But along with this envious interest, encouraged by those cheerfully unreal stories about twins written for children this century, there is a more negative type of inquisitiveness. What happened when both infants wanted attention at the same time? Was there a secret language which delayed ordinary speech? What about the eternal playing off of bad twin versus good twin, a staple ingredient of so many novels and movies? Put another way, how are you - the hapless twin object of all these questions - getting on with your own twin these days?
Penelope Farmer is a highly individual author who has written a children's classic, a study of myth, and several novels. She is also a twin; her sister Judith died five years ago. Within this anthology, she looks for answers to the questions she constantly asks herself about all twins everywhere. Her own running contributions also describe the stormy relationship she had with her sister. Haunting and provocative, they are material for someone else's anthology. Quotations elsewhere are drawn from more than 250 scientists and poets, anthropologists and novelists, with hardly any references to those most arid of all twin studies this century, performed by psychologists searching for the existence of an inherited IQ.
Farmer is after something much more important: an examination of the whole binary way in which humans have always tended to think, whereby evil is divided starkly from good, light from dark and right from wrong. In this dichotomous universe, twins can be seen both as a single supportive unit, forever fighting each other's battles in life, and as the divisive, potentially murderous couplings found in myth and the Bible.
Both images can be true. Twins in real life range from the Krays to the virtuous Bedser cricketing brothers, before whom an elderly Frenchman once sank to his knees following a local superstition that seeing identical twins brought good luck. Farmer herself is not an identical twin, but was often confused as one. She quotes from others detailing the way that such twins inevitably develop different personalities over time simply by being part of a couple within which certain roles, duties and personality traits usually come to be separated out.
Physiologically, however, identical twins may echo each other's development down to the smallest detail. Psychic convergence - reading minds, experiencing similar feelings at a distance - is more debatable, though some of the examples quoted here are difficult to explain on existing rational grounds.
The rest of us are born single and, if we are lucky, eventually find others with whom to share our lives as adults. Twins are born together, and as adults must become single - at least, from the other twin. Growing up will therefore always mean a sense of loss but also a feeling of liberation. No twin can ever be perfect for the other.
As Jennifer Gibbons - one of the famous "Silent Twins" of TV documentary fame - put it when writing about her sister June: "Somewhere I have a real twin in this world. J. can't be my real twin." From the sisters' childhood on an RAF base in Haverford West, and a tormented adolescence, their story ended unhappily in an early death after a spell in Broadmoor. The death of one, at least, brought some sense of relief for the other. The surviving twin wrote that she would like a banner reading "June is alive and well and has at last come into her own".
There is a surfeit of literary anthologies these days, often put together for no better reason than quick sales around Christmas. This one is different, assembled in an urgent spirit of enquiry. It draws on some fascinating and unfamiliar sources, and raises important questions about all individual experience. Excellently packaged by Virago, it remains compulsively readable for all its 482 pages.