"What’s an externality?"

"My friends tell me that in the Vondelpark, a delightful reserve in central Amsterdam, it is illegal to let your dog off a leash. But it’s perfectly legal to have sex in the park, so long as it is not in view of a children’s playground. The argument is that the dog may make a mess that imposes costs on the unwary walker, while the couple imposes no costs on other park users. I wonder which of these two actions would be more likely to be outlawed in the U.S. Implicitly, how do attitudes toward negative externalities, as expressed in city ordinances, differ between the U.S. and the Netherlands—or among other Western countries?"