Abstract
Differential mortality as a functon of birth weight was studied in Japan in the years 1969, 1974 and 1984. During the 15-year interval of observation a progressive equalization of mortality in the birth weight classes near the mean was found both for perinatal mortality as a whole and for its two components, stillbirth and mortality within the 1st week of life. The relaxation of selection on this metric trait is confirmed by the analysis of the variation over time of intensity of selection. A comparison of the Japanese populations with other populations so far studied is also presented.

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