Putnam noticed that bowling leagues had declined significantly in the last few decades of the twentieth century.
People still bowled, but as individuals and informal groups, not as part of a league.
Though social capital varies across many dimensions, according to Putnam.
the most important distinction is between bridging (inclusive) and bonding (exclusive) social capital.
And his vivid ten-pin metaphor — derived from the observation that “more Americans are bowling today than ever before, but bowling in organized leagues has plummeted” — set off hand-wringing from coast to coast about America’s declining sense of community.
Social capital is the mutual trust and cooperation that arises from the web of connections among people involved in organizations and community groups.
In doing so, he identifies a dominant theme: "For the first two-thirds of the twentieth century a powerful tide bore Americans into ever deeper engagement in the life of their communities, but a few decades ago--silently, without warning--that tide reversed and we were overtaken by a treacherous rip current" (p 27).
Thus, social capital increased in the US until the 1970s and then suddenly decreased right up to the present.
Many political scientists regard social capital as essential to democracy because social capital forges bonds between members of the community.
These bonds enable people to readily join together.