An aldol reaction catalyzed by a proline-thiourea host-guest complex in a nonpolar solvent shows excellent nonlinear effects. This proline-thiourea system has the ability to form a hydrogen-bonding network. The enantiomeric excess of proline in a solution can be significantly enhanced by its incorporation with a urea molecule into its solid racemate. This suggests a general and facile route to homochirality, which may be involved in the origin of chirality on earth. (C) 2010 Elsevier Ltd. All rights reserved.