Rachel Cooke in The Guardian:
You may not have heard of Robin Dunbar. But you will, perhaps, know of his work. Dunbar, now emeritus professor of evolutionary psychology at Oxford University, is the man who first suggested that there may be a cognitive limit to the number of people with whom you can comfortably maintain stable social relationships – or, as Stephen Fry put it on the TV show QI, the number of people “you would not hesitate to go and sit with if you happened to see them at 3am in the departure lounge at Hong Kong airport”. Human beings, Dunbar found when he conducted his research in the 1990s, typically have 150 friends in general (people who know us on sight, and with whom we have a history), of whom just five can usually be described as intimate.
In his new book, Dunbar revisits and unpicks this number, by which he stands; and he brings together several decades of other research in the area of friendship, some of it his own, some that of anthropologists, geneticists and neuroscientists with whom he has worked. It can’t be definitive: the possibilities in this field are surely limitless. But for the reader, it sometimes feels like it is. Why do most women have a best friend? Why do many men struggle to share confidences? Why is it so painful when we fall out with our friends? Above all, what effect do friends (or a lack of them) have on our mental and physical health? Think of any question you might have and you’ll find some kind of an answer to it here. What you may feel in your gut, it will back with science. Its central message, however, may be summed up in a sentence. In essence, the number and quality of our friendships may have a bigger influence on our happiness, health and mortality risk than anything else in life save for giving up smoking.