GIRLS aren't usually allowed to be badly behaved, so blaming your uncontrollable twin is the perfect excuse for being a bitch. It also provides a safe therapeutic outlet for female audiences bored with playing by the rules. Note how Bette Davis combines the sweetness of her Dark Victory persona with the malice of her Jezebel in A Stolen Life (1946) and Dead Ringer (1964). If she melts between the cultural extremes of angel and succuba with ease, it's because these films cannily represent femininity as a performance, a matter of hairstyle, cosmetic camouflage, clothes and rehearsed responses. Which explains why the star's male foils are slow to discern the obvious. After the bad Bette drowns, the good Bette dons her wedding band to woo the man (Glenn Ford) seized from her by her daring sister. Ford takes for ever to realise that his wife is . . . different. All women literally seem alike to him: enigmatic.
Or pathological. In The Dark Mirror (1946), the vindictive Olivia De Havilland's (above) yearning to be loved curdles when her virgin double constantly deflects male attention. The script assumes that passivity is what men truly desire: a truth that pushes the bad Olivia into insanity, murder and a scheme to kill off her good side. She too plans to take her sister's place. But then, as Hammer's Twins of Evil (1971) reminds us, through Peter Cushing's religious headbanger, women have a natural propensity toward sexual dysfunction - even before vampires lay teeth upon them.