[1] The aggregation problem is the difficult problem of finding a valid way to treat an empirical or theoretical aggregate as if it reacted like a less-aggregated measure, say, about behavior of an individual agent as described in general microeconomic theory[1] (see representative agent and heterogeneity in economics).
If this assumption is violated and the agents are subject to aggregated utility functions, restrictions on the latter are necessary to yield the law of demand.
The aggregation problem emphasizes: Franklin Fisher notes that this has not dissuaded macroeconomists from continuing to use such concepts.
Finally, if the distribution of income changes in favor of consumers who prefer the good in question, the demand will shift out.
The most important problem for micro- and macro-economics is the Sonnenschein–Mantel–Debreu theorem, which shows that almost no properties of the individual preference are inherited to the aggregate demand functions.
Microeconomist Hal Varian reached a more muted conclusion: "The aggregate demand function will in general possess no interesting properties".
[8] However, Varian continued: "the neoclassical theory of the consumer places no restriction on aggregate behavior in general".
[8] This means the preference conditions (with the possible exception of continuity) simply do not apply to the aggregate function.