"When we teach people about making friends," says Clarkson, "we always say, 'Don't expect to have too many close ones or to expect too much from them'." Grzyb agrees. "It takes a lot of time to develop the kind of non-sexual intimacy that real friendship requires. From a psychological point of view, that's why we can't have many."According to Grzyb what we look for in friendships varies throughout our childhood and adult life. "Often in your mid-twenties you're attracted to someone who's completely opposite to you It's as it that person fulfils the other side of you.

It's a crucial stage; you're searching for your own identity and often friends will define, rather than support, who you are."Susie, a 30-year-old designer, met Emily eight years ago through a friend at work. Emily was an extrovert, party girl-type while Susie was much quieter. On the face of it, they didn't have much in common but within months they were sharing a flat and doing everything together. Then four years ago, Susie met her current partner and moved out. "I can see now that we both envied each other terribly," says Susie.

"But I stopped wanting to be like Emily when I met my boyfriend - I became more confident being myself."That's when the friendship deteriorated; Emily became resentful of the new relationship and accused Susie of using her until something better came along "This can be quite a sad time for girls," says Clarkson. "I've seen people feeling quite bereft when a friend changes and their priorities shift, especially when they have babies. The single friend can feel a real loss; like a bereavement." When Susie stopped being the focus of Emily's feelings and thoughts, she felt completely rejected. "It can leave a gap in your life and it often feels like abandonment," says Grzyb.

"People could understand it more if their friend went to live in another country."Male friendships never seem to match up to this level of intensity, perhaps because they're conditioned to express emotions differently. Grzyb says: "Men have a much harder time making intimate friendships with other men. They have mates but not people they talk to in the same way that women do." The difference between male and female friendship is, as Grzyb says, "set in cement" around puberty. "Girls begin to change hormonally and there's a commonality that men don't have at that point."In adulthood, the male "mate" mentality tends to continue as does the lack of emotional candour This may result from the way men often select their friends. Rather than seeking an emotional affinity, they tend to establish ties based on shared hobbies. Grzyb explains: "They access each other through common interests. Conversationally men need to know where they stand and they talk about subjects on a less contentious level; football, politics, etc.