When my father was in the hospital in 2011, I watched an Animal Planet show called "Pit Bulls and Parolees," but I guess I'm not really part of the pit bull target demographic. Let me say upfront that I'm aware that put bulls are wonderfully loyal and they don't bite very much (the problem is that when they do bite they sometimes don't let go). In contrast, Labrador retrievers are bred to retract their teeth and gum rather than bite down. So, Labs are unlikely to maul your children's friends.
Audacious Epigone uses Google Trends to tabulate searches for "pitbulls" v. "labradors" by state. The most Labrador retriever-biased state is New Hampshire and the most pit bull-centric state is Mississippi: Moynihan's Law of the Canadian Border again.
Has there been much improvement in dogs in my lifetime? Ray Sawhill likes to point out how much better food has gotten in his lifetime, and that strikes me as close to indisputable. The board at Rice U. in the 1970s was awful by contemporary standards.
But I don't see much evidence that dogs are getting better. Perhaps the amazing gains of (roughly) the 19th Century when most modern breeds were more or less invented were a one time deal, and only marginal improvements are genetically possible anymore.
On the other hand, commercial animal breeding for specific traits continues: dairy cows produce twice as much milk as 40 years ago. Plant breeders apparently have made marijuana vastly stronger, too.
So, maybe animal breeding just doesn't attract the right type of hobbyist anymore. If you asked people 150 years ago to name somebody interested in animal breeding, they might have come up with Charles Darwin. Today -- Michael Vick.
Or maybe it's the worldview. Perhaps the Whiggish world view that produced Darwin is a delicate flower.