New Scientist, 27 May 2014

Find out the answer to the When Harry Met Sally question, why we have frenemies, plus seven more chum conundrums


1 Do we really need friends?
Yes. People with weak social relationships are 50 per cent more likely to die in a given period than those with strong social ties. Social isolation is as bad for you as drinking or smoking – by some estimates equivalent to smoking 15 cigarettes per day – and worse than inactivity or obesity.

Friendships also contribute to happiness – although quality, not quantity, is what counts. A study of 423 college students found that the quality of friendships had a big impact on how happy people were, whereas the number of friends they had made no difference.

However, the benefits of friendship probably vary from person to person, which would explain why some people say they feel happiest when alone.

2 What makes a good friendship?
We forge friendships with people who are similar to ourselves. The six most important criteria are language, profession, world view (political, moral and religious), sense of humour, local identity and education. Personality appears to be less important than cultural preferences – the bands you like, the books you enjoy, the jokes you find funny. In fact, the best predictor of how well you will get on with a stranger is whether you like the same music.

3 Why do some people have more friends than others?
It may be in their biology. Neuroimaging studies have found that people with more grey matter in areas such as the amygdalae – which are associated with memory and emotional processing – tend to have more friends. But it is not clear whether this is cause or effect. There are also cultural influences: people from big, extended families tend to have fewer non-kin friends than those from small families.

4 Can straight men and women be "just friends"?
Yes, but that doesn't mean they aren't attracted to each other. Numerous studies show that attraction is a frequent component of cross-gender friendships. A survey of US college students found that half had had sex with an otherwise platonic friend.

Young men tend to be more attracted to their female friends than the other way round. They are also more likely to become friends with a woman because they are attracted to her, and to overestimate how attractive their female friends find them.

Women are more interested in protection. They are also more likely than men to secretly test whether their opposite-sex friends fancy them. Researchers have identified 158 such tests – the most common are attempts to make a friend jealous, tests of fidelity and temporary physical separation.

5 Do male and female friendships differ?
Women are more likely to have a best friend, whereas men more often hang out with a group. Women tend to consider friendships more in terms of emotional connection. By contrast, men think about how much time they spend together or how long they have known each other.

Female-female friendships tend to be more intimate, and women make friends with similarly physically attractive women. That is a good mating strategy – their friends attract men who are likely to find them attractive too – but it also leads to competition.

6 Are all friendships good for you?
No. Relationships with "frenemies" can actually damage your health. These are people who bring us down but who we put up with anyway. About half of the people in your social networks are likely to be frenemies – most of them family members. Interacting with unreliable friends is stressful. Your blood pressure is likely to be more elevated when you are with a frenemy than it is with someone you do not like at all.

7 Does friendship change as we age?
Yes. Small children only really need one close friend – we don't develop the ability to juggle large numbers until our early 20s. Teenagers are hugely influenced by their friends, especially in behaviours such as substance use, violence and suicide.