I saw the environmental effect as an Ensign/JG in the Navy. The navy had to take 100000 group 4's, IQ level below normal. I had two men but did not notice anything until it appeared in their files. They seemed normal and performed difficult jobs well. Apparently, their IQ's suffered from isolation somewhere in Appalacia. Now, if a race were disproportionately living in such areas, someone of normal IQ might not be able to see the cause of the effect.