Will climate change ruin your sex life? Global warming could cool sexual passions and reduce birth rates.

• Author: Bailey, Ronald
• Position: Columns

"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 North-east, 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[degrees]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...