Studies of the volatility of the U.S. economy suggest a noticeable change in mid 1980s. There is some empirical evidence that the aggregate volatility of the U.S. economy has been decreasing over time. The response of firms to the change of economic volatility and economic fluctuation has been studied in terms of many margins a firm can adjust -capital, labor, capacity, material, etc. However, we have not studied the most important margin - the product.

My dissertation studies the effect of profit volatility on the firm/plant level product diversification. Chapter 2 profiles diversification and shows that there is a downward trend of aggregate diversification in many industries. Cyclicality of diversification is not clear at the aggregate or industry level. Firms change their diversification very frequently and very differently from one another. Chapter 3 verifies the trend of volatility at the aggregate, sectoral, and firm level and studies the relationship between diversification and volatility at the firm level. Firm level diversification decreases as the aggregate, sectoral and idiosyncratic volatility decreases.

Research on the volatility change is concentrated on recent U.S. history. However, new data allow us to study events of significant volatility change in early 19th century. Chapter 4 of my dissertation studies the causes and effects of the volatility change in early 19th century in U.S. and U.K.