The IQ values by the old definition did approximately fit the bell curve (but not exactly; there are more extremely stupid people than extremely smart people).
This part of the explanation needs the most followup. It’s often proposed that different subpopulations lie on different bell curves. These mixed normal distributions can be complicated. What did they look like in Germany in the 1910s, and were there really techniques available then for recognizing and analyzing them?
This part of the explanation needs the most followup. It’s often proposed that different subpopulations lie on different bell curves. These mixed normal distributions can be complicated. What did they look like in Germany in the 1910s, and were there really techniques available then for recognizing and analyzing them?