[modificated after david monroe's smart comments]Robert Putnam wrote about the decline of social capital. I like one of his index to show the decline of community life over the last 50 years. He used the "Bowling Alone" metaphor (it's the title of his book): he found that from 1980 to 1983, league bowling declined by 40 percent, while the number of individual bowlers rose by 10 percent. This indicator (among others: party enrollment, surveys, club memberships, and on and on) shows that people seems to be less inclined to be part of civic groups.

Anyway, academics like Richard Florida disagrees with this. [NO I should modify this statement]