From Sir Ken, as always. From his book the Element:
“IQ tests can even be a matter of life and death. A criminal who commits a capital offense is not subject to the death penalty if his IQ is below seventy. However, IQ scores regularly rise over the course of a generation (by as much as twenty-five points), causing the scale to be reset every fifteen to twenty years to maintain a mean score of one hundred. Therefore, someone who commits a capital offense may be more likely to be put to death at the beginning of a cycle than at the end. That’s giving a single test an awful lot of responsibility.
People can also improve their scores through study and practice. I read a case recently about a death row inmate who’d at that point spent ten years in jail on a life sentence (he wasn’t the trigger man, but he’d been involved in a robbery where someone died). During his incarceration, he took a series of courses. When re-tested, his IQ had risen more than ten points – suddenly making him eligible for execution.”