How to choose your inner circle: With friends like these...

When Aristotle was asked, in the 4th century BC, what defines a friend, he had no doubts. A friend is "one soul inhabiting two bodies", he said, adding: "Without friends no one would choose to live, though he had all other goods."

In 2006, it is a little more complicated. A startling in the American Sociological Review, found 25 per cent of Americans do not have a single friend. That is, nobody "with whom to discuss matters important to them", said the researchers. The average number of friends was two.

For others, the picture is different. Last week, Myspace.com, the labyrinthine international networking party, was pronounced the most popular website in the US. The site enables users to connect to friends, and their friends, and so on, meaning users such as Lyndon Blue from London, can blithely announce: "I have more than 4,080 MySpace friends now. I know about 1,000 of them in the real world." With marriage no longer a life-long institution and social mobility taking us far from family, you would think we have never needed our friends more.

Yet modern demands on our time can take a deep toll on friendships. The pressures have reached even the clergy. Last month, Radio 4's Thought for the Day, with the Rev Rosemary Lain-Priestley, considered the relative virtues of conducting a "friendship audit".

Two books to be published next month aim, in different ways, to help us through the maze. Vital Friends: The People You Can't Afford to Live Without, by Tom Rath, advises readers to evaluate the roles played by their friends, ensuring eight essential friendship types are represented. Rethinking Friendship: Hidden Solidarities Today by Liz Spencer and Ray Pahl looks at the different kinds of friendships in the internet age.

Rath is a New York Times bestselling author, and leads research and consulting at Gallup. He analysed more than five millions interviews to try to define what Plato said he never could: what is friendship? The result is Vital Friends, and it reaches startling conclusions. If you ask people why they became homeless, why their marriage failed or why they overeat, he says, they do not blame it on poverty or mobility; they blame it on a lack of friendships.

If your best friend eats healthily, he discovered, you are five times more likely to have a healthy diet yourself. His interviewees rated friendship as being more than five times as important as physical intimacy in the success of a marriage. He also found patients with heart disease are twice as likely to die if they do not have three or four close social connections.

Ray Pahl's Rethinking Friendships is based on in-depth interviews conducted over seven years. He finds the quality of our friendships has a huge influence on the amount of satisfaction we draw from life. "Individuals with no real friends at work have only a one in 12 chance of feeling engaged in their job," he says.

But the quantity, as well as the quality of friends, does matter. Some people can have three or four close friends and be very happy, he believes. Others more extrovert prefer to have 10 or 15. But he is certain you need to have more than one friend to be satisfied in life. "It is a very common mistake to expect one close friend to provide everything you need," he says. "And it can cause a lot of problems.

Dr Angela Carter, an occupational psychiatrist from Sheffield University, says: "We are social animals. We need friends to hold a mirror up to us and show us what our behaviour looks like. They provide companionship and support, but the most important thing friends do is help us to work out who we are. Families cannot do that in the same way."

She has found people can struggle to find the friends that they need. "We need to be quite strategic in our friendships. People think friends turn up from nowhere, and they grumble when they don't have any. You need to think, 'What do I need from friends and am I being a good friend in return?' "

In researching Rethinking Friendship, Pahl found little cause for the notion that society is becoming atomised and selfish. "Friendship takes such a variety of different forms," he says. "I don't think people have fewer friends now. What is new is that in the past 50 years we have become more and more used to choosing friends, rather than accepting them as given."

Both authors did not find transient communities are all about networking and getting on. "As people have learned to be consumers, maybe they have also learned to make choices among their friends," says Pahl. "But when people think about their close friends, those relationships are deep and trusting, not exploitative or self-seeking. People are probably loyal and decent, against what is cynically regarded as the current of the age."

So how many friends can one person reasonably support? "I don't think it is useful or meaningful to put a number on it," says Dr Carter. "Psychologists use the term 'affiliation need', and society tells us we have a lot of it. In truth, some people need a lot of friends, others few.

"It is fair to say people with a more diverse and mixed group of friends have a more robust defence against the way the world works," says Mr Pahl. "I don't want to be prescriptive, or the government will start saying that we need a Ministry of Mates. But if you can provide the circumstances for a more friendly society, then society will be happier and healthier."

All together now: "Get by with a little help from my friends".

EIGHT VITAL ALLIES

BUILDER motivate you, help you see your strengths and advise you on how best to use them and are generous with their time.

CHAMPION stand up for you and your beliefs and they praise you to everyone else they know.

COLLABORATOR are friends with similar interests, those with whom you are most likely to spend your time.

CONNECTOR get to know you and then instantly work to connect you with others who will share your interests or goals.

ENERGISER is fun, cheers you up when you're down and always available to boost your spirits.

MIND-OPENER stretch your viewpoint, introduce you to new ideas, opportunities, culture.

NAVIGATOR is the one you seek out when you need guidance and counsel; they're great at talking through your options.

COMPANION is the first person you call, with good news or bad. They are always there for you.

From 'Vital Friends' by Tom Rath, 1 August, Gallup Press

THE MATE WHO DONATED HER KIDNEY

Sandie Tiley, 54 and Jennifer Leaver, 49, met in Southend 25 years ago. Last year, Jennifer donated a kidney to Sandie

Sandie

Jen and I met about 25 years ago, and at first we didn't like each other. I had started going out with her boyfriend because I didn't know they were together, so for about a year we were daggers drawn. Then one day I saw her at the shops and said hello, and she turned back and said she felt awful that I had said hello and she hadn't responded. From that day, we were best friends.

I ended up marrying the boyfriend, but now we are divorced, and Jen and I are still friends. I don't know what drew us together. I can't even say we are similar, because we're not. She's a really lovely person. There's nothing nasty or horrible about her, I can't imagine her arguing with anyone. She's really quiet and I'm really mouthy.

She knew I wasn't well; I haven't been well since I was 20. I had a disease called polycystic kidneys. I was very ill, struggling to get upstairs and was going to have to start dialysis. My daughter had been to see if she could give me a kidney but she had the disease too.

One day, Jen phoned and said, "What blood group do you need to find a donor?" I told her, and she said she had just donated blood to find out her group and it was a match. I said, "No way". It's a hell of a lot of pressure, and she had three small children, and I didn't know what might happen in the future. But she kept saying, "Well, let's just find out, let's just try". Eventually, we found she was compatible.

Before the operation I was nervous. I thought, "How can I ever go out with her again and not offer to pay for dinner?" But I knew she would never ask for anything in return. She was giving me back my life.

The night before the operation last April we were in hospital and all ready for the next day. The doctors said, "Right, you two can go out now". So we went out and had a few drinks, and we laughed so much. We made the best of it really, and that's what we always do. That's why we're still friends.

Now my life is fantastic. I'm out at six in the morning, running and training. I always say I've got the best kidney in the world, and sometimes I'll be doing something and I'll stop and think, "Oh Jennifer, I can't believe you've done this for me".

She never makes me feel bad if I can't come out or if I don't ring for a while. I don't know anyone with a friend like that. I suppose most people are lucky; their friendships don't get tested as ours did.

Jennifer

Obviously, because of the way we met, we didn't particularly like each other at the beginning. But the day we got talking when we met at the supermarket we got on really well. After that, we became good friends.

She's great fun, and she has such a love of life, such a zest for life. A good friend has to be somebody you can trust and rely on, somebody who would be there for you if you needed them. But no close friends are anything like Sandie, so I obviously don't look for certain attributes in friends.

She is a lot more extrovert than I am and always good fun to be with. With some people, if you went on a night out and nothing was happening, you would be bored. But with Sandie we always find something funny and have a good time. She moved to Kent seven or eight years ago, but I don't think you need to see or speak to somebody every day to be a good friend. When you do you pick up where you left off, as if you saw them five minutes ago.

I always knew she had this kidney problem, but she never made a big thing about it and I didn't realise how serious it was. Occasionally, she would be ill for a few days. She never made a fuss. It was only over the past five or six years I realised she was getting progressively more unwell.

The doctors were thinking about dialysis and I knew her father had died of the same disease, and it became apparent it was pretty serious. From conversations about transplants I knew it was all to do with blood groups. I didn't know what mine was, so I had it tested. Then I told her I could be compatible. She was surprised, and very anti the idea. She thought she couldn't possibly put me through something like that, and she was worried about my young children. I said, "Well let's at least investigate and find out what our options are".

Sandie is a very generous, very giving person and she has always been good to me and my children. Normally, if somebody is not well there is nothing you can do about it, and I thought, "Well, if I can help, here's something I can do about it." I didn't know exactly what it would involve. I suppose I was quite naïve. I've had never had an operation, and there's always a minute of thinking, "Oh goodness, this is really it".

The night before the operation we did go out to a nice Malaysian restaurant. We felt like naughty schoolgirls, and we do joke about the things we're willing to do just to spend time together.

Now, we keep in touch even more than before, partly because Sandie feels much fitter. We're always talking on the phone, and it's nice to spend a day in hospital when we go back for a check-up.

If the transplant hadn't made a difference to her life would have been disappointed, but I wouldn't have regretted it. The best thing about it is how well she is now, and that she is able to enjoy life. It is wonderful for me to see that.

Sandie will compete in the British Transplant Games from 17 to 20 August in Bath

THE CHILDHOOD PLAYMATES REUNITED

Anne Scorfield and Andrea Riley, both 31, were childhood friends then didn't see each other for 17 years

Andrea

Anne and I grew up on the same street. We did everything together: infant school, junior school, Brownies. She was always the bubbly, confident one who would boss people around; she was never fazed by anything. We lost touch in 1986 when we went to different secondaries.

A couple of years ago I was cycling in Newcastle and I thought I saw her drive past. When I got home I emailed her through Friends Reunited and we arranged to see each other.

The sense of normality and intimacy was immediate , she never shuts up so she hasn't changed at all. She's the first person I think of if I need someone to talk to. When I had my first son I found her a great source of information and comfort. Shared history is important.

Anne

I remember going into her back garden and playing on her swing and her slide. In the house we used to play as shopkeepers and raid my mum's cupboards for all the tinned foods and sell them with our little toy till. It was always me that got my own way, and I think she used to go along with it.

When I got the email I invited her round to the house and it was amazing, just like we had seen each other yesterday.

Her friendship is very important to me now and it's lovely to have somebody in your life that knows you inside out. She's also there for me in a crisis.

THE FRIENDSHIP THAT'S LASTED 50 YEARS

Lydia Mazzotti, 77 and Phyllis Niemczyk, 80, have been friends for more than 50 years

Phyllis

We first met in the early Fifties when we were teaching at the same school in Rome. There was a big group of us and we would have lunch together every day and talk for a couple of hours so we became firm friends. From then on we called ourselves "the weird sisters".

Lydia often gives the impression of being very jittery but she's as cool as a cucumber. She is also highly intelligent and it's always interesting to hear her views. We share a love of plays, films, television and humour.

There aren't so many people in this world that really love you and accept you for who you are, but Lydia is one.

Lydia

My first memory of Phyllis is of this tall, very English-looking girl at the Berlitz School for Languages in Rome. We were both English women in Italy, so I suppose that drew us together.

Phyllis is one of the wisest people I know. She will always listen. She's also a wonderful cook and she feeds me so well that the serious talk I was considering just doesn't come because I am in a state of stupefied bliss.

She's a very good housewife whereas I am pretty hopeless, and she's also hardworking whereas I am very lazy.

Although we certainly don't look alike, there is equality. She listens to me but I also listen to her; there is a give and take. She gives me a protective feeling and when she hugs me, it's a wonderful enveloping hug.