Objective: Male live births slightly exceed female live births. This is usually expressed as M/F, the ratio of male to total live births. A multitude of external influences have been shown to reduce this ratio, including stress provoked through witnessing violent events; M/F dips occur three to four months later. The April 1992 Los Angeles riots constituted six days of extreme civil unrest in the city of Los Angeles. This study was carried out in order to ascertain whether M/F dipped in California following this event.
Methods: Monthly male and female live births for California were obtained from the Centers for Disease Control and Prevention for the state of California for 1992 and for January 1993.
Results: This study analysed 649 073 live births. M/F was lowest in August 1992, (0.5085). This was significantly lower than for the period after (September 1992 to January 1993, p = 0.044). The ratio of male to total live births was higher in January to July 1992 than in August 1992, but this difference was not statistically significant.
Conclusion: Stress has been shown to reduce M/F through an excess of male fetal loss during gestation and/or from gender-biased conception favouring females. Only the former mechanism is supported by these findings. This is the first time that violent events at state level have been shown to have potentially influenced M/F.