Dr William Masters and his wife Virginia Johnson , the pioneers of sex therapy, estimate that 50 per cent of married couples in America have sex problems which could lead to divorce. In Britain there are no statistics but, say Masters and Johnson, ' even if it's only a tenth as high, you'd still have a major problem.' Sex therapy is now big business in America. And since anybody can call themselves a sex therapist, clinics are mushrooming - much to the concern of reputable doctors who are worried about charlatans cashing in and causing more harm than good. John Pitman reports on 'the newest profession' and asks what we can learn from the American experience as sex therapy arrives in Britain.