Soccer world cup influences sex ratio at birth
Abstract
Objective: In man, the ratio of male divided by total live births (M/F) is expected to approximate 0.515. This study was carried out in order to analyse the effect of the World Cup on M/F birth ratio.
Methods: Annual male and female live births were obtained from a World Health Organisation database. Male and female births in World Cup years were compared to the aggregate of the previous and following two years. Countries included were those hosting and winning the event and the runners up. Years available were 1952-2008.
Results: There were 146,627,054 live births. There were significant increases in M/F birth ratio for World Cup years in 1978 and in 1994 for the winning countries, Argentina (p<0.0001) and Brazil (p=0.002) respectively. No significant changes were found for runners-up. Mexico hosted the event twice (1970, 1986), associated with significant declines in M/F birth ratio (p<0.0001).
Conclusion: Stress has been shown to decrease M/F birth ratio, and this may have reduced M/F birth ratio in Mexico as hosting country in anticipation of or just after the event. On the other hand, wars have been shown to increase M/F due to increased coital frequency resulting in higher conception rates at the beginning and end of the menstrual cycle, favouring male conceptions. It is possible that non-programmed copulation and/or higher coital rates just before/during the 1978 World Cup (hosted and won) in Argentina increased M/F birth ratio. This does not explain the Brazilian rise during the event year since it is held in June and conceptions would manifest in the following year.
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