Introduction

Although the human sex ratio at conception, i.e. the primary sex ratio, is not readily amenable to investigation, data on the karyotypes of spontaneous and induced abortions indicate that early in pregnancy there is a large surplus of males, estimated at between 123 and 130 males for every 100 females (Stevenson, 1959; Lee & Takano, 1970; Hassold et al., 1983). Selective attrition of male foetuses occurs throughout the gestational period and also is seen in excess numbers of male stillbirths, resulting in a sex ratio at birth (i.e. secondary sex ratio) in the Caucasian populations of Western countries of 105 to 106, with slightly lower values of between 102 and 104 for Blacks (Chahnazarian, 1988). However, considerable global variation has been reported in secondary sex ratios, ranging from less than 100 to over 110 (Visaria, 1967; James, 1987).

While the excess spontaneous loss of male foetuses must be considered a primarily biological phenomenon, related to the male phenotype but not significantly associated with the expression of X chromosome mutations in hemizygous males (Stevenson & Bobrow, 1967), in postnatal life both biological and social factors influence the tertiary sex ratio. By the third decade approximately equal numbers of males and females are observed in Western countries, the decline in proportion of males again being ascribed to their relatively greater biological frailty, in combination with higher exposure to fatal accidents and homicide (Macfarlane & Mugford, 1984).