Evidence for the decline of these phenomena comes from a number of independent sources. Surveys of average Americans in 1965, 1975 and 1985, in which they recorded every single activity during a day - so-called "time-budget" studies - indicate that since 1965 time spent on informal socialising and visiting has gone down (perhaps by one quarter) and time devoted to clubs and organisations is down even more sharply (by roughly half). Membership records of such diverse organisations as parent-teacher associations, the League of Women Voters, the Red Cross, trade unions and even bowling leagues show that participation in many conventional voluntary associations has declined by about 25 per cent to 50 per cent over the past two to three decades. Surveys show sharp declines in many measures of collective political participation, including attending a rally, a meeting about town or school affairs, or working for a political party.
Some of the most reliable evidence about trends comes from the US national opinion research centre in Chicago, the General Social Survey (GSS), conducted nearly every year for more than two decades. The GSS shows, at all levels of education and among both men and women, a drop since 1974 of roughly one quarter in group membership and a drop since 1972 of roughly one third in social trust. Slumping membership has afflicted all sorts of gatherings, from sports clubs to literary discussion groups.
Reversing this trend depends, at least in part, on understanding the causes of the strange malady afflicting American civic life. Many possible answers have been suggested for this puzzle, and they are worth looking at closely.
Studies have found that residential stability and home ownership are associated with greater civic engagement, but data from the US Bureau of the Census show that the number of people who have moved house has been remarkably constant over the past half century. In fact, to the extent that there has been any change at all, both long-distance and short-distance mobility has declined over the last five decades.
But if the sheer number of house moves has not eroded our social capital, could it be possible that we have moved to places, especially suburbs, less congenial to social connectedness? No: in fact, the downward trends in trusting and joining are virtually identical everywhere - in cities, in suburbs, in small towns, and in the countryside.
Americans certainly feel busier now than a generation ago. The proportion who report feeling "always rushed" jumped by half between the mid-1960s and the mid-1990s. And lurking nearby in the shadows are the economic pressures so much discussed nowadays, from job insecurity to declining real wages.
Yet, however culpable busyness and economic insecurity may appear at first glance, it is hard to find incriminating evidence. First, studies do not confirm the thesis that Americans are, on average, working longer than a generation ago.
But in any event, other data call into question whether longer hours at work lead to reduced civic life and social trust. The GSS figures show that employed people belong to more groups than those outside the paid labour force. Even more striking is the evidence that among workers, longer hours are linked to more civic engagement.
So hard work does not prevent civic engagement. Moreover, the nationwide fall-off in joining and trusting is perfectly mirrored among full-time workers, among part-time workers, and among those outside the paid labour force. If people are dropping out of community life, long hours do not seem to be the reason.
What about financial pressures? It is true that people with lower incomes are somewhat less engaged in community life and somewhat less trusting than those who are better off. On the other hand, the downtrends in social trust and civic engagement are visible among people of all incomes, with no sign whatever that they are concentrated among those who have borne the brunt of the economic distress of the past two decades.
The most significant social change of the past 50 years has been the movement of women out of the home and into the paid labour force. However welcome it may be, it is hard to believe that it has had no impact on social connectedness. Could this be the primary reason for the decline of social capital over the past generation?
Some patterns in the survey evidence seem to support this claim. Memberships among men have declined at a rate of about 10 to 15 per cent a decade, compared with about 20 to 25 per cent a decade for women. These trends, coupled with the obvious transformation in the professional role of women over this same period, led me in previous work to suppose that the emergence of two-career families might be the most important single factor in the erosion of social capital.
As we saw earlier, however, work status itself seems to have little net impact on group membership. Indeed, the overall declines in civic engagement are somewhat greater among housewives than among employed women. The central fact is that the overall trends are down for all categories of women, but women who work full time may have been more resistant to this slump than those who do not.
Another widely discussed social trend that coincides with the downturn in civic engagement is the breakdown of the traditional family unit - mom, dad, and the kids. Since the family itself is, by some accounts, a key form of social capital, perhaps its eclipse is part of the explanation for the reduction in joining and trusting in the wider community.
Married men and women do rank somewhat higher on both our measures of social capital. Married men and women are about one third more trusting and belong to about 15 to 26 per cent more groups than comparable single men and women. Thus, some part of the decline in both trust and membership is tied to the decline in marriage. On the other hand, changes in family structure cannot be a major part of our story, because the overall declines in joining and trusting are substantial even among the happily married.
Circumstantial evidence, particularly the timing of the downturn in social connectedness, has suggested to some observers that an important cause - perhaps even the cause - is big government and the growth of the welfare state. By "crowding out" private initiative, it is argued, state intervention has subverted civil society.
An empirical approach to this issue is to examine differences in civic engagement and public policy across different political jurisdictions to see whether enlarged government leads to shrivelled social capital. Among the US states, however, differences in social capital appear essentially uncorrelated with various measures of welfare spending or government size. Citizens in free-spending states are no less trusting or engaged than citizens in frugal ones.
In all our statistical analyses, however, one factor stands out as a predictor of all forms of civic engagement and trust. That factor is age. Older people are consistently more engaged and trusting than younger people, yet we do not become more engaged and trusting as we age. What's going on here?
There has been a long "civic" generation, born roughly between 1910 and 1940, a broad group of people substantially more engaged in community affairs and substantially more trusting than those who are younger. Since national surveying began, this cohort has been exceptionally civic: voting more, joining more, reading newspapers more, trusting more.
These patterns hint that growing up after the Second World War was a quite different experience from growing up before that watershed. It is as though the post-war generations were exposed to some mysterious X-ray which permanently and increasingly rendered them less likely to connect with the community. Whatever that force may be, it accounts - rather than anything that happened during the 1970s and 1980s - for most of the civic disengagement at the core of our mystery.
What could have been the mysterious anti-civic "X-ray" that affected Americans who came of age after the Second World War and whose effects progressively deepened at least into the 1970s?
There is only one prominent suspect against whom circumstantial evidence can be mounted. The culprit is television.
The timing fits. The long civic generation was the last cohort of Americans to grow up without television, which flashed into American society like lightning in the 1950s. In 1950, barely 10 per cent of American homes had sets, but by 1959, 90 per cent did - probably the fastest diffusion of a technological innovation ever recorded.
In the early years, viewing was concentrated among the less educated sectors of the population, but during the 1970s the viewing time of the more educated sectors of the population began to converge upward. By 1995, viewing per set-owning household was more than 50 per cent higher than it had been in the 1950s.
Most studies estimate that the average American now watches roughly four hours a day. Even a more conservative estimate of three hours means that it absorbs 40 per cent of the average American's free time, an increase of about one third since 1965. This enormous change in the way Americans spend their days and nights occurred precisely during the years of generational civic disengagement.
The links between civic engagement and television viewing can be instructively compared with the links between civic engagement and newspaper reading. Each hour spent viewing is associated with less social trust and less group membership, while each hour reading a newspaper is associated with more. An increase in viewing of the magnitude that the US has experienced in the past four decades might directly account for as much as one quarter to one half of the total drop in social capital, even without taking into account, for example, the indirect effects of viewing on newspaper readership or the cumulative effects of lifetime viewing hours.
Even though there are only 24 hours in everyone's day, most forms of social and media participation are positively correlated. Thus people who listen to lots of classical music are more likely, not less likely, than others to attend baseball games. Television is the main exception to this generalisation - the only leisure activity that inhibits participation outside the home. Viewers are homebodies.
An impressive body of literature suggests that heavy viewers are unusually sceptical about the benevolence of other people - over-estimating crime rates, for example. Heavy viewing may well increase pessimism about human nature. And it may increase passivity.
More than two decades ago, just as the first signs of disengagement were beginning to surface, the political scientist Ithiel de Sola Pool observed that the central issue would be whether the development represented a temporary change in the weather or a more enduring one in the climate. It now appears that much of the change whose initial signs he spotted did in fact reflect a climatic shift.
In an astonishingly prescient book, Technologies Without Borders, published in 1991 after his death, Pool concluded that the electronic revolution in communications technology was the first big technological advance in centuries which would have a profoundly decentralising and fragmenting effect on society and culture. He hoped that the result might be "community without contiguity". As a classic liberal, he welcomed the benefits of technological change for individual freedom - and in part I share that enthusiasm. Those of us who bemoan the decline of community in contemporary America need to be sensitive to the liberating gains achieved during the same decades. We need to avoid an uncritical nostalgia for the 1950s.
On the other hand, some of the same freedom-friendly technologies whose rise Pool predicted may indeed be undermining our connections with one another and with our communities. Pool recognised that social values can alter the effects of technology. This perspective invites us not merely to consider how technology is privatising our lives - if, as it seems to me, it is - but to ask whether we like the result; and if not, what we might do about it. Those are questions we should, of course, be asking together, not alone.
A fuller version of this essay appears in the March edition of `Prospect' magazine.