What I'm talking about is the backgrounds of the low-level fighters that they shared after they were captured and interrogated.
Let's set the clock back to 2013 when ISIS was all over the news. If you're a 20-year old ISIS recruit from Iraq, you were 10 when the US toppled Sadaam and removed the security that the Baathists provided.
What were those 10 years like for you? How did they affect your development as a person?
A good number of them had to stay inside when they were younger because there was fighting going on. Your house might have been raided. You or your family likely knew people that died.
You didn't get to go out and do what Iraqi teenagers got to do between 1992 and 2002, part of you was stunted.
Eventually, you got old enough to fight, and there were militias around for you to join. If you're Sunni, you knew about Sunni-majority cities getting shelled by the majority Shiite government, and ISIS rising as a response to them. They become an increasingly attractive group to join as they win.
So young guys with stunted social development joined an Islamist militia which offered training, or at least an opportunity to sacrifice their lives in a holy war. They were offered an opportunity to become men, or at least go straight to Heaven, such a deal.
Those that were totally unskilled would be the suicide bombers. Those who had more potential were given other jobs, some of them were the "infantry."
If you take a socially stunted "infantry" guy, give him the religious conviction that he has the right to capture or purchase a bride for himself if he helps capture a village, and his unit captures the village, what do you think he's going to do? If most of the guys in his unit are also like that, what do you think they're all going to do?