Intelligence is domain-specific. People who have put too many skill points in technical knowledge often have none left for common sense and street-smarts.
No, stupidity is very general. The statistical association between different areas of ability, which is reified in the concept of IQ, is because people who are bad at one thing are bad at everything. The Stanford-Binet test was developed as a way to measure mental disability, not giftedness.
(Einstein would have had an unexceptional score on an IQ test, had he ever taken one. His schoolteachers thought him destined for failure.)
To put it another way, polymaths are unusual but idiots are everywhere. And people who are outstandingly good at say, computer engineering can be mediocre at philosophy or business administration.
Psychometrists distiguish between crystallized and fluid intelligence. Expertise is a combination of knowledge and ability. But ability itself is multifaceted, and raw talent goes untapped without the motivation to study and the opportunity to work.
For most areas of human endeavor, being smart enough is all that is required. Being a genius helped Albert Einstein reimagine physics, but did not make him a better patent clerk.
I question the coherence of this notion of intelligence as continuum of domain-general ability.
Specific measures of ability tend to cluster at the low end more than they do at the high end. This explains both the correlations among abilities, and the fact that high abilities tend to be domain-specific.
An IQ score over 70 has a clinical meaning: it defines the absence of mental retardation. Scores of 90 and 160 are equivalent for this purpose.
"Giftedness" in a domain-general sense is an abstract concept that may or may not map onto something real. The validity of IQ testing derives from measuring the other end of the scale.
