Interacting agents in finance, entry written for the New Palgrave Dictionary of Economics, Second Edition, edited by L. Blume and S. Durlauf, Palgrave Macmillan, forthcoming 2006.

Interacting agents in finance represent a behavioral, agent-based approach in which financial markets are viewed as complex adaptive systems consisting of many boundedly rational agents interacting through simple heterogeneous investment strategies, constantly adapting their behavior in response to new information, strategy performance and through social interactions. An interacting agent system acts as a noise filter, transforming and amplifying purely random news about economic fundamentals into an aggregate market outcome exhibiting important stylized facts such as unpredictable asset prices and returns, excess volatility, temporary bubbles and sudden crashes, large and persistent trading volume, clustered volatility and long memory.

Heterogeneous Agents Models: two simple examples, forthcoming In: Lines, M. (ed.) Nonlinear Dynamical Systems in Economics, CISM Courses and Lectures, Springer, 2005, pp.131-164.

These notes review two simple heterogeneous agent models in economics and finance. The first is a cobweb model with rational versus naive agents introduced in Brock and Hommes (1997). The second is an asset pricing model with fundamentalists versus technical traders introduced in Brock and Hommes (1998). Agents are boundedly rational and switch between different trading strategies, based upon an evolutionary fitness measure given by realized past profits. Evolutionary switching creates a nonlinearity in the dynamics. Rational routes to randomness, that is, bifurcation routes to complicated dynamical behaviour occur when agents become more sensitive to differences in evolutionary fitness.

Economic Dynamics, Contribution to the Encyclopedia of Nonlinear Science, Alwyn Scott (ed.), Routledge, 2004.

Contribution to the Encyclopedia of Nonlinear Science, Alwyn Scott (ed.), Routledge, 2005, pp.245-248.

Evolution and Rationality of Budget Institutions in Colombia

A research project conducted by the Office of the Chief Economist at the Inter-American Development Bank has found preliminary but compelling evidence that the budgetary institutions in Latin America have a significant bearing on fiscal results. The researchers have developed an index of budget institutional development, which measures the strength of budget institutions. The results suggest that strong institutions have an impact on fiscal performance.

Heterogeneous Agent Models in Economics and Finance, In: Handbook of Computational Economics II: Agent-Based Computational Economics, edited by Leigh Tesfatsion and Ken Judd , Elsevier, Amsterdam 2006, pp.1109-1186.

This chapter surveys work on dynamic heterogeneous agent models (HAMs) in economics and finance. Emphasis is given to simple models that, at least to some extent, are tractable by analytic methods in combination with computational tools. Most of these models are behavioral models with boundedly rational agents using different heuristics or rule of thumb strategies that may not be perfect, but perform reasonably well. Typically these models are highly nonlinear, e.g. due to evolutionary switching between strategies, and exhibit a wide range of dynamical behavior ranging from a unique stable steady state to complex, chaotic dynamics. Aggregation of simple interactions at the micro level may generate sophisticated structure at the macro level. Simple HAMs can explain important observed stylized facts in financial time series, such as excess volatility, high trading volume, temporary bubbles and trend following, sudden crashes and mean reversion, clustered volatility and fat tails in the returns distribution.

Complexity, Evolution and Learning: a simple story of heterogeneous expectations and some empirical and experimental validation.

This note discusses complexity models in economics. A key feature of these models is that agents have heterogeneous expectations, disciplined by adaptive learning and evolutionary selection. Agents adapt their rules based upon past observations and switch between different forecasting heuristics based upon strategy performance. We discuss how these models match empirical facts as well as laboratory experiments with human subjects and how this approach may tame the ``wilderness of bounded rationality''.

Health and welfare of organic livestock

Animal health and welfare are important principles of organic animal husbandry. In the Netherlands organic animal husbandry has proven to perform better than the conventional sector on many aspects of animal welfare. The Dutch organic animal husbandry sector has recognised animal health and welfare as crucial and maintains a strong focus on further improvement. Dutch scientists are working to advance the health and welfare of organically kept animals. Additionally, they focus on innovations and directly applicable solutions to specific problems concerning diseases and behaviour. This research is often carried out in close collaboration with farmer

Prevention and control of weeds, pests and diseases

Weed control still requires major investments of money and labour in organic arable farming and field vegetable cultivation. For this reason, current research is focused to a large extent on the development of weed control strategies. These incorporate prevention as well as mechanical methods, and cover a range of approaches. From inexpensive techniques to very innovative technologies. Research into pests and diseases primarily focuses on improving systems and preventing infestation. However, even with maximum prevention measures some pests and diseases still cause unacceptable damage. For these residual problems control measures are being develope

Bounded Rationality and Learning in Complex Markets

This chapter reviews some work on bounded rationality, expectation formation and learning in complex markets, using the familiar demand-supply cobweb model. We emphasize two stories of bounded rationality, one story of adaptive learning and another story of evolutionary selection. According to the adaptive learning story agents are identical, and can be represented by an ``average agent'', who adapts his behavior trying to learn an optimal rule within a class of simple (e.g. linear) rules. The second story is concerned with heterogeneous, interacting agents and evolutionary selection of different forecasting rules. Agents can choose between costly sophisticated forecasting strategies, such as rational expectations, and freely available simple strategies, such as naive expectations, based upon their past performance. We also confront both stories to laboratory experiments on expectation formation. At the end of the chapter, we integrate both stories and consider an economy with evolutionary selection between a costly sophisticated adaptive learning rule and a cheap simple forecasting rule such as naive expectations.

People and society

Organic agriculture has excellent opportunities to create strong links between the environment it operates in, the people who live there and local nature and landscape. The Dutch organic sector aspires to strengthen these links and it is already well on its way. Together with researchers and stakeholders new concepts are being developed and put into practic