Is there a correlation between productivity and product variety? Certainly it appears that the rich countries are more productive and have more product variety than the poor nations. In fact, the relationship is quite strong when measured in levels. Does this same correlation hold up when measured in growth rates? If so, can poor countries imitate the success of the rich? Addison provides theoretical and empirical reasons to believe the answer to both questions is yes. Recent economic theory suggests that rising variety in factor inputs can help avoid diminishing marginal returns. Product variety can also sustain learning-by-doing which would otherwise be exhausted in a fixed number of products. Finally, invention or imitation adds to the stock of non-rival knowledge. There have been only two previous empirical tests of the correlation between growth in product variety and productivity growth. Both were affirmative but neither examined a wide range of developing countries and neither looked deeper to test what might drive product variety. This research is based on a cross-country sample of 29 countries (13 rich and 16 poor). The data display a statistically significant and positive relationship between growth in product variety and productivity growth when condition on other variables such as research and development (R&D) employment, macroeconomic stability, and domestic security. These results are robust to the addition and subtraction of various explanatory variables but fragile with respect to an influential data point for Venezuela. Industrial nations tend to generate most of their productivity gains through R&D employment in a stable environment that results in better production processes and product quality. In contrast, the largest source of productivity growth in developing countries is product variety imitation while instability takes away from productivity. Addison tests various explanations for growth in variety. The results show that nations furthest from the frontier of observable variety tend to imitate fastest, with the ability to imitate being improved by educational attainment and by productivity gains. This could be a source of hope for small, less developed nations. Growth in market size was not correlated with growth in variety, though this may be due to a rather short sample period of only eight years. In addition to the empirical testing, Addison also contributes to a general discussion of measurement concepts and measurement issues related to product variety and sets out an agenda for further research.