  We revisit a classical question of how individual consumer preferences and
incomes shape aggregate behavior. We develop a method that applies to
populations with homothetic preferences and reduces the hard problem of
aggregation to simply computing a weighted average in the space of logarithmic
expenditure functions. We apply the method to identify aggregation-invariant
preference domains, characterize aggregate preferences from common domains like
linear or Leontief, and describe indecomposable preferences that do not
correspond to the aggregate behavior of any non-trivial population.
Applications include robust welfare analysis, information design, discrete
choice models, pseudo-market mechanisms, and preference identification.
