderful things – interacted with these violent shocks.”

Mass armies and the spectre of communist revolution helped spur the extension of political and economic rights. Scheidel cited the precedent of ancient Greece, where “intense popular military mobilisation” and egalitarian institutions acted to constrain material inequality.

The neoliberal turn of the 1980s – encompassing tax cuts, privatisation and deregulation – has widened inequality in almost every Western state. But Scheidel pointed to another factor. “The traditional violent levellers currently lie dormant,” he told me.

He is doubtful that comparably strong alternatives will be found. “The taxation of capital income, of great wealth, including offshore holdings, that would make a big difference,” Scheidel said. “That requires co-operation and harmonisation, because if only one country does it, you’re not going to get very far. We have all these international bodies, including the EU, but they don’t seem to be terribly successful.”

Nor are the Four Horsemen poised to ride again. As a result of technological and medical advancements, the age of mass warfare and mass pandemics has mercifully passed. States, meanwhile, are less prone to collapse or violent overthrow.

“People keep asking me about climate change: isn’t this the Fifth Horseman?” Scheidel said. “In as much as that’s going to happen, it’s probably going to work its way through some of the traditional mechanisms. The Four Horsemen may return through the back door, triggered by severe climate change. Other than that, in the current globalised economic environment, it’s very hard to see how major reductions could be achieved.”

Scheidel warned that automation (by eliminating jobs) and greater ethnic and religious diversity (by reducing public support for redistribution) could also act to widen inequality. Critics of Scheidel have alighted on a sentence in The Great Leveller’s final chapter: “Only all-out thermonuclear war might fundamentally reset the existing distribution of resources.”

In the age of Donald Trump, I observed, his words may yet prove prophetic. “I’m not really looking forward to that,” Scheidel drily replied. “I wrote that sentence and it was ripped out of context. I was talking about future war, what kind of war would have an equalising effect. The only one is probably a thermonuclear war. But that doesn’t mean we all have to sit around and wait for that.”