People tend to have roughly the same idea of what a friend should be, according to where they live and how old they are. That is why so many thinkers, both eastern and western, have used friendship as a sort of litmus test to measure the mood of their times. Currently, social scientists say the idea of friendship is undergoing fundamental change - at least in the cultural hubs and latitudes subject to their scrutiny.
Friendship is getting both thinner and stickier, they say, meaning that we have more numerous, but shallower, friendships. Yet we depend on them more as a substitute for the social glue once generated by institutions like marriage, kinship and lifetime-employment corporations. Stickiness was exemplified by the surrogate family of chums in the US television series Friends. Thinness is evident in the unstable landscapes of interaction through which we wander during the day, scattering our attention as we go among face-to-face contacts, e-mails and mobile-phone calls.
This contrasts with the friendships among working-class people in our grandparents' day - a model of friendship that was long cherished and held up as an ideal. In those days, friendship bonds (and rifts) ran deep. But that was because people shared lives of relative deprivation, not because they were by nature superior in any way. In fact, those friendships tended to be romanticised by the researchers who wrote about them. The subjects of the studies couldn't wait to give themselves up to the 'evils' of modern life. They were quite right: the advantages of being lifted out of harsh living conditions far outweigh the mental drawbacks of modern living.
The orientation of Chinese society, too, is shifting from family to friends, according to Philip Pan, writing in The Washington Post. He sees the internet as partly responsible. More than anywhere else, a multinational survey showed, digital interaction in China is increasing the importance of relations between like-minded friends outside family networks.
In regular offline friendships, Chinese psychologists write, there is a stronger barrier between close and casual relations than in the west. Chinese are also less likely to disclose information about themselves - a fundamental feature of bonding - to anyone whose attitudes are not closely aligned to their own.
The average Briton, according to University of Essex sociologist Ray Pahl, has 18 friends. But it is the quality of such friendships that counts, judging by recent happiness studies. They show a decline in subjective happiness, and a very robust correlation between how socially supported people feel and how happy they say they are.