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.  … 

Cartesians had a response: any such human reaction was itself just a mechanical reflex. …  it’s true that many of those who have little experience of what goes on in an abattoir are repulsed by any kind of firsthand knowledge, or even by reading vivid accounts. But things are different on the other side of the slaughterhouse wall. Those who kill animals in the course of their working day may quickly become habituated to it, and to dismiss this effect as mere desensitization effectively discounts great knowledge of animal death in favor of slight knowledge.  Similarly, those who like to romanticize country people are frequently discomfited by their uncuddly ways with livestock. … Why is it "natural" not to know very much about "nature"? …

When the sixteen-year-old Ben Franklin converted to vegetarianism, he seemed to have been struck both by its health benefits and by moral sensitivity to animal suffering. But Franklin soon fell off the wagon. On his first sea voyage …:

Hitherto I had stuck to my Resolution of not eating animal Food; … But I had formerly been a great Lover of Fish, & when this came hot out of the Frying Pan, it smeled admirably well. … when the Fish were opened, I saw smaller Fish taken out of their Stomachs: Then thought I, if you eat one another, I don’t see why we mayn’t eat you. … So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do.

Some say innocents have a better grasp of moral truth, and that others have found convenient excuses for suppressing conscience.  Innocent insight is a common literary theme; consider all the stories where only children can see ghosts or do magic, adults being too closed-minded to believe. 

Others say ignorance is not insight, however much the innocent might want to flatter themselves.   Revulsion at meat is no more informative about moral truth than our intuitive hate of snakes, or kids’ disgust at the idea of sex; those with more experience and knowledge should have a better grasp of moral truth.   

Surely there must be a few topics where adults learn to fool themselves and so kids know better.   How can we distinguish those topics, where innocence contains insight?