Butch Cassidy (Paul Newman, right) and the Sundance Kid (Robert Redford).

Butch Cassidy (Paul Newman, right) and the Sundance Kid (Robert Redford).

A single good friend can make as much as a 10-year difference in life expectancy.

I first heard about the male deficit model, the sociological theory that men are lousy at friendship, a few months after my friend Matt moved to Seattle.

The male deficit model holds that men tend to drift apart whenever the activity they share ends. Matt and I, for instance, spent hours and hours surfing together in San Francisco. But then I became a father and no longer had much time to spend in the water, and we began to see less of each other. Our friendship rekindled only after Matt and his wife bought a fixer-upper in my neighbourhood, and we had a blast demolishing the walls of his old kitchen. Then, when he had a kid, we’d push our strollers around the neighbourhood.

So your social life could be better. Big deal, right? Actually, it's a bigger deal than you might know.

So your social life could be better. Big deal, right? Actually, it's a bigger deal than you might know.

Typically, when I got home from hanging with Matt, my wife, Liz, would say something like, ‘‘Well, what do you have for me?’’ Dish, she meant - like maybe some of that marital strife.

My reply almost always was, ‘‘Well, um...I don’t know.’’

‘‘Two hours you were gone!’’ she’d gripe, incredulous. ‘‘Think!’’

‘‘OK, Matt does want a new seven-footer, mostly for tube riding. Does that count?’’

Then Liz would let out a big theatrical groan that said, in essence, what kind of friendship is that?

I thought it was a great friendship, if I thought about it at all. But then Matt’s wife, Jodi, accepted a job offer in Seattle. Matt and I told ourselves nothing would change. Jodi and Liz even arranged a surprise ‘‘bromantic’’ (their word) surf trip to Baja that winter. When Liz told me, I had to laugh: it was like Matt and I were little boys, depending on our mums to plan a play date.

But then, in perfect accordance with the male deficit model, reality set in. First, Matt cancelled our farewell beers-and-barbecue session; he was too busy packing. Then, a month after the move, Matt came back to town on a three-day work trip but was too busy to drop by. A month later, he came down for a friend’s 50th birthday party, and I saw him for all of 10 minutes - at the party.

Feeling stung and sensing our friendship was toast, I told Liz I was thinking about cancelling the Baja trip.

The male deficit model is based on 30 years of research into friendship and relationships - most of which shows that male friendships are less intimate, more competitive, and less satisfying than female ones. ‘‘The older we get, the more we accept our essential friendlessness,’’ writes psychologist Stuart Miller in Men and Friendship.

So your social life could be better. Big deal, right? Actually, it’s a bigger deal than you might know.

That’s because nearly all research into healthy ageing has found that the key to a long, happy life is not diet or exercise but strong social connections - that is, friendships. Loneliness accelerates age-related declines in cognition and motor function, while a single good friend has been shown to make as much as a 10-year difference in overall life expectancy. A huge meta-study performed in part at Brigham Young University, which reviewed 148 studies with a combined 308,849 subject participants, found that loneliness is just as harmful to health as not exercising, smoking 15 cigarettes a day, and alcoholism, and fully twice as bad as being obese. Still more startling is a 2010 study published in the Journal of Clinical Oncology that looked at 2230 cancer patients in China. Social wellbeing, including friendship, turned out to be the number one predictor of survival.

Some of this stems from the fact that isolated people tend to exercise less, eat poorly, and drink too much. But some researchers believe that loneliness has a negative health impact all on its own. In numerous studies over the past 30 years, John Cacioppo, a professor of psychology at the University of Chicago and the pioneer of the biological study of loneliness, has found that lonely people have chronically elevated levels of the stress and fear hormones cortisol and epinephrine. In a 2007 paper published in Genome Biology, he even demonstrated a correlation between loneliness and the activity of certain genes associated with systemic inflammation, elevating risk for viral invasion and cardiovascular disease.

And yet the capacity of men to combat loneliness - and improve their health - by building strong friendships seems to be steadily eroding. Cambridge professors Jacqueline Olds and Richard Schwartz, writing in The Lonely American: Drifting Apart in the Twenty-First Century, point to a current tendency among adults to build stronger, more intimate marriages at the expense of almost all other social connections. In a study of contemporary childcare arrangements,  they found a deep sense of loneliness among many parents, especially men. ‘‘Almost every father we spoke with explained that he had lost contact with most of his male friends,’’ they write. And lest you believe family is company enough, the 2005 Australian Longitudinal Study of Ageing showed that family relationships have almost no impact on longevity. Friendships, by contrast, boosted life span as much as 22 per cent.

And don’t think you’re immune because of the great relationship you have with your wife. A team of researchers from around the world analysed a vast mobile phone database - 2 billion calls and 500 million text messages - and found that by the time married women hit 45 or so, they demote their husbands from first to second place among their most important relationships, typically in favour of a daughter or a younger female friend. It turns out our wives are going to dump us without even leaving the marriage.

To get some perspective, I put the question to my 75-year-old father and three of his friends. I asked their opinion of the male deficit model and discovered that they thought I had a male deficit all my own, just for entertaining such paleo-feminist horseshit. Two of these guys, Merv and Denis, had no memory of a single friendless year, nor of losing a friend for any reason other than abject betrayal - like the time one guy bedded another guy’s live-in girlfriend.

Of course, it wasn’t all Butch and Sundance. A public-interest lawyer named Steve described waking up at age 45 and saying, ‘‘Hey, what happened to all my friends?’’ My father escaped that fate only because Denis showed up twice a week at his office, dragging him off to the gym to lift weights. ‘‘I had to go right into your dad’s office and just sit and wait,’’ Denis told me. ‘‘At 5-of-5 his phone would ring again and I’d yell, ’Don’t answer!’’’

Several of these men said they continue to meet weekly for lunch. Which brings up a rival theory of male friendship - the alternate paths model. According to this line of thinking, the male deficit model is an historical aberration brought on by two unrelated cultural developments. The first is the rise of contemporary homosexual identity, which had the effect of pushing straight male intimacy into a closet of its own. At the same time, a wave of feminist sociologists and psychologists began describing female friendship, with all its confessional talk, as the optimal model.

Many feminist thinkers now see those views as overly simplistic. And as recent news about gay marriage shows, America is growing more comfortable with homosexuality.

Still others argue that in an era of dual-income households, the very idea of friendship is changing. ‘‘Fifty years ago, you wouldn’t know the other dads from childcare or school drop-off,’’ says Jon Miller, director of the Longitudinal Survey of American Youth, a vast survey of generation X lifestyle habits. ‘‘I’m not sure this whole logic that men and women have to have separate friendships makes sense anymore.’’

I buy this. I do think I came of age in a uniquely dumb time for male friendship, when we were all so freaked-out about seeming gay that we’d leave an empty seat between us at the movies. I’m also open to Miller’s idea that social life is changing, and not necessarily for the worse.

But I think men really do suck at friendship. So I decided to make a conscious effort to fix things with Matt. Not that I did a great job. My way of telling him I was angry was to ignore a couple of his emails. Then I ignored a couple of his voice mails. Finally, when I did answer the phone, he said, ‘‘Dude, are you boycotting me?’’

‘‘No. Why?’’ (Go ahead, judge me. It worked.)

‘‘Come on, man. I can’t take it,’’ he said. ‘‘You’re, like, my best friend.’’

‘‘Look,’’ I said, ‘‘you are my best friend. So I’m hurt! I mean, two weekends and no coffee? No beer?’’

Matt apologised and said he felt terrible about the whole thing. And just like that, our friendship was back on track. Christmas came, and Jodi surprised Matt with the Baja trip. He was psyched, and I decided to go for it, too. The surf wasn’t all-time, but it was shoulder-high and plenty of fun. In the evenings, we’d retire to a little desert town for dinner and beers. We chattered about waves and kids. It felt great and energising.

But then Matt flew back to Seattle and I flew back to San Francisco, and we settled back into pretty much never calling each other. So I decided to get some new friends. I started with an old climbing buddy and two guys I knew through family get-togethers. I started inviting the other dads over a little earlier than the wives and kids, so we could shoot the breeze while cooking a meat-centric meal. And my old climbing partner and I decided we’d ride a century together.

One morning, I got home from a long training ride with him just as Liz returned from dropping the kids at school. She said, ‘‘How was biking, honey?’’

‘‘Awesome. Gorgeous morning.’’

‘‘What do you got for me? Any dish?’’

‘‘Ummm . . .’’

‘‘Oh, god,’’ she said. ‘‘Forget I asked.’’

Q: How Many Friends Do You Really Need?

A: Three

According to Oxford University anthropologist Robin Dunbar, who has studied human relationships in societies from the Stone Age to Facebook, most people are capable of maintaining stable relations with about 150 others. Within that 150 - a figure now known as Dunbar’s number - he has identified smaller concentric ‘‘circles of intimacy.’’ Healthy people, he says, maintain a 10- to 15-member ‘‘sympathy group’’ (the death of any of whom would cause distress) and three to five close friends who can be relied upon in times of trouble.

If your own friend count falls within that range, you’re doing fine. Unfortunately, that’s not the case for most of us. According to data compiled by the University of Chicago’s General Social Survey, between 1985 and 2004, the median size of networks of personal confidants - groups of genuine intimates - decreased from 2.94 people to 2.08. Worse still, the survey found that in 2004, 25 per cent of Americans reported having nobody to talk to at all.