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 survived. 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].
