If it feels like your social calendar is packed with birthdays at the end of summer and early fall, you aren’t imagining it. Take a look at US birth rates, and you’ll notice that more babies are born in July through September than in any other month. After some quick mental math, it appears that these babies were all conceived during the winter months. But why? Sure, it seems easy to chalk it up to boozy holiday festivities that may lower inhibitions and lead to more sex. Or cold weather that...