Eliezer, are you familiar with Carol Dweck's research on intelligence, or has that corner of psychology eluded you? It matches up very closely with what you say here about maturity. Dweck says: some people (like your parents on maturity) have an "entity theory" of intelligence - they think of it as something fixed that you either have or you don't - while others (like you on maturity) have an "incremental theory" - they think of it as continually developing. Incremental theorists tend to learn better and be more eager to face challenges, while entity theorists are more threatened by challenges and care more about signaling that they have intelligence. More here.

Entity views may be a common source of bias, with intelligence and other qualities that people value.