Research involving human behaviour is tricky.
For example, a famous postwar study of London Transport workers was thought to have shown that people in sedentary jobs, such as bus drivers, were more likely to develop coronary heart disease than those whose work involved more vigorous physical activity, such as bus conductors (in the days when a human went round collecting fares from passengers.)
Much later, my GP friend told me, a flaw was discovered: it could be the case that those who intuitively felt their health less robust would choose sedentary roles. So it was possible that despite the large sample of people in the study and the fact that they had the same field of employment in common, like was still not being compared with like.
The notion that IQ is the most important element in success may also have its weaknesses. When the self-styled ‘Masters of the Universe’ bank traders have finally destroyed the economy and are dangling from lampposts like Il Duce the scorecard may read differently. During the Great Financial Crisis one broker is reported to have bought a flock of sheep from a local farmer in order to ensure his family’s survival. Your own imagination will supply a hundred practical difficulties and dangers that could follow from this decision. If he was that clever, why hadn’t he foreseen the crisis and planned for it well ahead of time?
Some maintain that IQ is heritable and that the average level varies according to ethnicity. As to the first bit, Sir Cyril Burt’s research on twins proved it - so people thought, until it was re-examined after his death and judged fraudulent; his notes and records were no help, as it turned out that they had all been burnt. Nevertheless, other studies appear to support the hypothesis.
As for the second assertion, the link just given says ‘The scientific consensus is that there is no evidence for a genetic component behind IQ differences between racial groups.’
This doesn’t stop some people from trying to show otherwise; historian Simon Webb recently released a vlog citing the indirect evidence of a spatial aptitude test applied to applicants to the Royal Air Force. In this white British scored - on average - higher than Afro-Caribbeans and black Africans, but - oh dear - not so well as Chinese. Those of mixed b/w ethnicity scored - on average - in between b&w.
Remember the London Transport study and look for flaws: were the applicants all aiming for the same roles in the RAF? With the same long-term career ambitions? Why did they apply, but not others of their peer group? Who was advising them on career options? Did they (as seems very unlikely) all come from the same kind of family upbringing and expectations, go to the same kind of school?
It may be possible to improve your IQ; though there may also be a ceiling to that, just as you may train to run faster without ever achieving Olympian standards.
But more significant may be factors that permanently lower the individual’s IQ ceiling: ‘poor prenatal environment, malnutrition and disease are known to have lifelong deleterious effects,’ says Wiki. Poor nurture in early years may also hobble the child, which needs both sensory and mental stimulation to foster its development.
Poverty - or relative poverty, inequality - may well be a meta-factor behind many of these factors.
Then there’s the social environment and the development of one’s self-image, but that’s for another day.