Earlier quoted context omitted.
Arguably the incarcerated, and more tenuously then unemployed, are not "average human brains". I'm not sure I'd actually argue that but it seems worth addressing.
Incarceration is much more highly correlated to your parents income level than their genetics. Unemployment is primarily a skillset mismatch problem. Becoming a newspaper pressman looked like a solid career choice in 1995, not so much twenty years later when those 18 year olds are 38 and unemployed. Not foreseeing the rise of the internet is hardly strong evidence that their brains are defective.
Nothing requires any stipulation that these minds be genetically inferior.