It's hard to say here in the US, since rehabilitation over retribution is poorly studied, particularly for violent offenders.
As I linked in response to your other comment asking a similar question, though, other countries have demonstrated quite a bit of success with the approach, Norway as one example having a low recidivism rate even among violent offenders. The Norwegian socioeconomic safety nets certainly help, too; crime in general is a function of both mental health and economic standing, so public policies aimed at addressing both of those things will naturally lead to both fewer incarcerations and fewer reincarcerations.
IQ is kind of a worthless measurement, given the numerous problems with its reliability and its rather arbitrary standards. Turns out measuring intellect is more complicated than some one-size-fits-all single numeric score (the meaning of which constantly fluctuates by design anyway).
> it's one of the most studied quantifiable concepts in pyschology.
A McDonald's hamburger is one of the most studied forms of food in gastronomy. That doesn't necessarily make it worthwhile to eat :)
See also: Stephen Jay Gould's The Mismeasure of Man, which describes at length why things like IQ and craniometry are fundamentally flawed due to their fallacious and unscientific insistence on reification and ranking - neither of which have been adequately proven to be applicable to something as complicated as human intelligence.
See also the very existence of the Flynn effect, and the fact that IQ is defined such that 100 is always the average IQ - making the comparison of IQs across time periods meaningless, since 100 IQ in 1900's America means something different from 100 IQ in 2000's America which means something different from 100 IQ in 2000's Norway which means something different from 100 IQ in 2000's Nigeria.
As I linked in response to your other comment asking a similar question, though, other countries have demonstrated quite a bit of success with the approach, Norway as one example having a low recidivism rate even among violent offenders. The Norwegian socioeconomic safety nets certainly help, too; crime in general is a function of both mental health and economic standing, so public policies aimed at addressing both of those things will naturally lead to both fewer incarcerations and fewer reincarcerations.