Image copyright AFP/Getty Images Image caption Researchers believe people in South Africa were less stressed during the World Cup in 2010, which increased the ratio of boys born

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.

'Better feelings'

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."

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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."