I wish I could find it, but there is a mini-essay on the chans from someone who did cognitive psychology research in the California prison system, within which (he claims) lower-than-population-average IQ predominates. Yes, I know IQ has its flaws, but the point of the post was what various IQ brackets meant for specific cognitive abilities. It turns out that certain kinds of reasoning that I personally take for granted are a struggle for people below, say, 95. For example, hypothetical reasoning: "How would you have felt if you had had X for lunch yesterday rather than Y?" "I don't know what you're talking about, I had Y for lunch." "Yes, but what if you had had X?" "I DIDN'T HAVE X I HAD Y!". Turns out recursive logic is difficult even for people around 100 (population average).

So, yes: a shocking number of people lack the cognitive engine power for any kind of self-awareness.

The conclusion of the post was that criminals may not lack empathy so much as lack any kind of theory-of-mind to support empathy. On the other side, you have high-IQ people who don't have much real empathy but are intelligent enough that they can (and know that they should) simulate it.