This paper reports the testing of hypotheses concerning: (i) whether the household is better viewed as planning over a single-period versus a multiperiod horizon; (ii) whether the household is better viewed as planning in a single-asset or a multiasset framework; (iii) the relative importance of substitution and wealth effects as sources of change in the stock demand for automobiles. The findings are that a multiperiod, multiasset model best describes stock demand, that the separation theorem which implies a zero wealth effect is rejected, and that substitution effects are seven times more important than wealth effects.