It is very interesting to me that from like from 1% to 10% wage percentile cognitive ability goes down very consistently. It is not clear what societal effect would cause this. My best guess is that among the people without the intelligence to completely pull themselves into the middle class, the smartest find ways of coping without necessarily making more money.
On the opposite side, from 95% to 100% cognitive ability also goes down, but not as consistently. My best guess for this effect would be people with connections / generational wealth are capable of squeezing out more qualified candidates.
0: https://academic.oup.com/view-large/figure/393964189/jcac076...