People who are fortunate have a tendency to explain their choices in language around personal merit, and people who are less fortunate have a tendency to explain their situations in language around personal demerit. Due to societal incentive structures being set up around income as a first-approximation, high-earners like lawyers, doctors, bankers, and software engineers fall in the former category, and low-earners like janitors, bakers, waiters, and taxi drivers fall into the latter. The positive language often centers around "high educational attainment", "complex job function", "hard work", "ambition", and "intelligence", and the negative language often centers around "lack of ambition", "poor grades", "inability", and "lack of intelligence". It's very tempting to explain the differences between people employed in these two classes of professions using IQ, and indeed there have been several highly questionable publications over the years correlating IQ with outcomes in life.
In order to do a sound correlation study leading up to a meaningful conclusion, it is necessary to prove that two variables move in tandem, independently of all other variables. Now, it is impossible to measure the IQ of a new-born child, so let's skip over the old "nature versus nurture" argument that's been beaten to death over the years with no conclusion †. The intelligence of a 8-year-old can be assessed using a reading test, which measures what the child has learnt since birth. As learning is ruthlessly cumulative, it should come as no surprise that intelligent children tend to have good outcomes later in life. Which children would do well in this reading test? A child who gets enough nutrition and medical care to develop fully; whose parents are able to afford the time and money to cook loving meals and take them to a good pediatrician; whose parents stimulate them by talking to them with a large vocabulary and correct grammar; whose parents nurture their curiosity with new and interesting topics; whose parents have enough free time outside work to spend with them; whose parents keep suitable toys and literature in the house; whose friends provide stimulating conversation; whose parents enroll them in a good kindergarten and school so that the child has access to these friends; whose friends' parents are friends with their parents so they go on play-dates; whose parents live in good neighborhoods with easy access to all those things — the list could go on forever, and all be bucketed into a simple "socio-economic factors of family". Perhaps the best proof of this in action is the Great Gatsby Curve correlating income inequality with inter-generational mobility across different countries — the socio-economic factors play an outsize role in unequal countries, and indeed, inter-generational mobility is higher in equal countries. Measuring and correlating adult IQ, which is the end variable arising from two decades of consequences, with outcomes in life, is hence tautological.
It follows from the discussion that the root-cause for the difference between a low-earner's avenue in life and a high-earner's avenue in life is privilege. The correct term for the job function of a high-earner is scarce, not hard or complex. They are compensated highly simply due to demand-supply market forces, and stories about their hard work are totally misplaced — low-earners work much longer hours and endure far more hardship. The inability of the vast majority of the people to perform a "complex" job function is the end result of being born unprivileged. If a barista or taxi driver had the same privilege as a high-earner, there is no reason to doubt that they would have educated themselves in top universities, and been productive in a high-skill job function. In order to subvert the privilege explanation and explain differences in terms of personal agency and individual merit, it is necessary to invoke a eugenics argument — discounting privilege, the only difference between two eight-year olds is genetics.