It's well known among urban design researchers that the average number of other people one is interacting with on a daily basis has been decreasing ever since the move to the suburbs that started 100 years ago. The movr to the suburbs has been turning Americans into asocial people, generation after generation, in a vicious cycle. If you look at other countries that are still social because they live in denser environments, there's a certain amount of people "naturally" introvert, but much fewer than in the US, where most of it is learned.