A recent New Yorker review of "The Bloodless Revolution: A Cultural History of Vegetarianism from 1600 to Modern Times" shows how bias theories are central to the vegetarian debate: Commentators argued that the habit of killing, like that of meat eating itself, hardened the heart and the nerves, both figuratively and literally. The squeamish human response to animal suffering was the authentic one; the callous reaction induced by familiarity was accounted artificial or false. …