I think they are implying that people with lower intelligence are more likely to be in jail. There's at least three reasons why I would consider this very plaus

I think they are implying that people with lower intelligence are more likely to be in jail. There's at least three reasons why I would consider this very plausible: 1. Intelligence is associated with higher impulse control. 2. Intelligent criminals are probably less likely to be caught. They might also have a better chance at working to lower their sentences after being caught. 3. It is harder to be successful with a lower IQ (although I would posit it is far from the most important factor), and economic hardship is strongly correlated with crime.