 The Fast and Frugal school and the Biases and Heuristics school both agree that heuristics are biased. Where they disagree, and disagree sharply, is whether those biases are necessarily a sign of irrationality. For the Fast and Frugal program the question is under what environmental conditions, if any, does a particular heuristic perform effectively. If the heuristic’s structural bias is well-suited to the task environment, then the bias of that heuristic may be an advantage for making accurate judgments rather than a liability  (section 4).  We saw this adaptive strategy before in our discussion of Brunswik’s lens model  (section 3.2),  although there the bias in the model was to assume that both the environment and the subject’s responses were linear. The aim of the Fast and Frugal program is to adapt this Brunswikian strategy to a variety of improper models.