Great achievements in knowledge are produced by older innovators today than they were a century ago. Using data on Nobel Prize winners and great inventors, I find that the mean age at which noted innovations are produced has increased by 6 years over the 20th Century. I estimate shifts in life-cycle productivity and show that innovators have become especially unproductive at younger ages. Meanwhile, the later start to the career is not compensated for by increasing productivity beyond early middle age. I further show that the early life-cycle dynamics are closely related to variation in the age at Ph.D. and discuss a theory where accumulations of knowledge across generations lead innovators to seek more education over time. More generally, the results show that individual innnovators are productive over a narrowing span of their life-cycle, a trend that reduces, other things equal, the aggregate output of innovators. This drop in productivity is particularly acute if innovators’ raw ability is greatest when young.

Assassinations are a persistent feature of the political landscape. Using a new data set of assassination attempts on all world leaders from 1875 to 2004, we exploit inherent randomness in the success or failure of assassination attempts to identify assassination’s effects. We find that, on average, successful assassinations of autocrats produce sustained moves toward democracy. We also find that assassinations affect the intensity of small-scale conflicts. The results document a contemporary source of institutional change, inform theories of conflict, and show that small sources of randomness can have a pronounced effect on history.

A response by Stefan Wuchty to a letter to the editor about the mechanisms and interpretations of letters is presented.

This paper presents novel analysis of the relationship between temperature and income. First, using sub-national data from 12 countries in the Americas, we provide new evidence that the negative cross-country relationship between temperature and income also exists within countries and even within states. Second, we provide a theoretical framework for reconciling the substantial, negative association between temperature and income in the cross-section with the even stronger short-run effects of temperature estimated by panel models. The theoretical framework suggests that half of the negative short-term effects of temperature may be offset in the long run through adaptation.

This paper investigates the remarkable extremes of growth experiences within countries and examines the changes that occur when growth starts and stops. We find two main results. First, virtually all but the very richest countries experience both growth miracles and failures over substantial periods. Second, growth accelerations and collapses are asymmetric phenomena. Collapses typically feature reduced investment amidst increasing price instability, whereas growth takeoffs are primarily associated with large and steady expansions in international trade. The results show that even very poor countries regularly grow rapidly, but sustaining growth is difficult and may pose a very different set of challenges than starting it.

This paper investigates a possibly fundamental aspect of technological progress. If knowledge accumulates as technology advances, then successive generations of innova- tors may face an increasing educational burden. Innovators can compensate through lengthening educational phases and narrowing expertise, but these responses come at the cost of reducing individual innovative capacities, with implications for the organization of innovative activity - a greater reliance on teamwork - and negative implications for growth. Building on this "burden of knowledge" mechanism, this paper first presents six facts about innovator behavior. I show that age at first invention, specialization, and teamwork increase over time in a large micro-data set of inventors. Furthermore, in cross-section, specialization and teamwork appear greater in deeper areas of knowledge while, surprisingly, age at first invention shows little variation across fields. A model then demonstrates how these facts can emerge in tandem. The theory further develops explicit implications for economic growth, providing an explanation for why productivity growth rates did not accelerate through the 20th century despite an enor- mous expansion in collective research e¤ort. Upward trends in academic collaboration and lengthening doctorates, which have been noted in other research, can also be ex- plained in this framework. The knowledge burden mechanism suggests that the nature of innovation is changing, with negative implications for long-run economic growth.

We have used 19.9 million papers over 5 decades and 2.1 million patents to demonstrate that teams increasingly dominate solo authors in the production of knowledge. Research is increasingly done in teams across virtually all fields. Teams typically produce more highly cited research than individuals do, and this advantage is increasing over time. Teams now also produce the exceptionally high impact research, even where that distinction was once the domain of solo authors. These results are detailed for the sciences and engineering, social sciences, arts and humanities, and patents, suggesting that the process of knowledge creation has fundamentally changed.

Economic growth within countries varies sharply across decades. This paper examines one explanation for these sustained shifts in growth—changes in the national leader. We use deaths of leaders while in office as a source of exogenous variation in leadership, and ask whether these plausibly exogenous leadership transitions are associated with shifts in country growth rates. We find robust evidence that leaders matter for growth. The results suggest that the effects of individual leaders are strongest in autocratic settings where there are fewer constraints on a leader’s power. Leaders also appear to affect policy outcomes, particularly monetary policy. The results suggest that individual leaders can play crucial roles in shaping the growth of nations.

This paper uses annual variation in climate to examine the impact of temperature and precipitation on national economies. We find three primary results. First, higher temperatures substantially reduce economic growth in poor countries. Second, higher temperatures appear to reduce growth rates, not just the level of output. Third, higher temperatures have wide-ranging effects, reducing agricultural and industrial output, investment, innovation, and political stability. Decade or longer increases in temperature also show substantial negative effects on poor countries’ growth. These findings inform debates over climate’s role in economic development and suggest substantial negative impacts of future climate change on poor countries.

This paper presents a model where human capital differences - rather than technology differences - can explain several central phenomena in the world economy. The results follow from the educational choices of workers, who decide not just how long to train, but also how broadly. A "knowledge trap" occurs in economies where skilled workers favor broad but shallow knowledge. This simple idea can inform cross-country income differences, international trade patterns, poverty traps, and price and wage differences across countries in a manner broadly consistent with existing empirical evidence. The model also provides insights about the brain drain, migration, and the role for multinationals in development. More generally, this paper shows that standard human capital accounting methods can severely underestimate the role of education in development. It shows how endogenous educational decisions can replace exogenous technology differences in a range of economic reasoning.