Most of us, too, will react with decent horror when told about the malice and violence that used to be directed against twins. For generations, it was believed that a woman could only bear twins if she had been inseminated by two men; to avoid charges of adultery, the unfortunate younger siblings would often be killed at birth.
Yet over the same decades in which Western societies have grown more kindly and rational towards their twins, the representation of twins in the arts has become weirder and more atavistic. Though we are now justifiably repelled by the idea of Chang and Eng, the original Siamese twins, being shown off to gawping rubes by Barnum, remarkably few artists since the Romantic period have treated identical twins in the cheerful, charming manner of, say, Twelfth Night or The Comedy of Errors.
In art, thoroughly modern twins will tend to be morbid, neurotic, self- destructive... in a word (Barnum's word), freaks.
True, there are the jocular and popular likes of Schwarzenegger and De Vito - the ultimate non-identicals - in Twins, Bette Midler and Lily Tomlin doubling up for Big Business, and Elvis auto-duetting in Double Trouble, but there is something embarrassingly old-fashioned about such uncomplicatedly comic treatments of complicated situations.
It is in less popular, more grimly comic films that the true contemporary note of freakishness is clearest: the doomed gynaecologists played by Jeremy Irons in David Cronenberg's Dead Ringers; the equally doomed former Siamese pair in Peter Greenaway's A Zed and Two Noughts; the separated- at-birth gangster and wimp in Alan Rudolph's Equinox (set in the twin cities of Minneapolis-St. Paul); the East End thugs of The Krays, written by Philip Ridley, whose plays and paintings have also dwelled on the matter of twins. Hence his screenplay's nods to the myth of Leda, who gave birth to Castor and Pollux, now enshrined in the heavens as Gemini.
The lineage of such films is a literary and dramatic tradition in which sibling symmetries tend to be fearful, and often charged with unwholesome eroticism. To name just a few unhappy couples: Roderick and his undead sister in Poe's "The Fall of the House of Usher"; the incestuous Siegmund and Sieglinde in Wagner's Die Walkure, and the pampered, etiolated young brother and sister of the same names in Thomas Mann's story "The Blood of the Walsungs".
Why all the morbidity? A few easy answers: because of a burgeoning taste for the outre over the last couple of centuries, because of novel worries about the integrity of the personality, even, perhaps, because of Darwinism and a newly informed concern with genetics. The rise of the bizarre twin is roughly contemporary with the flourishing of the "double" theme in literature, and though Karl Miller devotes only one short chapter to twins in his critical study Doubles - that chapter discusses Lewis and Benjamin Jones in Bruce Chatwin's On the Black Hill, and Arthur and Waldo Brown in Patrick White's The Solid Mandala - it's plain enough that twins were being widely recruited for those spooky stories about humanity's divided nature that thrived from James Hogg's Confessions of a Justified Sinner to Oscar Wilde's The Picture of Dorian Gray.
Moreover, what more snappy a vehicle for fictional debates about nature versus nurture - in English, those complementary concepts actually look like twins - than stories of identical siblings raised in different households? While the idea of the twins who stand for good and evil, yin and yang, is as old as Genesis, it becomes freshly pertinent when there are angry debates about whether criminality is the product of slums or poisoned heredity; and so bad-penny twins were bound to become ever more widely circulated.
As long as that social argument remains unsettled, and as long as we follow hand-me-down fashions from the Romantics, it is unlikely that artists will renounce their penchant for twins; indeed, the Irish philosopher Richard Kearney has just published a reflective novel about rivalrous twin brothers, Sam's Fall. And, as long as we are able to separate fantasies about twinhood from reality, the twin motif need not be pernicious, though it is encouraging to note that there are now quite a few twins who work in tandem as artists - the Wilson sisters, the Quay brothers - and are therefore ideally placed to criticise or contradict it.
It might almost be tempting to say that, in the arts, twins are a rising minority group, were it not that there is something paradoxical about attributing minority status to people who, compared to singletons, always enjoy a 100 per cent majority.
'Mixed Doubles', a Kaleidoscope feature about twins in the arts, presented by Kevin Jackson and produced by Paul Quinn, is on
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