This is not to say that modular relationships entail no risks or that this is the best of all possible worlds. There are, in fact, profound risks in the situation, as we shall attempt to show. Until now, however, the entire public and professional discussion of these issues has been badly out of focus. For it has overlooked a critical dimension of all interpersonal relationships: their duration.

Sociologists like Wirth have referred in passing to the transitory nature of human ties in urban society. But they have made no systematic effort to relate the shorter duration of human ties to shorter durations in other kinds of relationships. Nor have they attempted to document the progressive decline in these durations. Until we analyze the temporal character of human bonds, we will completely misunderstand the move toward super-industrialism.

For one thing, the decline in the average duration of human relationships is a likely corollary of the increase in the number of such relationships. The average urban individual today probably comes into contact with more people in a week than the feudal villager did in a year, perhaps even a lifetime. The villager's ties with other people no doubt included some transient relationships, but most of the people he knew were the same throughout his life. The urban man may have a core group of people with whom his interactions are sustained over long periods of time, but he also interacts with hundreds, perhaps thousands of people whom he may see only once or twice and who then vanish into anonymity.

All of us approach human relationships, as we approach other kinds of relationships, with a set of built-in durational expectancies. We expect that certain kinds of relationships will endure longer than others. It is, in fact, possible to classify relationships with other people in terms of their expected duration. These vary, of course, from culture to culture and from person to person. Nevertheless, throughout wide sectors of the population of the advanced technological societies something like the following order is typical:

Long-duration relationships. We expect ties with our immediate family, and to a lesser extent with other kin, to extend throughout the lifetimes of the people involved. This expectation is by no means always fulfilled, as rising divorce rates and family break-ups indicate. Nevertheless, we still theoretically marry "until death do us part" and the social ideal is a lifetime relationship. Whether this is a proper or realistic expectation in a society of high transience is debatable. The fact remains, however, that family links are expected to be long term, if not lifelong, and considerable guilt attaches to the person who breaks off such a relationship.

Medium-duration relationships. Four classes of relationships fall within this category. Roughly in order of descending durational expectancies, these are relationships with friends, neighbors, job associates, and co-members of churches, clubs and other voluntary organizations.

Friendships are traditionally supposed to survive almost, if not quite, as long as family ties. The culture places high value on "old friends" and a certain amount of blame attaches to dropping a friendship. One type of friendship relationship, however, acquaintanceship, is recognized as less durable.

Neighbor relationships are no longer regarded as long-term commitments – the rate of geographical turnover is too high. They are expected to last as long as the individual remains in a single location, an interval that is growing shorter and shorter on average. Breaking off with a neighbor may involve other difficulties, but it carries no great burden of guilt.

On-the-job relationships frequently overlap friendships, and less often, neighbor relationships. Traditionally, particularly among white-collar, professional and technical people, job relationships were supposed to last a relatively long time. This expectation, however, is also changing rapidly, as we shall see.

Co-membership relationships – links with people in church or civic organizations, political parties and the like – sometimes flower into friendship, but until that happens such individual associations are regarded as more perishable than either friendships, ties with neighbors or fellow workers.

Short-duration relationships. Most, though not all, service relationships fall into this category. These involve sales clerks, delivery people, gas station attendants, milkmen, barbers, hairdressers, etc. The turnover among these is relatively rapid and little or no shame attaches to the person who terminates such a relationship. Exceptions to the service patterns are professionals such as physicians, lawyers and accountants, with whom relationships are expected to be somewhat more enduring.

This categorization is hardly airtight. Most of us can cite some "service" relationship that has lasted longer than some friendship, job or neighbor relationship. Moreover, most of us can cite a number of quite long-lasting relationships in our own lives – perhaps we have been going to the same doctor for years or have maintained extremely close ties with a college friend. Such cases are hardly unusual, but they are relatively few in number in our lives. They are like long-stemmed flowers towering above a field of grass in which each blade represents a short-term relationship, a transient contact. It is the very durability of these ties that makes them noticeable. Such exceptions do not invalidate the rule. They do not change the key fact that, across the board, the average interpersonal relationship in our life is shorter and shorter in duration.

Continuing urbanization is merely one of a number of pressures driving us toward greater "temporariness" in our human relationships. Urbanization, as suggested earlier, brings great masses of people into close proximity, thereby increasing the actual number of contacts made. This process is, however, strongly reinforced by the rising geographical mobility described in the last chapter. Geographical mobility not only speeds up the flow of places through our lives, but the flow of people as well.

The increase in travel brings with it a sharp increase in the number of transient, casual relationships with fellow passengers, with hotel clerks, taxi drivers, airline reservation people, with porters, maids, waiters, with colleagues and friends of friends, with customs officials, travel agents and countless others. The greater the mobility of the individual, the greater the number of brief, face-to-face encounters, human contacts, each one a relationship of sorts, fragmentary and, above all, compressed in time. (Such contacts appear natural and unimportant to us. We seldom stop to consider how few of the sixty-six billion human beings who preceded us on the planet ever experienced this high rate of transience in their human relationships.)

If travel increases the number of contacts – largely with service people of one sort or another – residential relocation also steps up the through-put of people in our lives. Moving leads to the termination of relationships in almost all categories. The young submarine engineer who is transferred from his job in the Navy Yard at Mare Island, California, to the installation at Newport News, Virginia, takes only his most immediate family with him. He leaves behind parents and in-laws, neighbors, service and tradespeople, as well as his associates on the job, and others. He cuts short his ties. In settling down in the new community, he, his wife and child must initiate a whole cluster of new (and once more temporary) relationships.

Here is how one young wife, a veteran of eleven moves in the past seventeen years, describes the process: "When you live in a neighborhood you watch a series of changes take place. One day a new mailman delivers the mail. A few weeks later the girl at the check-out counter at the supermarket disappears and a new one takes her place. Next thing you know, the mechanic at the gas station is replaced. Meanwhile, a neighbor moves out next door and a new family moves in. These changes are taking place all the time, but they are gradual. When you move, you break all these ties at once, and you have to start all over again. You have to find a new pediatrician, a new dentist, a new car mechanic who won't cheat you, and you quit all your organizations and start over again." It is the simultaneous rupture of a whole range of existing relationships that makes relocation psychologically taxing for many.