Patrick Markey, a psychology professor at Villanova University, found a similar trend when he studied Google search words as an indicator of sexual interest. Searches for prostitution, pornography, and dating websites all increased in the summer months.

But it’s not exactly clear what’s behind this uptick in sexual interest.

“We don’t know if it’s some kind of leftover mechanism from our ancestral past that we have this mating cycle, or if it’s something that results from circumstances,” says Markey. “When you see the pattern across so many different data sources it shows, ok, there are these cycles of sex. They’re definitely there, we just don’t know why they’re there.”

It’s plausible that there’s an evolutionary purpose behind mating in the summer, or else annual variations in sperm quality and hormonal fluctuations. But Markey says he leans toward simpler theories, which point toward changes in our environment.