The 2010 World Cup in South Africa triggered an increase in the number of boys born in the country nine months after the event, a study says. The ratio of boys to girls born in that period was the highest recorded between 2003 and 2014, the study in the Early Human Development journal found.
People were more relaxed and probably had more sex during the World Cup, increasing the ratio, say researchers. South Africa hosted its first World Cup between 11 June and 11 July 2010.
Dr Gwinyai Masukume, from the University of the Witwatersrand, was involved in the study and told News Day on BBC World Service: “The World Cup caused less stress, people were happier, there has been published research done that people had better feelings, positive feelings about themselves and their country.
“People also probably had more sexual intercourse during the World Cup. “It has been known that if people have sexual intercourse more frequently there is a tendency to have more boys born than females.”
The study showed the ratio of boys born nine months after the tournament was 0.5063, compared with an average during the period from 2003-2012 of 0.5029, which represented about 1,088 extra boys.
Because the ratio of sexes shows “significant cyclic trends”, researchers compared the data with the same months in previous years.
The medical reasons for the altered sex ratio could be because of unimpaired sperm mobility, increased frequency of sexual intercourse and, or, decreased male foetal loss during pregnancy, the report’s authors concluded.
They added it was “unlikely to be due to chance or a seasonal effect”.
‘No one knows’
Allan Pacey, Professor of Andrology at The University of Sheffield, said it had been known for many years that the proportion of males born could be altered temporarily by external factors like military conflict and natural disasters.
But he said the reasons were still unclear. He said theories included the thought that men might produce more Y chromosome (male-bearing) sperm for a short period of time, or that a woman’s body can “sort” the sperm in some way after intercourse and therefore alter the proportion of X or Y sperm which reach the egg.
Professor Pacey added: “All these are credible biological mechanisms, but no-one knows which of them, if any, is responsible for altering the number of boys born in a population.”
In humans in the absence of significant stress the sex ratio at birth [males/(males+females)] is in favor of more male than female live births.
This study sought to determine the influence of the 2010 Fédération Internationale de Football Association (FIFA) World Cup in South Africa on the sex ratio at birth in that country specifically 9months afterwards. Publicly available data from Statistics South Africa was utilized detailing recorded live births. Analysis was carried out by Chi-squared tests.
February and March 2011 about 9months after the World Cup, had the highest observed sex ratio at birth (relatively more male births) of 0.5063 for the period 2003 to 2012. The observed sex ratio at birth in the considered two months of 2011 was 0.63% (p=0.02) greater than the sex ratio at birth for corresponding periods from 2008 to 2012. The increase noted in 2011 corresponds to more than 1000 extra male births than expected for February and March 2011.
The 2010 FIFA World Cup was followed about 9months afterwards by a significant increase in the sex ratio at birth. The main mechanism driving the observed increase in the sex ratio at birth in South Africa is most likely more frequent sexual intercourse at population level during the tournament.