This article extends the recent literature on the Prebisch–Singer hypothesis of a long-run decline in the

relative prices of primary commodities. Our main innovation is testing for and estimating nonlinear

alternatives to a secular deterioration. Specifically, we use bootstrap procedures to test the linear unit

root model against models belonging to the family of smooth transition autoregressions (STARs) for

twenty-four commodities, 1900–2003. In nineteen cases we reject the linear null at usual significance

levels. In sixteen cases we are able to successfully fit STAR-type models. Simulation results show there

is little support for the Prebisch–Singer hypothesis.