(a)The number of persons originally enlisted or inducted to serve on active duty (other than active duty for training) in any armed force during any fiscal year whose score on the Armed Forces Qualification Test is at or above the tenth percentile and below the thirty-first percentile may not exceed 20 percent of the total number of persons originally enlisted or inducted to serve on active duty (other than active duty for training) in such armed force during such fiscal year.
(b)A person who is not a high school graduate may not be accepted for enlistment in the armed forces unless the score of that person on the Armed Forces Qualification Test is at or above the thirty-first percentile; however, a person may not be denied enlistment in the armed forces solely because of his not having a high school diploma if his enlistment is needed to meet established strength requirements.
An AFQT score is derived from the ASVAB(essentially the militaries’ IQ test). IQ scores are based on a normal distribution of scores from the general population with a mean of 100 and a standard deviation of 15. So the 30th percentile represents an IQ score of 92 while the 10th percentile would correlate with an IQ of 81.
There are probably ways to correlate the military test with a standardized IQ test, and which point the military test might be a rough proxy for IQ. If that was the case, the 80 IQ rule might be roughly accurate.
I don’t know if that’s been done though. Just playing devil’s advocate.
I think there have been studies on the correlation between ASVAB scores and IQ scores. The correlation is supposedly 0.8.
This stack exchange post has a few sources linked
https://psychology.stackexchange.com/questions/20220/to-what-extent-does-afqt-correlate-with-iq