Density functional theory computations have elucidated the mechanism and origins of stereoselectivity in McGlacken's aldol-Tishchenko reaction for the diastereoselective synthesis of 1,3-amino alcohols using Ellman's t-butylsulfinimines as chiral auxiliaries. Variations of stereochemical outcome are dependent on the nature of the ketone starting materials used, and the aspects leading to these differences have been rationalized. The intramolecular hydride transfer step is the rate- and stereochemistry-determining step, and all prior steps are reversible.