The sex ratio at birth (1).

This and the next series of Best Practise Guidelines will revisit the main theories that attempt to explain the influences on the sex ratio at birth (M/F), which was similarly reviewed in this journal in 2015. The papers include a historical review of the classical theories and initial work pertaining to the study of M/F, an analysis of 88,875,750 live births in an attempt to quantify M/F, the effect of earthquakes and climate change and an attempt to approximate the primary sex ratio (at conception). Another theoretical paper provides several potential explanations for autism spectrum disorder that may be correlated with M/F. Yet paper tests the hypothesis that RhD positive heterozygous mothers have a male-skewed sex ratio. A final paper provides a systematic review of the factors that influences M/F. It is hoped that this set of papers will continue to provide stimulation and food for thought on this intriguing topic.