Robert L. Axtell

Bio: Robert L. Axtell is an academic researcher from George Mason University. The author has contributed to research in topics: Population & Medicine. The author has an hindex of 26, co-authored 65 publications receiving 7677 citations. Previous affiliations of Robert L. Axtell include The Turing Institute & Santa Fe Institute.

Zipf distribution of U.S. firm sizes.

Abstract: Analyses of firm sizes have historically used data that included limited samples of small firms, data typically described by lognormal distributions. Using data on the entire population of tax-paying firms in the United States, I show here that the Zipf distribution characterizes firm sizes: the probability a firm is larger than size s is inversely proportional to s. These results hold for data from multiple years and for various definitions of firm size.

Growing Artificial Societies: Social Science from the Bottom Up

Abstract: How do social structures and group behaviors arise from the interaction of individuals? Growing Artificial Societies approaches this question with cutting-edge computer simulation techniques. Fundamental collective behaviors such as group formation, cultural transmission, combat, and trade are seen to "emerge" from the interaction of individual agents following a few simple rules. In their program, named Sugarscape, Epstein and Axtell begin the development of a "bottom up" social science that is capturing the attention of researchers and commentators alike. The study is part of the 2050 Project, a joint venture of the Santa Fe Institute, the World Resources Institute, and the Brookings Institution. The project is an international effort to identify conditions for a sustainable global system in the next century and to design policies to help achieve such a system.

Growing Artificial Societies: Social Science from the Bottom Up

Abstract: Part I Introduction: "Artificial Society" models life and death on the sugarscape sex, culture and conflict - the emergence of history sugar and spice - trade comes to the sugarscape disease agents a society is born artificial societies versus traditional models artificial societies versus a life toward generative social science - can you grow it?. Part II Life and death on the sugarscape: in the beginning - there was sugar the agents artificial society on the sugarscape wealth and its distribution in the agent population social networks of neighbours migration summary. Part III Sex, culture and conflict - the emergence of history: sexual reproduction cultural processes combat the proto-history. Part IV Sugar and spice - trade comes to the sugarscape: spice - a second commodity trade rules markets of bilateral traders emergent economic networks social computation, emergent computation summary and conclusions. Part V Disease processes: models of disease transmission and immune response immune system response disease transmission digital diseases on the sugarscape disease transmission networks. Part VI Conclusions: summary some extensions of the current model other artificial societies formal analysis of artificial societies generative social science looking ahead. Appendices: software engineering aspects of artificial societies summary of rule notation state-dependence on the welfare function.

Aligning Simulation Models: A Case Study and Results

Abstract: This paper develops the concepts and methods of a process we will call "alignment of computational models" or "docking" for short. Alignment is needed to determine whether two models can produce the same results, which in turn is the basis for critical experiments and for tests of whether one model can subsume another. We illustrate our concepts and methods using as a target a model of cultural transmission built by Axelrod. For comparison we use the Sugarscape model developed by Epstein and Axtell. The two models differ in many ways and, to date, have been employed with quite different alms. The Axelrod model has been used principally for intensive experimentation with parameter variation, and includes only one mechanism. In contrast, the Sugarscape model has been used primarily to generate rich "artificial histories," scenarios that display stylized facts of interest, such as cultural differentiation d0ven by many different mechansims including resource availability, migration, trade, and combat. The Sugarscape model was modified so as to reproduce the results of the Axelrod cultural model. Among the questions we address are: what does it mean for two models to be equivalent, how can different standards of equivalence be statistically evaluated, and how do subtle differences in model design affect the results? After attaining a "docking" of the two models, the richer set of mechanisms of the Sugarscape model is used to provide two experiments in sensitivity analysis for the cultural rule of Axelrod's model. Our generally positive experience in this enterprise has suggested that it could be beneficial if alignment and equivalence testing were more widely practiced among computational modelers.

Thinking fast and slow.

Abstract: In 1974 an article appeared in Science magazine with the dry-sounding title “Judgment Under Uncertainty: Heuristics and Biases” by a pair of psychologists who were not well known outside their discipline of decision theory. In it Amos Tversky and Daniel Kahneman introduced the world to Prospect Theory, which mapped out how humans actually behave when faced with decisions about gains and losses, in contrast to how economists assumed that people behave. Prospect Theory turned Economics on its head by demonstrating through a series of ingenious experiments that people are much more concerned with losses than they are with gains, and that framing a choice from one perspective or the other will result in decisions that are exactly the opposite of each other, even if the outcomes are monetarily the same. Prospect Theory led cognitive psychology in a new direction that began to uncover other human biases in thinking that are probably not learned but are part of our brain’s wiring.

Agent-based modeling: Methods and techniques for simulating human systems

Abstract: Agent-based modeling is a powerful simulation modeling technique that has seen a number of applications in the last few years, including applications to real-world business problems. After the basic principles of agent-based simulation are briefly introduced, its four areas of application are discussed by using real-world applications: flow simulation, organizational simulation, market simulation, and diffusion simulation. For each category, one or several business applications are described and analyzed.

Statistical physics of social dynamics

Abstract: Statistical physics has proven to be a fruitful framework to describe phenomena outside the realm of traditional physics. Recent years have witnessed an attempt by physicists to study collective phenomena emerging from the interactions of individuals as elementary units in social structures. A wide list of topics are reviewed ranging from opinion and cultural and language dynamics to crowd behavior, hierarchy formation, human dynamics, and social spreading. The connections between these problems and other, more traditional, topics of statistical physics are highlighted. Comparison of model results with empirical data from social systems are also emphasized.

Export Versus FDI with Heterogeneous Firms

Abstract: Multinational sales have grown at high rates over the last two decades, outpacing the remarkable expansion of trade in manufactures. Consequently, the trade literature has sought to incorporate the mode of foreign market access into the “new” trade theory. This literature recognizes that Ž rms can serve foreign buyers through a variety of channels: they can export their products to foreign customers, serve them through foreign subsidiaries, or license foreign Ž rms to produce their products. Our work focuses on the Ž rm’s choice between exports and “horizontal” foreign direct investment (FDI). Horizontal FDI refers to an investment in a foreign production facility that is designed to serve customers in the foreign market. Firms invest abroad when the gains from avoiding trade costs outweigh the costs of maintaining capacity in multiple markets. This is known as the proximity-concentration tradeoff. We introduce heterogeneous Ž rms into a simple multicountry, multisector model, in which Ž rms face a proximity-concentration trade-off. Every Ž rm decides whether to serve a foreign market, and whether to do so through exports or local subsidiary sales. These modes of market access have different relative costs: exporting involves lower Ž xed costs while FDI involves lower variable costs. Our model highlights the important role of within-sector Ž rm productivity differences in explaining the structure of international trade and investment. First, only the most productive Ž rms engage in foreign activities. This result mirrors other Ž ndings on Ž rm heterogeneity and trade; in particular, the results reported in Melitz (2003). Second, of those Ž rms that serve foreign markets, only the most productive engage in FDI. Third, FDI sales relative to exports are larger in sectors with more Ž rm heterogeneity. Using U.S. exports and afŽ liate sales data that cover 52 manufacturing sectors and 38 countries, we show that cross-sectoral differences in Ž rm heterogeneity predict the composition of trade and investment in the manner suggested by our model. We construct several measures of Ž rm heterogeneity, using different data sources, and show that our results are robust across all these measures. In addition, we conŽ rm the predictions of the proximityconcentration trade-off. That is, Ž rms tend to substitute FDI sales for exports when transport * Helpman: Department of Economics, Harvard University, Cambridge, MA 02138, Tel Aviv University, and CIAR (e-mail: ehelpman@harvard.edu); Melitz: Department of Economics, Harvard University, Cambridge, MA 02138, National Bureau of Economic Research, and Centre for Economic Policy Research (e-mail: mmelitz@ harvard.edu); Yeaple: Department of Economics, University of Pennsylvania, 3718 Locust Walk, Philadelphia, PA 19104, and National Bureau of Economic Research (e-mail: snyeapl2@ssc.upenn.edu). The statistical analysis of Ž rmlevel data on U.S. Multinational Corporations reported in this study was conducted at the International Investment Division, U.S. Bureau of Economic Analysis, under an arrangement that maintained legal conŽ dentiality requirements. Views expressed are those of the authors and do not necessarily re ect those of the Bureau of Economic Analysis. Elhanan Helpman thanks the NSF for Ž nancial support. We also thank Daron Acemoglu, Roberto Rigobon, Yona Rubinstein, and Dani Tsiddon for comments on an earlier draft, and Man-Keung Tang for excellent research assistance. 1 See Wilfred J. Ethier (1986), Ignatius Horstmann and James R. Markusen (1987), and Ethier and Markusen (1996) for models that incorporate the licensing alternative. We therefore exclude “vertical” motives for FDI that involve fragmentation of production across countries. See Helpman (1984, 1985), Markusen (2002, Ch. 9), and Gordon H. Hanson et al. (2002) for treatments of this form of FDI. 3 See, for example, Horstmann and Markusen (1992), S. Lael Brainard (1993), and Markusen and Anthony J. Venables (2000). 4 See also Andrew B. Bernard et al. (2003) for an alternative theoretical model and Yeaple (2003a) for a model based on worker-skill heterogeneity. James R. Tybout (2003) surveys the recent micro-level evidence on trade that has motivated these theoretical models. 5 This result is loosely connected to the documented empirical pattern that foreign-owned afŽ liates are more productive than domestically owned producers. See Mark E. Doms and J. Bradford Jensen (1998) for the United States and Sourafel Girma et al. (2002) for the United Kingdom.

Collective Action and the Evolution of Social Norms

Abstract: With the publication of The Logic of Collective Action in 1965, Mancur Olson challenged a cherished foundation of modern democratic thought that groups would tend to form and take collective action...