When I was a tragic knock-kneed, bucktoothed youngster, I longed to be friends with the coolest girl in the school. This goddess, named Sophie, toyed with my affections – allowing me thus far into her gang, but no further; bestowing the warmth of her approval upon me one day, then casting me into Siberia the next; and, on one occasion during assembly, testing my devotion by plunging a sharpened pencil into my thigh and snapping off the lead, to see whether I would squeal (I didn’t). Although I never told my mother about any of this, she knew instinctively that I was being tortured, and by whom. “That girl is poison,” she would say, shaking her head in despair. “I can’t understand why you’re so determined to be friends with her.”

That is one of many mysteries to which, quite unexpectedly, an answer can be found in The Sister Knot. Psychologist Terri Apter believes that the neglected science of “sistering”, as she calls it, can explain a great deal of apparently baffling female behaviour, including the anxiety and envy that drives us to engage in doomed power struggles with other females, from the playground to the office.

Sibling relationships generally have far more impact on our psyches than was once imagined, but the relationship between sisters, says Apter, is unique in its intensity. It creates extremes of love, fear, rivalry and protectiveness that affect us for the rest of our lives. The sister knot begins, she says, with the trauma common to all sibling relationships: that of displacement. This is usually assumed to be the predicament of the firstborn, who, having enjoyed her parents’ undivided love and attention, suddenly finds herself elbowed aside by a mewling new bug. But, in fact, the dread of displacement is common to almost all siblings, regardless of birth order. Sisters, especially, constantly measure themselves against each other, terrified of being “annihilated” by the other’s superior qualities. They feel an urgent need to demonstrate their distinctness from each other – “I am me, I am not you” – which often tips over into cruelty.

The fact that these ignoble instincts are matched by an equally fierce protective love only makes them more distressing. Many of the 76 sisters interviewed by Apter described this emotional conflict. “I wanted everyone to see how hateful she was,” recalls Donna of her little sister, “but when she did get shouted at, I’d feel a different sort of terror and was always rushing to comfort her. She blubbered and sobbed and put her disgusting hand, wet with all her slobber, into mine, and I hugged her, and I felt this . . . Oh, it was awful: I really wanted to comfort her, but I also wanted to punch her.”

Because they understand each other so well, siblings are able to manipulate each other with Machiavellian cunning. One of Apter’s interviewees describes how she exploited her older sister’s sweet tooth, secretly supplying her with forbidden treats in the (gratified) hope that she would get fat. Another recalls telling her little sister that there were spiders in the school lavatory, knowing that she would try to hold it in until she wet her knickers. Sweetly, the victims of these Darwinian manoeuvres hardly ever remember them: each sister, preoccupied by her own guilty feelings, tends to think of herself as the “bad” one and her sibling as a model of kindness.

The other piece of good news is that, once the primal terrors of childhood and adolescence have passed, sisters become nicer to each other. Closeness in adulthood varies according to circumstance (how far apart they live, and whether they get on with each other’s partners), but generally the power struggles give way to a more generous, accepting bond of shared memories and understanding. Sisters sometimes become closest at the end of their lives, living together after their spouses have died.

Other people’s lives are endlessly fascinating, and the interviews that provide the backbone of this book are no exception. Apter’s analysis of why sisters behave the way they do (and how this behaviour is echoed in their future relationships with men, women and school bullies) is cool-headed and wise. She uses sisterhood to unpick certain myths of femininity: putting paid, for instance, to the notion that females are naturally less combative than males.

There are moments when one craves a slightly more empirical, less therapeutic approach, especially when it comes to the fundamental question: Why do girls seem predisposed to this sort of emotional intensity? It would be good, for example, to hear the scientific explanation, as put forward recently by the American neuropsychiatrist Louann Brizendine in her book The Female Brain. Brizendine’s explanation (it’s all about hormones, and the size of your communication cortex) is satisfyingly authoritative, whereas Apter sometimes lapses into woolly Freudianism. On the whole, however, she is lucid, funny and illuminating on a subject that has been kept in the dark for much too long.

Norton £16.99 pp304

Available at the Sunday Times Books First price of £15.29 (including p&p) on 0870 165 8585

Read on . . .

My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bonds by Dorothy Rowe (Routledge £9.99)

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