Jörgen Weibull

The accumulation of individual fitness or wealth is modelled as a population game in which pairs of individuals are recurrently and randomly matched to play a game over a resource. In addition, all individuals have random access to a constant background resource, and their fitness or wealth depreciates over time. For brevity we focus on the well-known Hawk-Dove game. In the base-line model, the probability of winning a fight (that is, when both play Hawk) is the same for both parties. In an extended version, the individual with higher current fitness or wealth has a higher probability of winning. Analytical results are given for the fitness/wealth distribution at any given time, for the evolution of average fitness/wealth over time, and for the asymptotics with respect to time and population size. Long-run average fitness/wealth is non-monotonic in the value of the resource, thus providing a potential explanation of the curse of the riches.

Individual and national wealth accumulation is here modelled as a recurrently played game between randomly matched pairs of individuals from a large population. The simple game here studied represents exogenously and spontaneously arising productive opportunities, and the drawn individuals may seek cooperation or conflict over each opportunity. How does national wealth and the evolutionarily stable propensity to cooperate depend on the natural and institutional environment? We show that the steady-state level of national wealth is not monotonically increasing in the richness of the environment. We also study the evolution of the full wealth distribution. When the population is large, the distribution of individual wealth converges over time to a sqewed distribution that is well approximated by a Weibull distribution.

What preferences will prevail in a society of rational individuals when preference evolution is driven by their success in terms of resulting payo§s? We show that when
individualsí preferences are their private information, a convex combinations of selfishness and morality stand out as evolutionarily stable. We call individuals with such
preferences homo moralis. At one end of the spectrum is homo oeconomicus, who acts so as to maximize his or her material payo§. At the opposite end is homo kantiensis, who does what would be ìthe right thing to do,î in terms of material payo§s, if all others would do likewise. We show that the stable degree of morality - the weight placed on the moral goal - equals the index of assortativity in the matching process. The motivation of homo moralis is arguably compatible with how people often reason, and the induced behavior appear to agree with pro-social behaviors observed in many laboratory experiments.

We develop a mathematical model of obesity in which individuals value consumption but also have a concern for their body weight, a concern that may be inuenced by peers and that may be hampered by a lack of self-control. Our model is thus focused on the interplay between economic, social and psychological factors. It is general but yet tractable enough to permit analysis of a range of factors that have been put forward as relevant to obesity. The model sheds light on stylized facts about the obesity epidemic of the last thirty years and can be used to simulate policy effects and as a workhorse in theoretical and empirical research of obesity.

We explore whether competition between firms owned and run by entrepreneurs favors overconfident entrepreneurs. We study this question in a variety of settings, all based on Cournot duopoly in the product market. In the basic model, entrepreneurs choose their own firm’s output and may have more or less optimistic beliefs about their own firm’s (random) production costs. We study both the case of complete and incomplete information about the competitor’s type. We also analyze a model with endogenous costs in the complete-information setting in which entrepreneurs make efforts to reduce their firm’s production costs. For each of the model versions, we show that, if market selection is driven by firms’ absolute and/or relative profit performance, somewhat overconfident entrepreneurs will be selected for, and that this tendency is stronger the more emphasis is placed on relative performance.