Abstract. It is well known that nonlinear diffusion equations can be interpreted as a gradient flow in the space of probability measures equipped with the Euclidean Wasserstein distance. Under suitable convexity conditions on the nonlinearity, due to R. J. McCann [12], the associated entropy is geodesically convex, which implies a contraction type property between all solutions with respect to this distance. In this note, we give a simple straightforward proof of the equivalence between this contraction type property and this convexity condition, without even resorting to the entropy and the gradient flow structure.We consider the nonlinear diffusion equationwhere f (r) is an increasing continuous function on r ∈ [0, +∞) and C 2 smooth for r > 0 such that , with u t ≥ 0 and mass u t (x)dx = M for all t, such thatii) u weakly satisfies equation (1), i.e., it satisfies the identitythere exists a unique such solution to (1) with initial datum u 0 , see [16, Sect. 9.8]. Let us normalize the mass M to unity in the rest of the introduction for convenience. Equation (1) admits the mapas a Liapunov functional. Here the map U ∈ C([0, +∞)) ∩ C 3 ((0, +∞)) is defined in a unique way by the relations f (r) = rU ′ (r)−U (r) on (0, +∞) and U (0) = U ′ (1) = 0. The map f being increasing is equivalent to U being strictly convex, since f ′ (r) = rU ′′ (r), and f being positive on (0, +∞) is equivalent to ψ :