A nonequilibrium migration model is proposed and applied to genetic data fr
om humans. The model assumes symmetric migration among all possible pairs o
f demes and that the number of demes is large. With these assumptions it is
straightforward to allow for changes in demography, and here a single abru
pt change is considered. Under the model this change is identical to a chan
ge in the ancestral effective population size and might be caused by change
s in deme size, in the number of demes, or in the migration rate. Expressio
ns for the expected numbers of sites segregating at particular frequencies
in a multideme sample are derived. A maximum-likelihood analysis of indepen
dent polymorphic restriction sites in humans reveals a decrease in effectiv
e size. This is consistent with a change in the rates of migration among hu
man subpopulations from ancient low levels to present high ones.