The number of teenage births is considerably lower in the united states than it was 50 years ago because

i dont know sorry why dont you looking it up on google

an informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.