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ANNALS OF HUMAN BIOLOGY, 1978, VOL. 5, NO. 5, 487--489 Natural selection and birthweight N. BLURTON JONES Department of Growth and Development, Institute of Child Health, University of London [Received 23 March 1978; revised 6 May 1978] Summary. Mean birthweight, even before induced births became commonplace, is slightly lower than the birthweight at which peri- natal mortality is lowest. This finding, once hard to explain by natural selection, is shown to be exactly in line with predictions from natural selection theory. 1. Introduction Karn and Penrose (1952) showed that mean birthweight in man was slightly lower than the "optimum" birthweight, defined as the weight at which perinatal