The conclusion casts doubt on conventional wisdom that religious doctrine is the main lure for joining groups like Boko Haram, al-Shabaab and the al-Qaeda-linked Support Group for Islam and Muslims (GSIM).
The UN Development Programme (UNDP) in 2021 interviewed nearly 2 200 people across eight countries torn by jihadist or other violent groups - Burkina Faso, Cameroon, Chad, Mali, Niger, Nigeria, Somalia and Sudan.
The interviews included nearly 1 200 former members of extremist groups, of whom nearly 900 had joined voluntarily while the others had been coerced.
Of those who had voluntarily joined, a quarter said the prospect of paid work had been their primary motive, UNDP said in a report.
That marks a 92% increase for that justification, compared to its previous report on the issue in 2017.
UNDP chief Achim Steiner told journalists:
In many countries... the lack of income, the lack of job opportunities, livelihoods, desperation is essentially pushing people to take up opportunities with whoever offers them.
Another 22% said they joined to be with family or friends.
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But religious motivations came in third, cited by just 17% as the main reason for joining.
In contrast, nearly half the respondents cited a specific trigger event that also pushed them to join - 71% pointed to an abuse, often by state security forces - as "the tipping point".
"It is one of the sad realities that in the context of trying to push back on violent extremism, often the state itself becomes a trigger factor," Steiner said.
Properly defining what is driving violent extremism in sub-Saharan Africa is vital, at a time when the region is seeing surging numbers of attacks.
Deaths worldwide from terrorism have declined over the past five years, but attacks south of the Sahara have more than doubled since 2016, the UNDP said.
Between 2017 and 2021, there were 4 155 attacks in the eight countries listed in the report, UNDP said, putting the number of resulting deaths at over 18 400.
In 2021, nearly half of all terrorism-related deaths were in this region, with more than one-third in just four countries: Somalia, Burkina Faso, Niger and Mali.
"Sub-Saharan Africa has emerged as... the global epicentre of violent extremism in recent years," Steiner warned.
The shift has garnered relatively little international attention, at a time when the world is reeling from the Covid-19 pandemic, climate crises and the war in Ukraine, the agency said.
The UNDP called for a preventive strategy to take the shine away from recruiters' promises.
It urged investment in child welfare and education and help to rehabilitate those wishing to leave the ranks of extremists.
Nirina Kiplagat, UNDP's technical lead on preventing violent extremism in Africa, said:
Research shows that those who decide to disengage from violent extremism are less likely to re-join and recruit others.
"This is why it's so important to invest in incentives that enable disengagement."
"Security-driven counter-terrorism responses are often costly and minimally effective, yet investments in preventive approaches to violent extremism are woefully inadequate," Steiner said.