The bell curve is a thing, but nobody knows why, or why jews are, in the aggregate, a standard deviation higher than most other groups.
Bell curves are a statistical phenomenon (thoughtco.com). A bell curve/normal distribution is an approximation. Why people cluster around averages rather than extremes in IQ probably has a few reasons. One is probably because low IQ is detrimental to life outcomes. Another is that high IQs correlate with larger brains which means the brain consumes more energy and human baby heads (which are already relatively large) would be larger.
Also, hypotheses about Jewish IQ exist. It may be due to genes also linked to diseases (vdare.com) (quite the double-edged sword) and/or being permitted in banking and little else long ago (commentarymagazine.com) (which acted as a selection pressure).
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