"Twins threaten us because they undermine our notion of identity. We think we are who we are because of the life we have lived. We think we form the character and values of our children by the way we raise them.... But when we read about twins who have been separated at birth and reunited in middle age only to discover that in many respects they have become the same person, it suggests that life is a charade, that the experiences that we presume have...