By Carina Storrs
Special to CNN
(CNN) — There are so many things that can dampen your sex drive: You have a headache, you’re tired, it’s too hot outside.
According to a study, the last of those mojo-killers — and the climate change that is causing more scorching hot days — could be bringing down the birth rate in the United States.
Although the number of births in the United States went up last year for the first time since 2007, the U.S. birth rate has mostly been on the decline for at least a century. More couples have access to birth control and work opportunities for women have increased. Economic downturns, such as the recent 2007 recession, also contribute to baby busts. But the new study suggests that really hot days could also take a toll.
Researchers used historical vital statistics and other sources to look at the number of babies born about nine months after really hot days, which they defined as above 80 degrees, based on National Climate Center Data from weather stations across the United States.
The researchers found that, for every day that soared above 80 degrees — and in many cases above 90 degrees — between 1931 and 2010, there were 0.4% fewer births nine months later. The impact of one of these scorching days was that about 1,165 fewer babies were born across the United States.
Over a larger period, this could mean about 100,000 fewer births in the United States every year, based on climate change models that predict the number of these really hot days will increase from the current number of about 30 a year to about 90.
“I wouldn’t say it is the end of human civilization, but I would suggest it is going to add to the cost of climate change,” said Alan Barreca, associate professor of economics at Tulane University. Barreca is the lead author of the study, which was published Wednesday by the National Bureau of Economic Research, a nonprofit economic research organization.
Although previous research has supported the idea that heat hurts fertility, one of the advances of the current study, Barreca said, is that it asks the question whether fertility ramps back up when temperatures cool down. The answer? Not much.
There was an uptick in babies born between 11 and 13 months after heat spells, suggesting that couples put off their procreation by a couple of months, but this increase only made up for 32% of the decline at nine months.
“People might be constrained to conceiving in certain calendar months because they have time off work,” Barreca said. And if those precious few days in the month that a woman is fertile are thwarted by hot weather, that could be it for the year, he added. The authors did not look at whether birth rate rebounded more than the initial 32% in the years after a heat spell.
The fact that birth rates drop nine months after temperatures spike suggests that hot weather is having a direct effect on fertility, Barreca said. Although the study notes that rising temperatures can wreak havoc on income levels and food prices, both of which could discourage couples from having children, these factors would probably have more of a delayed effect on birth rate. It might take several months, for example, before a heat spell affects harvest and drives up food prices, for example.
It is easy to guess how things could cool down in the bedroom as they heat up outside. Just feeling uncomfortable could decrease, as the authors call it, “coital frequency.” But studies suggest high temperatures can also hurt reproductive health, by impairing sperm function, reducing testosterone levels and interfering with menstruation.
However, temperature probably has a lot less effect on when people have children than it did decades ago, when fewer people had air conditioner units and more people worked outside, said Kevin Bakker, a graduate student in ecology and evolutionary biology at the University of Michigan, who was not involved in the current study.
Instead, factors such as the type of work people do have probably had more of an effect on decreasing the U.S. birth rate, Bakker said.
Barreca agrees with this assessment.
“Temperature’s role has probably been pretty negligible compared to other things like access to birth control (and) increasing labor opportunities for women … but it would suggest that, if anything, it’s adding on to the other things going on,” he said.