Secondary sex ratios in Micronesian populations.

Secondary sex ratio (SSR) values in excess of 109 male for every 100 female livebirths have been reported from several Micronesian populations. Analyses of linked family records dating from before 1942 for the native people of Guam reveal a highly masculinized SSR even at high birth orders and parental ages of married fertile couples. These findings contrast with reported declines in SSR with parity and parental age among historical and modern populations in Europe and North America where such declines often have been ascribed to the effects of decreasing mean coital frequencies associated with marital duration and spousal age. This study challenges implicit assumptions about the supposed universality of Euroamerican patterns of marital sexual behavior and supports the coital frequency hypothesis of SSR determination.