Nine months after South Africa hosted the 2010 football World Cup a disproportionately higher number of male babies were born compared with the previous eight years. There are two reasons for this: people were having more sex and they were happy.

In our study, published in the journal Early Human Development, we found that nine months after the World Cup, the sex ratio at birth increased. The ratio is defined as the proportion of male babies in the total population of babies born.

The sex ratio at birth can be used to assess the stress a population is under.

When there is stress due to natural events such as earthquakes, floods or man-made events such as terrorist attacks, fewer male babies on average are born, showing a decreased sex ratio at birth. When there are no events adding stress to people, more male babies than female babies are born alive. This means the sex ratio is increased.

Why more boys were born

Between February and March 2011—nine months after the World Cup—the proportion of boys born compared to girls was 50.6%. This is the highest proportion recorded for that time of year since 2003. In previous years, the average proportion for that time of year was about 50.3%. Although this is only a 0.3% difference, which might appear small, it translates to an extra 1,100 male births.

From our statistical analysis, we are 98% certain that this increased birth rate nine months after the tournament was not a coincidence. There are a combination of two factors in the context of our study that result in more male than female children being born. This includes sperm motility, or the ability of sperm to move forward, and how frequently people have sex.

Gwinyai Masukume and Victor Grech, "The sex ratio at birth in South Africa increased 9 months after the 2010 FIFA World Cup"

When people have sex more often, on average more boys are born. This relates to the fertile period of a woman’s menstrual cycle. If conception takes place at the beginning or the end of the fertile period, the child is more likely to be a boy. If it takes place in the middle of the fertile period, the child is more likely to be a girl.

In terms of sperm motility, the consequences of negative natural events is that sperm motility is low. This translates into fewer boys being born.

Following the Kobe earthquake in Japan in 1995, sperm motility reduced in men who experienced the severe consequences of the earthquake. Nine months after the devastating earthquake, relatively fewer boys were born than was usual for that time of the year.

Similarly, research shows the death of Princess Diana of Wales in 1997 was followed by a significant fall in the sex ratio at birth in Britain. The research suggests her death was a stressful event for the population.

Had the World Cup caused severe stress, sperm motility would have declined in men and there would have been a subsequent decline in the proportion of boys born. This suggests that sperm motility was unimpaired at population level by the World Cup.

Other studies shows that South Africans were less stressed during the World Cup period.

One conducted in Cape Town during the tournament showed that emergency admissions of those 18 years and younger (the paediatric population) was 37% less than usual relative to periods before and after the event.

Another study showed that up to eight months after the tournament South Africans said they felt good about themselves and their communities. This could be attributed to the country hosting the tournament.

Watching sport does things to the body

There is ample evidence that watching major sporting events can have a biological effect on the population. People have increased emotions when they watch sport.

Research shows that during the 2003 Rugby World Cup semi-final when New Zealand lost to Australia, there was an increase in New Zealand women being admitted to hospital for heart failure and there was an increase in cases of abnormal heart rhythms and heart rates among men in the immediate aftermath of the game. There was a 50% relative increase in admissions for heart failure and a 2.6-fold increase in admissions for abnormal heart rhythms and rates compared to periods when there was no rugby match.

Separate research shows that during the 2006 football World Cup in Germany, when the German national team took to the field, heart attacks and other heart disorders of sudden onset became more common due to emotional stress. Cardiac emergencies nearly tripled in Germany on the days their national team was playing.

And analysing the football World Cups between 1998 and 2010 shows that heart attacks increased in Brazil when the Brazilian national team played in the tournament.

Similarly, in 2010 when the men’s Winter Olympic games ice hockey final between Canada and the US was broadcast, emergency department visits in Ontario—the most populous Canadian province—increased significantly for those classified as experiencing a severe cardiac condition. The games were arguably the most watched television event in Canada’s history.

In 2000, global anti-apartheid stalwart and former South African president Nelson Mandela said:

Sport has the power to change the world. It has the power to inspire, it has the power to unite people in a way that little else does. It speaks to youth in a language they understand. Sport can create hope, where once there was only despair. It is more powerful than governments in breaking down racial barriers. It laughs in the face of all types of discrimination.

These words are clearly supported by biological phenomena.

This post originally appeared at The Conversation. Follow @TC_Africa on Twitter.