Militants of the Islamic State group hold up their weapons and wave flags as they ride in a convoy. (Militant website via AP, File)

Since publishing his much-acclaimed book "Capital in the Twenty-First Century" last year, Thomas Piketty has ignited a renewed debate on the root causes of terrorism and the rise of the Islamic State. In a recent op-ed for French newspaper Le Monde, he argued that inequality was a "major driver of Middle Eastern terrorism, including the Islamic State attacks on Paris earlier this month — and Western nations have themselves largely to blame for that inequality," as my colleague Jim Tankersley described it.

Piketty writes that the Middle East's political and social system has been made fragile by the high concentration of oil wealth into a few countries with relatively little population. If you look at the region between Egypt and Iran — which includes Syria — you find several oil monarchies controlling between 60 and 70 percent of wealth, while housing just a bit more than 10 percent of the 300 million people living in that area.

Piketty, of course, is not the first academic who has tried to explain the factors that have contributed to the rise of terror groups, such as the Islamic State. So, what do the others say?

In a report, the United States Agency for International Development came to a similar conclusion in 2009: "One such finding is that while individual terrorists tend to be better off than the average citizen in the societies to which they belong, impoverished countries tend to generate more terrorism than wealthier ones."

A Pew Research study from 2005 appeared to confirm Piketty's thesis, as well: "Income is also a significant determinant of support for bin Laden, with wealthier individuals holding a more negative view of the al Qaeda leader," Richard Wike and Nilanthi Samaranayake wrote.

A closer look at the Pew Research survey complicates the subject even more: In some countries, including Pakistan, Morocco and Lebanon, middle-income citizens were slightly more likely than low-income citizens to say that attacks against civilians and suicide bombings in Iraq were justified.

"Empirically, it is not true that inequality by itself is a major causal factor," said Paul Davis, a senior researcher at the Rand Corp. think tank.  "Inequality can’t help, to be sure, and we may be concerned about it for many reasons, but it’s a poor factor to highlight as 'the cause.'"

Although Davis emphasized that inequality, in combination with state repression, could in fact motivate some to become terrorists, he also acknowledged that other factors appear to be equally or even more decisive.

"Today’s Jihadists have a sense of themselves that entangles ethnicity, nation origin, culture, family, a sense of oppression, and religion," Davis said.

Many academics who research terrorism-related issues say that single-root cause explanations are not particularly helpful.

"The reality is much more complex," said Kim Cragin, a senior research fellow at the National Defense University. To emphasize that complexity, Cragin and Davis published a diagram showing the various factors that play into public support for terror groups.

In his op-ed, Piketty also discusses how equality could help eradicate the problem of homegrown radicalization. "It's austerity which has led to the rise of national egoism and identity tensions. It's social development and equality that could succeed over hatred," he writes.

However, other researchers are far less confident that equality alone would stop homegrown radicalization.

"There are usually multiple reinforcing factors, and what those factors are varies from one case to another and across individuals," Davis said.

"One of these factors, more often than not, was a close association with other radicals either in-person or online. The difference today is that online peer groups have become more widespread," Cragin added.

In a recent study, Cragin argued that more attention should be given to what prevents people from joining terrorism groups. "Family influence represents the single most important factor for inhibiting radicalization," she wrote.

"It suggests that, rather than try to eliminate a single cause of radicalization, a more effective approach would be to strengthen factors that cause individuals to reject violent extremism."