This paper proposes a simple method for guiding researchers in developing quantitative models of economic fluctuations. We show that a large class of models, including models with various frictions, are equivalent to a prototype growth model with time varying wedges that, at least on face value, look like time-varying productivity, labor taxes, and capital income taxes. We label the time varying wedges as efficiency wedges, labor wedges, and investment wedges. We use data to measure these wedges and then feed them back into the prototype growth model. We then assess the fraction of fluctuations accounted for by these wedges during the great depressions of the 1930s in the United States, Germany, and Canada. We find that the efficiency and labor wedges in combination account for essentially all of the declines and subsequent recoveries. Investment wedge plays at best a minor role.

We look for the scale effects on growth predicted by some theories of trade and growth based on dynamic returns to scale at the national or industry level. The increasing returns can arise from learning by doing, investment in human capital, research and development, or development of new products. We find some evidence of a relation between growth rates and the measures of scale implied by the learning by doing theory, especially total manufacturing. With respect to human capital, there is some evidence of a relation between growth rates and per capita measures of inputs into the human capital accumulation process, but little evidence of a relation with the scale of inputs. There is also little evidence that growth rates are related to measures of inputs into R&D. We find, however, that growth rates are related to measures of intra-industry trade, particularly when we control for scale of industry.