[Jach]

Sure, plenty are smart, or at least not dumb. But the difference doesn't even have to be so simple as smart/dumb. If your hiring pool consists of two distinct sources, one with an average IQ of 98 (i.e. not dumb), and the other an average IQ of 105, the second source will probably have more hires. Increase the difference (say, to a full standard deviation) and it's only worse for the lower group.

[querulous]

Why do you assume plumbers or electricians let's say have a lower average iq than programmers? Until ~10 years ago plumbers and electricians far outearned programmers.

[Jach]

The main assumption fell on coal miners. Plumbers and auto mechanics (and now electricians?) and other blue-collar work came up as a generalization, though I'd generally accept the same bet -- that is, there probably won't be much mobility in out-of-work individuals in those fields to software (and IQ-gating is just one of several reasons why that would be since admittedly some BigCo tasks could be done by someone with an IQ between 80-90 but those people will almost never get in the usual way because to invert a tree on a whiteboard or whatever stupid proxy is in fashion probably requires an IQ of >100). But the main reason I assume most of those fields have a lower average IQ is because I assume their job tasks are less cognitively complex in a technical sense than those of a software engineer and I've read https://www.udel.edu/educ/gottfredson/reprints/1997whygmatte... Additionally an average programmer (maybe ignoring freelance web programmers?) has a CS degree and the average CS degree requires a certain level of rigorous math among other things that require above average intelligence to do well, whereas blue collar work typically requires nothing beyond a GED and sometimes various certifications or apprenticeships/training.

But why assume when we have data? Here's just one set. http://www.iqcomparisonsite.com/occupations.aspx If you find another that contradicts it please share.