Just what is it that makes the bond so special, and often so fraught? As a new book sparks debate, Rebecca McQuillan tries to find out.

It's the one jarring element in an otherwise perfect work of literature. How could someone like Elizabeth Bennet, the heroine of Jane Austen's Pride and Prejudice - a woman with a fierce independence of spirit, quick wit and keen intelligence - be so devoted to a sister like the bovine Jane? A more insipid creature you'd be hard pressed to find, even parodied by French and Saunders, and yet Elizabeth dotes on her and coos over her and wants "only her happiness". In real life, surely, Elizabeth would roll her eyes in exasperation at Jane's long-suffering meekness; would urge her to flash some steel at that bitch Miss Bingley. "Dear, sweet, kind-hearted Jane," you can imagine her thinking, "pleeease would you get a grip!"

Do Jane and Elizabeth bear any resemblance to real-life sisters?

Dr Terri Apter is a senior tutor in psychology at Newnham College, Cambridge, and author of the book The Sister Knot, which was published this month and has already provoked widespread comment. Deep mutual affection is certainly a hallmark of sisterhood, she confirms. After talking to 76 sisters in 37 family groups during her research, she declares that the bond between sisters tends to be closer than between brothers, or between brother and sister. Even during the competitive, insecure teenage years, a "real best friend is often a sister".

But of Jane and Elizabeth Bennet's relationship she says: "I think it is a little bit idealised." That's because Apter's research has shown that bickering, envy and judgments on each other's character flaws are also a normal part of being sisters. Being riddled with self-doubt because your sibling passed her driving test when you failed first time: during the childhood and teenage years especially, such competitive thoughts are common, says Apter, who has a sister herself. "With sisters we learn one of the most disturbing lessons about human connections - that we can love and hate the same person."

Hate? That's a strong word, but it's one that comes easily to children, whose relationships with their siblings are often akin to that of warring generals. "Since the beginning of human history siblings have represented potential threats to each other's claim on a unique position in the family," says Apter. Children fight with their siblings in order to carve out a secure niche in the family, and it's a time-consuming business: "Children have on average nearly five fights with siblings every day; in very early childhood, a younger sibling has a conflict incident' at the rate of about eight an hour," Apter notes. Phew.

You could say those things about any siblings. But sister relationships have certain specific features.

Girls, who are typically strong on empathising, may use their intimate understanding of each other's insecurities to score points. Some episodes Apter recounts from her research are darkly comic, like that of Gina, a teenager who could give Machiavelli a run for his money: she "generously" gave her elder sister Sam her pocket money, knowing the grateful Sam would spend it on cakes and put on weight.

Where does such behaviour come from? Well, says Apter, no matter where they are in the family, siblings naturally try to differentiate themselves from each other to fulfil a deep psychological need that we all share - to feel unique. Among sisters, though, it's made harder because they must live with constant comparisons. Sisters tend to identify very strongly with one another, which leads to very close bonds but also to mutual sizing-up. If one sister does very well, the other will often feel insecure by comparison. Love and envy are both very much part of that hoary tangle of emotions that Apter christens the "sister knot".

"Throughout life, it is the person who has been like us - who takes a step forward or makes a sudden gain - who triggers the poisonous anxiety of envy," she says. No wonder sisters often seek to free themselves from comparisons by taking different paths in life.

Apter is anxious to stress, though, that this talk of envy and conflict should not be seen as negative. "One thing that has given me a lot of pause for thought is that the book has been interpreted as talking about the ugly side of sisterhood," she says. "What I thought I was talking about was a close and passionate bond, but one that has a darker side. I think understanding envy is very important - understanding it not as a morale malaise or meanness of character, but as a primitive fear of annihilation." Understanding it removes its sting.

Psychologists, she feels, have focused too much on women's supposed empathy and "kindness", and wrongly assume that aggression and establishing status are not really natural female behaviours, when they are. She says: "There may be conflict but it's not all bad."

The important thing is that competitiveness between sisters typically fades when one sister is in need, overcome by intense loyalty and love. Even in childhood, sisters have strong protective instincts towards one another and are prepared to get themselves into trouble or danger to help the other out. The spats are almost invariably remembered in later years as being funny, if they are remembered at all.

In adulthood, sisterly relationships are an enormous source of fun and comfort. Sisters continue monitoring each other's progress through life, but ultimately "a sister offers mutual understanding and support and a long-term knowledge of one's disappointments and hopes".

The harmony between Jane and Elizabeth is wholly realistic, then. It's just that Austen must have left out the bit about how, when she was eight, Lizzy cut off Jane's hair as a joke. Mrs Bennet never did learn to stop telling everyone that Jane was the pretty one.

The Sister Knot by Terri Apter is published by WW Norton & Co, priced £16.99.

Females in the family best friends or bitter rivals?

We've always been a bit of a package'
Gillian Zaoui, 35, a modern studies and guidance teacher, and her sister Joanne O'Neill, 30, also a teacher, live a mile apart in Glasgow. They have an elder sister and an elder brother. Gillian says: "We've been best friends as well as sisters. We see each other at least twice a week and speak on the phone every night. It makes you lazy on the friends front. She even turned up when me and my husband were on our first date. We've always been a bit of a package. "Our older sister was seen as the intelligent one - she got straight As. We were given the same label - we were seen as the entertainers. Envy has never been an issue; nor has competition. "I'm very, very protective; it's all right for me to be critical, but not other people. "Joanne and my husband are both very important to me: I couldn't live without either one of them." Joanne says: "She's definitely one of the most important people in my life. "I've always liked being with Gillian. I wanted to be like her when she was a teenager. I saw it as a good thing when we were compared. I do look to her as a role model. She always gives me the bald truth and knows I'll give her the truth. We really identify with each other's sense of humour. I know what she's going to say - I finish her sentences. "I couldn't move away from her."

We used to bicker a lot'
Fiona Mackenzie, 42, a paediatric MRI radiographer, and her sister Eleanor, 38, an administrative assistant, live in Leith. They both run with Jogscotland groups. They have another sister, who is the middle one. FIona says: "We see each other at least once a week. Eleanor comes round to the house for her tea on Tuesday after running. She's pretty important to me and is closely involved with my daughter. We did used to bicker quite a bit, the three of us, and we'd take sides. Do I feel protective towards her? Oh yes!" Eleanor says: "We run together, but when we're in a group, she's in the middle or front and I'm further back. We definitely don't compete; I'm quite happy plodding. Most people can see a resemblance between us. Character-wise, Fiona is a lot more into going out and doing things, while I'm a lot more happy to let things happen. I'm more likely to sit back, being the younger one. But we've always had shared interests; we enjoy being active."

Being apart is so hard'
Shelley and Suzanne Grant, 22, play football for Hibernian Ladies and are Scottish football internationalists; Suzanne recently played in the Cyprus International Friendly Tournament. They lived near one another until December, when Suzanne moved to East Kilbride, where she works in retail. Shelley, a clerical assistant, lives in Strathpeffer. They have a younger and an elder sister. Shelley says: "Suzanne's the party animal and I'm the more sensible one. We see each other once a week in Edinburgh when we play football. But there's not a day goes by when we don't talk. "Being apart is probably the hardest thing that's happened to us. "There are days when we fight like mad, but you forget about it within minutes. Sometimes I'll come off the phone and think, I wish I hadn't said that'. I can't see her to say sorry, but I phone her back. "When we were younger, we were competitive. But say Suzanne gets selected - I'm totally delighted. We always want at least one of us to do well. If anyone criticises her, I bite." Suzanne says: "I miss her a lot. "Shelley's quieter than me. We have different dress sense - I'm a bit more the crazy one. We're so used to being compared, I don't mind it. When we were kids, Shelley was the better swimmer. People would say: does it bother you? I would say, of course not. We've never been the jealous types. "If I'm having a bad game, Shelley will say head up' and I might snap at her. But because she's my sister, she loves me. She never shouts. After the match I say I love you and I miss you' and that's it."