From Aristotle to present-day soap operas, friendship has always fascinated. But how much do we actually know about contemporary friendship? How can we make sense of statistics telling us how many friends we are supposed to have - is it three, or ten, or do the thousand names on our database count? Does friendship still matter, or is "true" friendship a thing of the past? Is there any evidence to support gloomy prognoses that friendships today are fleeting, a pale shadow of what they used to be, another casualty of the individualisation that has made long-term commitment impossible?
In order to address these questions, we need to discover what people mean when they call someone a friend. Knowing that the word "friend" may be reserved for long-term relationships, or used to apply to anyone we know by their first name, helps us decode seemingly contradictory "findings". But it is not just a question of terminology: friendship takes many forms, as Aristotle claimed 2,500 years ago and recent research confirms. Some friendships are essentially light-hearted and casual - they help us relax and cope with the stresses of modern life. Others are more serious and intimate - they are the "rocks" on which we rely and in which we confide, which act as a vital safety net in addition to (or, sometimes, in place of) family.
Vague claims that "friendship" is in terminal decline make no sense. The sheer diversity and vibrancy of the relationships enjoyed by friends today belie such claims. Friendships may be relationships that we choose, but this does not mean they lack commitment: "chosen" and "committed" are not mutually exclusive categories.
Yet it is not necessarily the case that, in this age, friends are more significant than family. We know that, for some people, family loyal-ties are still paramount. And historical records and diaries going back five centuries show that friendships could be among the closest relationships people had. Rather than seeking to determine the direction of social change, we should focus on the continuing importance of friends and friend-like ties.
People do not always live near their families; families are not always in a position to offer help and support; family members do not always get on well with each other. In these circumstances, friends can act as a flexible source of social glue, a non-territorial form of "community" and a vital source of social support.
Friendship matters: there is now a growing body of evidence linking the support of friends as well as family to individual health and well-being. It is not a case of either friends or family - indeed, family members and partners may also be our friends - but a question of making time for friendship in its many forms.
"Rethinking Friendship: hidden solidarities today" by Liz Spencer and Ray Pahl is published by Princeton University Press (£22.95)