Stabilizing selection (not to be confused with negative or purifying selection) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. Karn & Penrose (1951) showed that birth weight follows a normal distribution, that mortality for newborns is greater for those either under- or over-weight, and that the mean birth weight (7 lbs) coincides with that showing minimum mortality. That is, natural selection reduces the variance in birth weight, in such a way that the distribution has become stabilized around the optimum size for maximum survival.