We study an economy with traders whose payoffs are quasilinear and their private signals are informative about an unobserved state parameter. The limit economy has infinitely many traders partitioned into a finite set of symmetry classes called types. It has a unique rational expectations Walrasian equilibrium (REE) whose price reveals the state. Total monotonicity, a property that limits heterogeneity across types, determines whether an efficient social choice function (SCF) is attainable using mechanisms in a class that includes auctions. An average crossing property on the primitives is a sufficient condition for total monotonicity. The REE is an efficient SCF so it is attainable by an auction if and only if it satisfies total monotonicity. REE with total monotonicity s not only attainable, but also implementable: it is approximated by the equilibrium outcomes of auctions with finitely many traders of each type and fine grids of the state, signals and bids.
