Operating systems such as Communities that Care, operating principles such as collective efficacy, and much else connected with successful service design and delivery make assumptions about the aspirations of the societies they work within.

The biggest concerns "social capital" – the core intuition that the goodwill that others have toward us – and we toward them – is a valuable and trustworthy resource.

But there is a widespread contrary intuition that social capital is declining and has been dwindling even faster during the 15 years since Harvard Professor of Public Policy Robert Putnam wrote pessimistically about its condition in the US, and sent a tremor of anxiety through other Western societies.

One Putnam allusion, which seemed to encapsulate the spirit of his argument (and has since seen heavy use by community arts lobbyists), was to choral singing. During a 20-year quasi-experimental study of civic society in Italy, he had found that alongside involvement in soccer clubs and co-operatives, membership of choral societies was one of best predictors of a robust and effective local democracy and economy.

"Communities don't have choral societies because they are wealthy;" he wrote. "They are wealthy because they have choral societies, or, more precisely, the traditions of engagement, trust and reciprocity that choral societies symbolize."

Putnam's 1995 essay, which re-emerged in book form in 2000 as Bowling Alone: The Collapse and Revival of American Community assembled the social capital down-curves. Since 1973 the number of Americans who reported attending a public meeting on town or school affairs in the previous year had fallen by more than a third. Much the same went for attending a political rally or speech, serving on a committee of some local organization, and working for a political party.

By almost every measure, he wrote, Americans' direct engagement in politics and government had nose-dived, despite the fact that average levels of education – the best individual-level predictor of political participation – had risen sharply.

Similarly, and just as in the UK, union membership had been falling for nearly four decades, and interest in parent-teacher associations "an especially important form of civic engagement in twentieth-century America" had dipped sharply, before rallying somewhat.

He went on to acknowledge other rises, for example in the numbers attending small, mutual support and self-help groups, but he dismissed their potential contribution to trends in social connectedness.

More concerning was the "massive evidence of the loosening of bonds within the family" and the loss of ordinary neighborliness – ammunition for electioneering politicians everywhere.

All of this is reviewed – with a twist – in a fifteenth anniversary article published, like its predecessor, in Johns Hopkins University's Journal of Democracy.

Putnam and his Harvard colleague Thomas Sander reflect on how the original had struck a chord with readers who had "watched their voting precincts empty out and their favorite Elk lodges close".

They add new signs of decline:

As of 2004, a quarter of those polled in the United States reported that they lacked a confidant with whom to discuss important personal matters and nearly half of all respondents reported being only one confidant away from social isolation. Since social isolation (that is, the lack of any confidants) strongly predicts premature death, these are sobering statistics.

The twist is 9/11:

Whether they were in college, high school, or even grade school when the twin towers and the Pentagon were hit, the members of the 9/11 generation were in their most impressionable years and as a result seem to grasp their civic and mutual responsibilities far more firmly than do their parents …

For example, young collegians' interest in politics has rapidly increased in the last eight years, an increase all the more remarkable given its arrival on the heels of thirty years of steady decline. From 1967 to 2000, the share of college freshmen who said that they had "discussed politics" in the previous twelve months dropped from 27 to 16 percent; since 2001, it has more than doubled.

But to bring light to this ironic reversal in the health and social capital of the nation, which fueled the optimism that swept Barack Obama to power, is not the reason for the journey.

Putnam and Sander regard it as only one aspect of an ominous larger and longer-term prospect "whose main feature is a growing civic and social gap between upper-middle-class young white people and their less affluent counterparts".

On indicator after indicator—general and academic self-esteem, academic ambition, social friendships, and volunteering – the kids who could be described as the "haves" grew in confidence and engagement while their not-so-well-off contemporaries slipped farther into disengagement with every year.

The widening gaps that we are seeing in social capital, academic ambition and self-esteem augur poorly for the life chances of working-class youngsters. If these gaps remain unaddressed, the United States could become less a land of opportunity than a caste society replete with the tightly limited social mobility and simmering resentments that such societies invariably feature.

See Sander T H and Putnam R D, "Still Bowling Alone? The 9/11 Split," Journal of Democracy 21, 1 January 2010 pp 9-16 and Putnam R D "Bowling Alone: America's Declining Social Capital" Journal of Democracy 6, 1, January 1995, pp. 65-78.

• for gloomy prognostications about economic inequalities in the UK, see the BBC news story, Rich-poor divide "wider than 40 years ago".