There’s upside in a down economy — if you’re in the condom business. According to an interview with the condom manufacturer Trojan , their sales peak is the week between Christmas and New Years. Come Santa season, everyone seems to jump in the sack. (Here’s further proof of the “Santa effect.”)

What is it about Christmas that revs up the sex drive? The increase in sexual activity at at the end of December harks back to Saturnalia, a pagan holiday when people feasted on boar flesh and indulged in a lot of prehibernatory sex. Disapproving early Christians coopted the sexfest and turned it into the celebration of Jesus’s birth. “Why either/or?” ask modern day revelers, and now many both celebrate the baby Jesus and also enjoy conceiving (or preventing the conception of) their own babies. It’s unsurprising that people have more sex during the weeklong holiday: we have time to relax, drink, and socialize more. And not everyone wears Trojans — the birthrate in the United States peaks in the summer and in September, nine months after the winter bacchanalia.

Even more interesting, there may be a biological reason why people’s sex lives heat up as the seasons cool down. In BLONDES I describe the connection between hormone levels and waning daylight hours in autumn. And we’re not alone: other animals in the northern hemisphere also have their rutting season in the late fall — including reindeer.

