The origins of fluctuations in the human secondary sex ratio

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Abstract
Summary: A probabilistic model of the human reproductive process is used to assess the success of hypotheses designed to explain human sex ratio fluctuations. It is shown that theories based on different maturation rates or effective lifetimes of X- and Y-bearing spermatozoa cannot account for many of the major sex ratio fluctuations.A specific physiological model is proposed, based on differential motility of the two types of spermatozoa in the female reproductive tract, which together with differential fetal mortality, accounts successfully for most of the ratio fluctuations. On this theory the principal factor determining sex ratio is the physiological state of the female reproductive tract which in turn is governed by complex physiological and psychological parameters.