Growing up in Finland, New Zealand-based designer Minna Pesonen and her friends spent a lot of time sheltering from the freezing winters in cocoon-like Finnish saunas.

Something about the heat, the quiet, and the vulnerability of sitting, naked, with people she knew well, encouraged long, honest conversations.

Now 37, she has always treasured close friendship. When she was 23, she had to call on her closest friends like never before after her world fell apart, when her husband of two years passed away from melanoma.

"I wasn't even walking on this planet," she says. Her closest friends dropped everything to take care of her. They listened while she talked and cried, called in regularly to make sure she was eating, took her on all-expenses-paid outings to change her environment. They carried her until she could do life again.

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Before he died, her husband made her promise to carry on and make herself a happy life. And she has.

Since settling in New Zealand she has made a few very close friends, one of whom has returned to Germany to live. Pesonen says it was tough to lose the face-to-face element. "It's the little things, the smile, the bright eyes, talking about something and laughing together, really sharing the moment," she says. The friends use Skype once a week to preserve their friendship.

Pesonen says for her closest friendships, which straddle three countries on opposite sides of the globe, distance is no barrier. "When we catch up every couple of years it's like there hasn't been any time in between. That connection stays, regardless of what happens in life or how much you change as a person."

She acknowledges that really opening up to others brings the possibility of being misunderstood or judged, and she can understand why some people wouldn't want to risk it. But that's not how she wants to live. "I think if you show your heart, it makes you strong," she says.

Close friendships like these pack a punch in terms of emotional and physical benefits. They radiate a sense of wellbeing through our lives, profoundly influencing our mental, emotional and physical health. They shield our immune systems, digestive systems, hearts, insulin levels and sleep patterns from the corrosive effects of everyday stress. They protect our brain function from dementia as we age. Our close friendships also trigger their own stress-reducing hormones, keeping us calmer and happier.

But the picture is bigger than this. Friendship is rocket fuel for human kind.

The theory goes that as a species, one of our most effective survival strategies has been to work in groups. Everything is more effective in groups: catching food, warding off predators, building shelter, caring for children.

Through the millennia, those who preferred to go it alone were much less likely to survive, whereas the more sociable among us, motivated to co-operate with others, had a higher chance of thriving and passing on their group-oriented genes.

The resulting evolution of our DNA has left us hard-wired to be intensely social. We have a visceral, negative emotional response to rejection and prolonged isolation that funnels us back into the group. We have a complicated network of mechanisms including embarrassment, feelings of loneliness, peer pressure and a sense of social responsibility that ensure we're fitting, we're belonging, we're connecting with others.

As a survival strategy it's been a huge success - our population has increased from 5 million about 10,000 years ago, to almost 7.5 billion today - and counting.

Unequivocally, friendship is front and centre for us. But the tiny clutch of true, intimate friendships we tend to make over our lifetimes means these closest bonds are almost absurdly rare in comparison with the approximately 400 new friendships many of us will make over the course of our lives. In terms of our most intimate friendships it seems we're more panda than puppy.

Relationship scientist Dr. Nickola Overall of the University of Auckland confirms what we intuitively know, that this closest kind of friendship is usually built on many layers of similarity, and is earned over time.

We start deepening a friendship by sharing small truths about ourselves, and gauging how they are received. If things go well and are reciprocated, we open up more. It happens intuitively, rather than consciously, and ideally develops eventually into an inner-circle friendship.

While inner-circle friendships are crucial, they are also a huge investment of our time and energy, and are emotionally intense. This is why we can maintain only a few. And a few, Overall says, is all we need to fulfill our deepest needs for belonging.

Since moving to New Zealand, Minna Pesonen (right) has made some very close friends, including Anita Totha. Photo / Brett Phibbs
Since moving to New Zealand, Minna Pesonen (right) has made some very close friends, including Anita Totha. Photo / Brett Phibbs

Relationship guru Dr Brene Brown, author of

Daring Greatly

, says that empathy is the key to a high-quality friendship. Empathy is the glue in that drawing closer process. Whether the other person receives our most private confidences with empathy - the "I get it, I've been there" response - dictates the quality of the friendship. The more empathetic the response, the higher the quality of the friendship.

It's a double-edged sword. Showing our real selves makes us vulnerable to the possibility of rejection. And the more we open up, the deeper the friendship becomes, meaning we are more dependent on it and have more to lose if things go wrong.

But it's only when we take that risk that we usher in the possibility we'll be accepted for who we really are.

"If you have a vulnerable moment and someone will listen with empathy, if you have that friend you are very lucky," Brown says. "If you have two or three of those, you've won the lottery."

I'm familiar with the popularised "Dunbar's Number" theory of friendship proposed by Dr Robin Dunbar of Oxford University, which maps our friendships in concentric circles of closeness - one to five besties in the centre, surrounded by 10 close friends, then 35 acquaintances, and lastly 100 people we know as part of our community.

But I can't help feeling, wading through the science, that it reduces friendship to an impersonal, species-protecting equation forced on us by our genes.

There's a magic about those closest, earned-over-time friendships that surely transcends the biology. While Dunbar's Number confirms the age-old saying that you can count your real friends on one hand, I want to test the theory.

I start asking people around me how many intimate, core friends they have. I ask everyone from the most self-sufficient introverts to gregarious extroverts. It's hardly scientific but, as the weeks go by, not one of the people I ask has more than five very close, intimate friends. Most have less. When I suggest interviewing them for the story, people are reticent to talk about it. Some turn me down.

The reasons are always the same: they feel embarrassed that they somehow don't have enough friends or not the right kind, or that it's too personal. It's a subject that seems to make us skittish.

It's possible this is because there's not a lot of information out there shedding light on the reality of friendship. There are a lot of friendship articles which offer helpful "friendship tips". But there's less about how friendship is messy, how it takes work and commitment, how some of the closest friendships don't work out, and how much that hurts.

Rather than basking in the glow of their inner-circle friendships, many people I speak to about the subject jump straight to a time things have gone irrevocably wrong.

One man told me that all through school he had no friends at all. Another woman said she felt hostaged in her own home every night by a close friend's relentless drunken phone calls. A third told me she had been dumped, by text, from a friendship lasting 45 years. Another just said grimly "I could tell you a story about friendship..." But she didn't.

The dark twin of healthy friendships, toxic friendships come at a high cost to our health and wellbeing. They elevate stress, which leads to a host of health problems including compromised immune system, heart problems, digestive issues, headaches and disturbed sleep.

A study released in 2016 by UCLA showed that stressful friendships set up a processes of inflammation in the body which, over time, can lead to diabetes, heart disease, and cancer.

Toxic friendships are no easier on our psyches. While high-quality friendships contribute to a strong base level of self-worth, meaning we can get out there and interact with others without needing their validation, toxic friendships are lonely, and tend to undermine our sense of self-worth.

The majority of our friendships, however, are light, "proximity friendships", based around a shared activity. These are our workmates, our soccer buddies, our book club crew. We tend to spend a lot of time with our proximity friends, the frequency driven by our shared activity.

There's a certain revolving-door quality to these friendships, because when the shared activity that binds the connection ends, the friendships usually die away and are replaced with new ones.

That means while the total number of friends we have tends to stay fairly stable, over a seven-year period half of our friendships can be replaced via this drifting away process.

Thirty-nine-year-old Stephen Richardson, a designer based in Auckland, leaned heavily on proximity friendships in his early 20s when he was modelling in New York. He met most of his "friends" at the apartments provided for models to use while on assignment.

"You'd turn up at an apartment and you just didn't know what it was going to be like," he laughs. "It was like Zoolander, but worse."

There was one apartment - a two bedroom - that had 14 guys crammed into it. There were people sleeping on the floor, there was garbage everywhere. "You met every kind of cliched personality," he says, "from the jock to the nerd. It was a lion's den of personalities."

But sifting through, he always found people he connected with. They would bond and within a week he had new proximity friends. He says without them his experience of New York would have been entirely different.

At the time he didn't think of his proximity friends as temporary. He remembers talking with one, an Australian who was moving back home. He said Richardson should come with him back to Australia. "It was almost like we were building this imaginary friendship life together," he says. "It was young, naive, buddies forever."

Reality intervened and over time the emails between them dwindled.

"You have space in your life for a certain amount of friends," he says. "With everyone else, it can only go so far."

He has three close friends, one he has known since he was 16. "He knows me inside-out," says Richardson. "Our friendship's durable to the ups and downs, and to being an idiot. It's just there."

While researching this story, I randomly picked up a novel by Haruki Murakami in which, ironically, the lead character is dumped by his four best friends without explanation. Grief-stricken, he disappears inside an emotional concrete bunker, ceasing all but the necessary interactions with other people. More than a decade later a girl finally coaxes him out of the bunker, encouraging him to repair the damage his friends' rejection caused.

I can't stop thinking about his years without friendship, and I'm happy when he resolves it. That might largely be my DNA talking, but it's something else too - relief he is being human, trading all the risks for the possibility of gaining real friendship.

CONNECTING UP

Ideally, we would all be operating within cosy nests of high-quality friendships. But more and more, that's not the case. Research has found that 20 per cent of Americans are lonely - meaning they are less connected to others than they want to be - and relationship scientist Dr Nickola Overall of the University of Auckland estimates numbers are similar in New Zealand.

Severe loneliness is becoming increasingly problematic as we live in larger societies, partly due to the anonymity of city living. Also, says Overall, our contemporary cultural focus on career and financial security as the key determinants of a happy life can make intangibles like quality friendships seem less important.

But prolonged loneliness is no joke for our health. "The very lonely die earlier, they're less healthy, they're more depressed, more likely to commit suicide," says Overall. "All of the bad stuff."

And online friends, while useful for other reasons, aren't able to reproduce the benefits of flesh-and-blood friendship. This is because online friendships can't offer attributes like shared experience and touch, which, says Dr Robin Dunbar of Oxford University, are some of the main building blocks of genuine connection.

It can be daunting to take the first step in making friends, but friendship websites Meetup, Huggle, Girlfriend Social and others have come to the rescue, revolutionising how we make connections with other people. New-kid-on-the-block Huggle, launched in 2016, is a swipe-app that matches prospective friends based on the shared places they visit.

Meetup, the pioneer of friendship websites, was founded in 2002 by New Yorker Scott Heiferman when he realised, after the 911 attacks, that he knew almost no one in his neighbourhood. Since its inception, Meetup has ballooned to 28 million members in around 180 countries.

Based around the idea that people bond over shared activities, Meetup is a member-run collection of themed groups that meet regularly.

New Zealand has an active Meetup community. Within 25km of Auckland are hundreds of groups with names like "Basketball Lovers", "Waterfalls Explorers", "Social Girls Auckland", "Not Tinder", "ArtExplore", "Auckland Adrenalin Junkies", "Sneaker Sisters - Learn to Run", "Drink 'n' Draw", and "Auckland Ladyboss Chats". Similar groups exist around the country.

Jonty Crane heads the Auckland 20s and 30s Social Group on Meetup, which has 1500 active members and held 400 get-togethers last year. He says as well as being a good way to connect with people sharing a common interest, it can also help if you need to assimilate quickly after a lifestyle change like a break-up or if you're new to town.

He tells me of a British nurse who was coming to live in New Zealand and knew no one here. She joined his Meetup group before she arrived, landed at 2pm on a Saturday afternoon, and was out at drinks with them that evening.

"It's friends on demand," he says.