HEROIN was a scourge of America’s cities in the 1960s and 70s. But then it seemed to go out of fashion. By the 1990s it was less widely used than crack cocaine. In Europe its use has continued to decline, with the number of addicts falling by about one-third in the past decade. In America, by contrast, it is resurgent. Last year nearly 700,000 Americans took the drug, twice as many as a decade ago. It is now more popular than crack, by some measures. What explains heroin’s return?