They are the children who are supposed to look alike, dress alike, behave alike and carry their almost supernatural closeness to their graves; twins, two babies from the same egg who have a bond that outsiders cannot break and who, consequently, live for each other. But, as Penelope Farmer's exhaustively-researched anthology on the subject shows, that judgment comes partly from the fascination, and in some parts of the world, horror, that twins hold for outsiders. When it does hold true, the consequences are not as cosy as we would imagine; many twins have a love-hate relationship in which hate has the upper hand. First, you must define your twin. Identical twins are, Larousee observed in Science of Life, 'the same individual of whom two copies have been printed', or 'one river flowing in two beds' according to Anatoly Pristavkin in The Inseparable Twins. So much for the snap conclusions. Science proves more interesting. 'A study of twins reared apart has reported several cases where genes seem to be controlling not just broad tendencies, such as being intelligent or outgoing, but surprisingly detailed aspects of personal behaviour,' Jerome Burne wrote in The Times of London in 1993. 'For some time now the study [by the University of Minnesota] has been finding twins, separated at birth and reunited only in middle age, shared such curiously specific things as: being brilliant storytellers, refusing to express controversial opinions, being habitual gigglers, and always wearing seven rings. 'There was even a pair who always entered the sea backwards and then only on their knees.' Then there are fraternal twins - separate eggs, same womb - two individuals born at the same time, but with different characteristics; 'I must trust the substance, not the shadow; you are most like me, but are not the same,' wrote William Rider in the 17th century. After getting the basics out of the way, offering some startling statistics as in the case of the woman who gave birth to 44 children in 33 years, including 13 pairs of twins and six sets of triplets, and discussing famous childhoods - the criminal Kray twins in Britain, the Dionne quins in Canada - Farmer moves into deeper waters. Love and jealousy, twins who swap sexual partners without their lovers' knowledge, ghostly goings on when a dead twin returns to collect his living brother, separation, where children are capable of pining to the point of death, and the ever-present risk of hatred developing when two children grow so closely into adults with shared desires. My favourite, by L A G Strong, in an anthology that by its nature is full of quotable extracts, involves twin sisters who die within hours of each other and whose enmity goes beyond the grave. They are at war even in death. In their part of Scotland, tradition dictates that the last person buried in the local graveyard has to keep watch over the other dead until the next burial occurs, which is not expected for a very long time. Their supporters engage in an unseemly race to bury one before the other - and so excuse one twin from such an onerous chore before an angry priest appears. 'Murtagh seized one of his arms, Ellen's factor the other and all began to shout at once, each urging that he should bury their corpse first . . . Before he knew what was happening they were pulling each of his arms, tugging him, jerking him, trying by main force to drag him to their own corpse. 'At this crowning outrage, the priest recovered himself . . . plunging forward he shook his right arm free and with a swing of his fist sent Murtagh a back-hander across the mouth that knocked him backwards over a gravestone.' The twins were buried simultaneously and were finally united; standing joint guard over the dead. For Penelope Farmer, there was a tragic motivation for this book. Her twin sister (fraternal) died of cancer in adulthood and Farmer intersperses others' observations with her own on the breaking of a bond that she had not always appreciated. For this reviewer, Two holds a special interest. As the father of young, fraternal twins, I am both bemused and alarmed at what the future may hold for them, and for me. But for all the fascination that the more serious sections of Farmer's book holds, it is the humour that twins have attracted that appeals the most, especially from a wit with the skill of Mark Twain: 'My twin and I got mixed up in the bathtub when we were only two weeks old and one of us drowned, but we didn't know which. 'Some think it was Bill and some think it was me. One of us had a peculiar mark - a large mole on the back of his left hand; but that was me. That child was the one that drowned.' Two or The Book of Twins and Doubles by Penelope Farmer Virago $340