HERE IS ANOTHER REASON to call your old buddy to catch up: new findings show that it is not just fun or socially enriching to maintain solid relationships with close friends—it also helps you stay healthier and may extend your life span.

Psychologists have long known that having a set of cherished companions is crucial to mental well-being. In addition, a recent study by Australian investigators Lynne Giles, Gary Andrews and Mary Luszcz of Flinders University and Gary Glonek of the University of Adelaide concluded that our chums even help to prolong our lives. The scientists analyzed data from a decade-long survey called the Australian Longitudinal Study of Aging, which was initiated in 1992. It concentrated on the social environment, general health, lifestyle and age of death of 1,477 persons older than 70 years. Study participants were asked how much personal and telephone contact they had with friends, children, relatives and acquaintances.

Researchers were surprised to learn that friendships increased life expectancy to a far greater extent than, say, frequent contact with children and other relatives. This benefit held true even after these friends had moved away to another city and was independent of factors such as socioeconomic status, health and way of life.

What exactly underlies this effect on longevity? Apparently, the scientists posit, it is not merely the mutual buoying of spirits that occurs among associates. What is more important is that the support given and received by friends is voluntary and pleasurable and not just the result of a sense of duty or convention. In contrast to our own families, we are able to choose our friends.

According to the Australian scientists, the ability to have relationships with people to whom one is important has a positive effect on physical and mental health. Stress and the tendency toward depression are reduced, and behaviors that are damaging to health, such as smoking and drinking, occur less frequently. The investigators speculate that in times of calamity in particular, our support networks can raise our moods and feelings of self-worth and offer helpful strategies for dealing with difficult personal challenges.

In fact, the physiological pluses of such social interaction have already been demonstrated and include alleviating cardiovascular disease, high blood pressure and gastrointestinal problems. For example, physiologist and pharmacologist Eric B. Loucks of the Harvard School of Public Health discovered that considerably smaller amounts of a molecule called interleukin-6 circulate in the blood of older men with an extended set of connections than in that of loners. The elevated presence of this inflammation mediator is a risk factor for cardiovascular disease; it appears to favor the development of arteriosclerosis, a condition in which fatty deposits build up on blood vessel walls.

In addition to the benefits of friendship for individuals, our species as a whole has gained from the experience. It seems that social interaction contributed greatly to the evolution of our brain, making it the high-performance organ that it is today. Anthropologist and evolutionary psychologist Robin Dunbar of the University of Liverpool in England came to this conclusion a few years ago. It occurred to him that brain size and group size seemed to be correlated in apes, our closest relatives in the animal world. The more members there are on average in the extended families of a particular primate type, the larger the cerebrum is likely to be in those animals. Humans, with the bulkiest brains, have the biggest social networks.

From this notion, Dunbar derived his hypothesis of the “social brain.” According to him, the development of social structures accelerated the evolution of the brain. The reason, presumably, is that the greater the size of the group, the more information the brain must process about each of its members so that the social unit will be able to function. By the same token, the processing capacity of the brain also limits the size of our immediate social circles—to about 150 persons [see box above].

Reasons enough to ring your pal—and perhaps even to renew a few dormant acquaintances from childhood or college.