You're going to have the Three Felonies A Day problem there, where in practice everybody commits crimes all day long and the people "not getting caught" is really everybody, even including people currently incarcerated who are still guilty of many other crimes they haven't been convicted of.
But if you want to talk about, shall we say, "real" crimes then that's another story. The solve rate for murders is actually pretty high (because they're given significant investigative resources), to the point that the population of convicts is probably not a terribly unrepresentative sample, and the lower intelligence of the convicts is pretty well established.
It also depends how you measure intelligence. The IQ of people who commit politically-motivated bombings is often significantly above average, but they also choose to commit a crime that attracts a hugely disproportionate level of investigative resources and correspondingly has quite a high solve rate despite the perpetrators' supposed intelligence, so maybe there are different kinds of stupid too.
But if you want to talk about, shall we say, "real" crimes then that's another story. The solve rate for murders is actually pretty high (because they're given significant investigative resources), to the point that the population of convicts is probably not a terribly unrepresentative sample, and the lower intelligence of the convicts is pretty well established.
It also depends how you measure intelligence. The IQ of people who commit politically-motivated bombings is often significantly above average, but they also choose to commit a crime that attracts a hugely disproportionate level of investigative resources and correspondingly has quite a high solve rate despite the perpetrators' supposed intelligence, so maybe there are different kinds of stupid too.