The sex ratio at birth (SRB) in Spain jumped abruptly in the late 1970s and temporarily reached values of more than 109 boys per 100 girls in the early 1980s. This article shows that health care system expansion increased the likelihood of male births in Spain between 1975 and 1995. By facilitating the delivery of preterm and dystocic babies and improving overall maternal conditions, these developments increased the survival chances of male fetuses, who are biologically weaker than females. However, biological factors alone cannot explain the biased SRB. Our analysis shows that the availability of prenatal sex determination technologies and a strong son preference nurtured by the Francoist dictatorship fostered gender-biased behaviors that resulted in an excessively high SRB. The lack of evidence on sex-specific abortions suggests that women took better care of themselves when carrying a son. The spread of gender-egalitarian values brought about by the end of the dictatorship and the transition to democracy undermined son preference and returned the SRB to normal levels.