"Not tonight, dear. It's too damned hot." That's a phrase lovers are likely to hear more frequently as the climate warms, according to a new National Bureau of Economic Research (NBER) study. The paper's authors looked for a causal link between heat and coital frequency using U.S. seasonal and regional temperature and birth data. Their conclusion: If people are already hot and sweaty, they're less likely to want to get hot and sweaty.
That, in turn, raises a worry about global warming's effects on future fertility. If couples indulge in intercourse less frequently when it's hot, economists Alan Barreca of Tulane, Melanie Guldi of the University of Central Florida, and Olivier Deschenes of the University of California, Santa Barbara reason, then they are less likely to conceive and give birth. But before we consider whether that conclusion makes sense, let's take a look at their empirical evidence.
The study examines the seasonality of birth rates in four U.S. regions—the South, the Northeast, the Midwest, and the West. Using vital statistics data from 1931 through 2010, the researchers confirm the well-known fact that U.S. birth rates peak in August and September. They're about 10 percent higher in those months than at their trough in April. From this it seems reasonable to conclude that couples enjoy more intimacy in the cooler months of November, December, and January. The authors also compile temperature and humidity data from 1931 through 2010 by county, against which they match subsequent birth rates by state.
After crunching the numbers, the researchers find that days above 80 degrees Fahrenheit do correlate with fewer subsequent births. Specifically, they report that "each additional >80°F day causes birth rates to fall by approximately 0.06% 8 months later, 0.39% 9 months later, and 0.21% 10 months later." This implies that on average, from 1931 to 2010, a day with temperatures greater than 80 degrees results in 1,165 fewer births across the whole United States.
Not surprisingly, they find that the effect of greater-than-80-degree days is about double in "cold states" versus "hot states," suggesting those of us who dwell in balmier climes are less likely to forgo romance just because it's a bit warm. In fact, a 2011 Degrees of Pleasure survey for the Trojan condom company found that Americans living in the South generally reported having more sex than those living farther North. For example, folks living in Miami claimed an average for sexual intercourse of 102 times per year compared to just 64 times annually for inhabitants of Minneapolis. Still, a majority agreed that 68 degrees is the ideal indoor temperature and 70 degrees is the ideal outdoor temperature for sex. Some 35 percent also said that they had passed on sex until temperatures cooled down. For what it's worth, the most recent figures show that the crude birth rate for Miami stands at 12.8 per 1,000 people compared to 14.8 per 1,000 for residents of Minneapolis.
The researchers do also detect a rebound effect in which births increase in the months after an 80-degree-or-above temperature shock. In other words, coital frequency apparently increases once temperatures drop. The rebound in births, however, is only half of what the average would have been if temperatures had been sustained at a lower level. Their analysis suggests that 30 additional days at 85 degrees Fahrenheit or above would result in a 12 percent decline in births.
But what about the high fertility rates found in some tropical countries? High temperatures don't seem to prevent people there from enjoying a bit of copulation. Or do they? In fact, researchers have noticed a birth seasonality that corresponds with changes in average temperature.
Way back in 1978 demographers associated with the International Fertility Research Program analyzed births in India. "The conception rate in Baroda and Manipal, India, is lowest during the hottest weather," they reported. "The greatest extremes are found in Baroda and, following high summer temperatures, there is a prolongation of low conception rates." The researchers speculated that this could be due to elevated scrotal temperatures inhibiting sperm production, but also suggested that high temperatures might also result in less frequent coitus.
A 2000 study similarly reported that in West Bengal "the distribution of conceptions over calendar months was negatively associated with average monthly temperature." Birth seasonality studies for southwestern Nigeria in 1985 and a 1994 study for Ho Chi Minh City in Vietnam similarly found conceptions increased during cooler times of the year.
The authors of the recent NBER paper acknowledge that beginning in the 1970s, the correlation between temperature and subsequent births in the U.S. weakens somewhat. "In the 1940s, 1950s, and 1960s, exposure to one additional >80°F day consistently causes a 0.6 percent reduction in the birth rate 9 months later," the researchers report. They find that by the 2000s that one additional >80°F day causes birth rates 9 months later to decline by only 0.2 percent. What happened? Simple: The percentage of U.S. households with air conditioning rose from 21 percent in the 1960s to about 87 percent today. Apparently, manufactured cool air is a turn-on. Despite the heat, a 2007 review found that summer is the most popular season for American adolescents to lose their virginity.
So what is the future of intimacy and fertility in a warming world? The authors cite climate models that project the number of 80-degree-plus days in the U.S. will increase to about 64 per year from a baseline of 31 per year now. Most of those additional days will occur in May and September, although June, July, and August will supposedly each experience 10 more days above 80 degrees by 2100, too.
Recognizing the inherent uncertainties in such long-term projections, the researchers make a back-of-the-envelope estimate using an econometric model that takes post-1970s (that is, post-air conditioning) data into account. They calculate that as a result of higher temperatures due to climate change, annual U.S. births will decline by 2.6 percent, meaning we'll see about 107,000 fewer babies born per year than there would have otherwise been. They also reckon that the proportion of births during the August peak will increase by 4 percent relative to the April trough.
As amusing as it is to contemplate the effect of warmer temperatures on future sexual activity, the researchers recognize that technological innovation could make their projections moot. "Providing low-cost access to air-conditioning may be an effective tool for mitigating the fertility costs of climate change throughout the world," they note. But that concession is way too limited.
By the end of this century, technology will have radically reshaped the terrain of human sexuality and fertility. Extensions and elaborations of virtual pornography, teledildonics, sexbots, and sexual neuroenhancers will change how people experience and enjoy sex. Similarly, people will gain ever more control over their fertility, up to and including the use of artificial wombs. Whatever minor effects climate change might have on making love and making babies, they will be overwhelmed by technological improvements in service of the ceaseless human quest for healthy children and healthy orgasms. At the end of this century, lovers will still likely croon, "Baby, it's hot outside, but not as hot as you."
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