Harvard University political scientist Robert D. Putnam promoted a new concept, called "social capital," in his book "Bowling Alone." It refers to bonds of trust and mutual concern that arise through volunteering, socializing, and taking part in organizations such as church and civic groups, bowling leagues, PTAs, and professional associations.

Among other benefits, such interactions help transmit new ideas and improve children's education -- thereby enhancing the efficiency of labor and capital markets.

But Putnam and others suggest that many activities that build social capital have been declining in the U.S. in recent years. A study by Dora L. Costa of the Massachusetts Institute of Technology and Matthew E. Kahn of Tufts University focuses on such trends.

• Among them are the huge rise in the proportion of women who work, and the growing racial, ethnic and income diversity in communities.

• They identify a five-percentage-point drop since the early 1970s in the share of prime-age adults doing volunteer work.

• Growing income inequality plays a role in the decline of social capital since people are more likely to volunteer to help others whose economic status they can identify with.

• They found an 11-percentage-point decline from the early 1970s to the early 1990s in membership in organizations ranging from youth, sports and church to literary and hobby clubs.

They even found a decline in the frequency with which people entertain at home or visit friends and neighbors.

They identified income inequality as the largest contributor to these trends -- but also included increased ethnic diversity as a possible explanation.

Source: Gene Koretz, "Economic Trends: Why Americans Grow Apart," Business Week, July 23, 2001.