  Unbiased models are foundational in the archaeological study of cultural
transmission. Applications have as- sumed that archaeological data represent
synchronic samples, despite the accretional nature of the archaeological
record. I document the circumstances under which time-averaging alters the
distribution of model predictions. Richness is inflated in long-duration
assemblages, and evenness is "flattened" compared to unaveraged samples. Tests
of neutrality, employed to differentiate biased and unbiased models, suffer
serious problems with Type I error under time-averaging. Finally, the
time-scale over which time-averaging alters predictions is determined by the
mean trait lifetime, providing a way to evaluate the impact of these effects
upon archaeological samples.
