A concept of demographic separability is proposed that formalizes the notion that there are groups of goods (adult goods) that have little or no relationship to specific classes of household demographics (the numbers or ages of children).That there exist adult goods demographically separable from children is a necessary but not sufficient condition for the validity of Rothbarth's method for measuring child costs. We propose two different methods for testing demographic separability and present results from a 1981 survey of Spain. The econometric evidence is in fair agreement with the theoretical presuppositions.

For thirty years it has been accepted that consumption is smooth because permanent income is smoother than measured income. This paper considers the evidence for the contrary position, that permanent income is in fact less smooth than measured income, so that the smoothness of consumption cannot be straightforwardly explained by permanent income theory. The paper argues that in postwar U.S. quarterly data, consumption is smooth because it responds with a lag to changes in income.

A concept of demographic separability is proposed that formalizes the notion that there are groups of goods (adult goods) that have little or no relationship to specific classes of household demographics (the numbers or ages of children).That there exist adult goods demographically separable from children is a necessary but not sufficient condition for the validity of Rothbarth's method for measuring child costs. We propose two different methods for testing demographic separability and present results from a 1981 survey of Spain. The econometric evidence is in fair agreement with the theoretical presuppositions.