Journal article: An ideologically biased expert faces trade-offs in model design. The perceived model must be autocoherent—its use by all agents delivers a self-confirming equilibrium. Policy may be influenced by manipulating the Keynesian multiplier or the Phillips curve parameters. Ideological bias may arise in a way that resembles well-known historical controversies. A larger reported Keynesian multiplier is favored by more left-wing economists, as is a flatter inflation output trade-off. Some combinations of parameters must be truthfully revealed, illustrating the tight link between parameter identification and the scope for bias that is implied by the autocoherence conditions.