Karn & Penrose
(1951) showed that the distribution of human birth weights
(grey curve) closely matches the modal weight for maximum infant
survival at ~7 lbs (green curve). Infant mortality increases
sharply below ~4 lbs and above ~10 lbs: only about half of infants
with a birth weight of
< 3 lbs survive. Natural selection appears to stabilize mean
birth weight at the optimum for survival. [These data
are from 1951; survival of low-weight 'preemies' is now much
higher with medical intervention].
