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If you're lucky enough to have a best friend, you'll know they're worth their weight in Tim Tams.

She's been there through break-ups and breakdowns, celebrated your successes, and held your hair back when you puked. She knows secrets even your husband doesn't (she'll take them to her grave). And now there's scientific research to back the fact that hanging with her makes you feel good - female friendships lower women's stress levels, and are one of the reasons we live longer than men.

But new research shows that, compared to our mums, there's been a huge drop in the number of friends 30-something women can rely on during a crisis. Twenty years ago, women reported having two very close friends. In 2005, that number fell to one.

"That's a 50 per cent drop, which makes for a public health crisis," says the man who conducted the research, Dr Paul Dobransky, psychiatrist and author of The Power of Female Friendship. "It's not completely clear why, but it's likely related to the [fact that] people are spending more time on their careers."

But in these times of uncertainty, we need mates more than ever.

Tend and befriend

Making buddies as a kid is as effortless as breathing. In your teens, your friends are the centre of your universe, and in your 20s the world is your playground, where your mates come along for the ride. But in your 30s, there never seems to be time to catch up with friends for a drink, let alone enough to spend hours choosing lipsticks together. Yet studies show that when you're stressed, it's the perfect time to turn up for mojitos.

A US study conducted in 2000 found female company lowers levels of stress chemicals in women's bodies. Study authors Dr Shelley Taylor and Dr Laura Cousino Klein discovered the fairer sex copes with stress differently to the male "fight or flight" response - something they've dubbed the "tend and befriend" mechanism.

Instead of heading for the hills, women are wired to gather with other women in times of trouble. In other words, while men might let off steam with some solo time on the Xbox, you're more likely to organise emergency tapas with your señoritas.

And it's not just the effect of gossiping it all out; the nurturing and calming hormone oxytocin is released when a woman gathers with friends. Dr Terri Apter, co-author of Best Friends: The Pleasures and Perils of Girls' and Women's Friendships, says this female skill could be one reason we live longer than men: "The greater number of friendships women have compared to men, and the greater work women tend to do to maintain friendships, may contribute to women's longer life span."

Women who clock up more hours with girlfriends are also less likely to have health issues later in life. A 10-year study from Adelaide's Flinders University found that having friends around in old age can up life expectancy more than having family around. And, in 2006, a US study of almost 3000 women with breast cancer found ladies without close friends were four times as likely to die from the disease as those with 10 or more female mates.

"When women talk to each other, the daily stresses of their lives are soothed," says Dr Apter. "Sharing problems relieves pressure. Friends reassure one another that their views make sense and that they are not alone. Recent studies show that friend-talk changes the chemical balance in the brain and reduces the effects of stress."

Divine sisterhood

Social researcher and writer Hugh Mackay believes the friendship crisis is due to Australia's falling birth rate. In the past, he points out, "women relied on friends and neighbourhood relationships to help them bring up their children and keep their sanity. But, as women become more career-oriented - often having no children or just one - bonding over children happens less often. Women go back to work, children go into childcare and women's friends are work colleagues."

All this means we should be putting our girlfriends higher on our list of priorities. "We often take female friendships for granted, placing them after family and career," says Dr Irene Levine, author of Best Friends Forever. But friendships require nurturing to survive and thrive. "If you don't make time for your friendships, they'll disappear."

Posse power

Some friends come and some friends go, and some stick around forever, but all have an impact. Here, how your different groups of friends fit in your life, why they're all important - and how to make the most of each bunch...

Your childhood friends

"Having an intense, shared history with someone, and having gone to the same school, makes for a very rich friendship," says Dr Levine. "Childhood friends form the scrapbook of our lives."

Mackay adds that the current generation of schoolgirls looks set to cling to their first friendships more tightly than ever: "They're the offspring of the most divorced generation of parents; they've learnt that they really need each other."

Your 9 to 5.30 friends

Studies show that women find navigating workplace friendships more difficult then men, perhaps due to trust issues. Recent research by the US Workplace Bullying Institute found 40 per cent of workplace bullies are women, and that they target their own gender 70 per cent of the time. How to figure out who you can confide in and who's strictly business?

"Take your time getting to know someone at work before revealing anything too personal," advises Dawn Bertuca, co-founder of Girlfriend Celebrations.

"Assume that business will come first, unless you've become very good friends outside the office. We have different kinds of friends throughout our lives - they all don't have to be our closest confidants."

Your best friends

Close mates allow us to be ourselves and feel accepted without fear of ridicule, says Dr Levine: "Best friends help us transcend being alone and make us feel part of something larger than ourselves. They also help us feel more confident and secure."

A recent study of high school girls by the University of Texas, US, found girls scored well in maths and science more frequently if their best friend did well in those subjects, too. While schoolboys competed against each other, resulting in only one winner, girls worked together and pushed each other to succeed. And don't feel guilty if you and a bestie doesn't last the distance - it's natural for friends to come and go over the course of our lives.

"Most times, very close friendships, even with best friends, don't last forever," says Dr Levin.

Your online friends

Not just for socially inept compudorks anymore, social networking sites like Facebook and Twitter have boomed. But has talking via a screen changed the essence of friendship? Not really, says Dr Will Reader from Sheffield Hallam University, UK, who reckons connecting with friends using technology doesn't affect how often we see them.

"We researched whether using social networking sites leads to people seeing each other face-to-face less, and we found no relationship," he says. "Close friendship requires some old fashioned face-to-face time; that will never change."

Click here to read three real-life girlfriend experiences.