Less sex after heat waves
A heatwave means less sex, which sees a fall in births nine months later Getty Images

Researchers have found out that a rise in temperature makes people less inclined to have sex, which in turn leads to a drop in birth rates.

The study, which was carried out in the US by National Bureau of Economic Research, reveals that following a hot day, the birth rate falls 0.4% within nine months.

"Extreme heat leads to a sizeable fall in births," the study says. "Temperature extremes could affect coital frequency. It could affect hormone levels and sex drives.

"Alternatively, high temperatures may adversely affect reproductive health or semen quality on the male side or ovulation on the female side"

The report's author, Alan Barreca, told The Independent: "Although the study relates to the US, the consequences are likely to be even more pronounced in the developing world – while the effect in the UK is likely to be similar to North America.

"The decline in birth rates is a very serious issue for countries, like the United States and the UK, which have below-replacement birth rates," said Barreca, an associate professor at the Tulane University in New Orleans.

"This will put a lot of strain on social insurance programmes, like social security, because it will create large imbalances in the make-up of the population."

The study also said that birth rates did not return to normal after the nine months following a heat wave – it found that only 32% of the gap in births was closed after the nine-month period.

Moreover, it said that declining birth rates could be a worry for countries such as China, which recently dropped its one-child policy in favour of two children per family. It also meant that in the US, the would be less people of working age to pay for the Social Security of retirees.

The research was conducted by three economists from Tulane University, the University of California and the University of Central Florida, who studied 80 years of US fertility and temperature data.