I think you might be over-estimating the skills of the bottom 10% (or under-estimating the skills needed to be a plumber). The US Army currently rejects applic

I think you might be over-estimating the skills of the bottom 10% (or under-estimating the skills needed to be a plumber). The US Army currently rejects applicants who score lower than 31 on the ASVAB (roughly 92 on the IQ scale, about 30th percentile). I’m pretty sure there are Army jobs that can be done with less need for intellect than that needed to be a plumber. I don’t think that the lowest 10% of the population is generally capable of being a plumber. They are capable of doing some of the work done by plumbing companies, but the plumbing code is lengthy, in large part because people’s lives are literally at stake and the actual plumber (the license-holder) has to keep that straight in their head, apply the relevant parts to the varied job site requirements, turn blueprints into working plumbing systems, etc. Plumbing is a lot more than remembering “shit flows downhill, don’t lick your fingers, Friday is payday.” Sample plumber exam: https://www.tests.com/practice/plumbing-practice-test https://www.tests.com/practice/plumbing-practice-test