Despite ISIS's claims of ruling over an Islamic "caliphate" in line with Sharia law, a large number of the group's fighters joined for reasons having little to do with religion, according to a defector from the group.

Instead, people are joining the organisation because they are desperate for money and are struggling to find a way to survive in Syria, where four years of civil war have decimated the economy.

The ISIS defector, who goes by the pseudonym Abu Khaled, spoke with Weiss about the group's internal dynamics, and what it was like to live under ISIS's rule, Business Insider reports quoting The Daily Beast.

According to Abu Khaled, a large number of people are joining ISIS because they need money. After joining the militants, people are paid in US dollars instead of Syrian liras. Abu Khaled said that ISIS also runs its own currency exchanges.

ISIS members receive additional incentives to fight for the group. "I rented a house, which was paid for by ISIS," Abu Khaled, who worked for ISIS's internal-security forces and "provided training for foreign operatives," told Weiss. "It cost $50 per month. They paid for the house, the electricity. Plus, I was married, so I got an additional $50 per month for my wife. If you have kids, you get $35 for each. If you have parents, they pay $50 for each parent. This is a welfare state."

And those financial benefits are not just limited to the organisation's fighters. According to Abu Khaled, any member of ISIS, ranging from construction workers to doctors, receives similar compensation. In war-torn Syria, these salaries are a powerful lure for people who might not otherwise be able to support their families — or for people just hoping to get rich, Business Insider reports.

"I knew a mason who worked construction. He used to get 1,000 lira per day. That's nothing," Abu Khaled told Weiss. "Now he's joined ISIS and gets 35,000 lira—$100 for himself, $50 for his wife, $35 for his kids. He makes $600 to $700 per month. He gave up masonry. He's just a fighter now, but he joined for the income."

Other Syrians who have fled from ISIS's rule have corroborated Abu Khaled's reports, confirming that one of the only ways to accumulate wealth and status under ISIS's rule is by joining the organisation. Yassin al-Jassem, a Syrian refugee from near ISIS's de facto capital of Raqqa, Syria, shared his experience with The Washington Post.

"There is no work, so you have to join them in order to live," al-Jassem told the Post. "So many local people have joined them. They were pushed into Daesh by hunger."

According to Newsweek, there is a widening gap in living standards for those under ISIS rule. Members of the organisation have access to food, free medical care, and desirable housing. In contrast, people who aren't ISIS members suffer under a barely functioning economy with rapidly increasing prices.

ISIS can afford to pay people seeking to join its ranks through four main sources of income: oil, the sale of looted antiquities, taxation, and kidnapping ransoms, reports Business Insider.

The militant group either controls or has an operational presence around a number of oil wells in Iraq and in the majority of oil-producing areas in Syria. This allows the group to earn a steady income from oil production and smuggling that helps it to continue its daily operations.

The New York Times estimates that ISIS can make upward of $40 million a month through oil-related activities. In a bid to cut the group's income, the US conducted its first airstrikes against ISIS oil trucks on November 16.

ISIS's main source of income is significantly more difficult for the US and other coalition partners to target by air. According to Foreign Policy, ISIS makes the majority of its money through extortion and taxation of people living under the group's rule.

ISIS taxes nearly every possible economic activity, with the revenue ultimately covering the expenses of waging continuous war along multiple fronts. Foreign Policy notes that taxes are put in place for militants who loot archaeological sites. Non-Muslims must pay religious taxes, and all ISIS subjects pay a base welfare and salary tax in support of the fighters. All vehicles passing through ISIS territory — which may carry the only food available to those living under ISIS control — must pay taxes often totaling hundreds of dollars.

This ad hoc war economy means that ISIS has little money to spend on improving the lives of those who are forced to live under its rule. But as Abu Khaled's account confirms, it still finds the money for conducting military operations and incentivizing militants to join the group.

That money and the other benefits that ISIS fighters receive means that Syrians join ISIS out of desperation — and not necessarily out of religious or ideological conviction.