We show that preference-homogeneity and loss-aversion are necessary and sufficient for the value function to have the power form with identical powers for gains and losses and for the probability weighting functions for gains and losses to be identical.

Loewenstein and Prelec (1992) explain the 'magnitude effect' and the 'sign effect', respectively, by using increasing elasticity of the value function and a higher elasticity for losses as compared to gains. We provide a value function with these two properties.