The Role of Choice in Social Dilemma Experiments

On the basis of problems related to asymmetric information, self-governance has been proposed and often empirically found to be superior to the external imposition of rules in social dilemma situations. The present paper suggests and experimentally analyses a different line of argument, namely to what extent behavioral aspects can explain these findings. We study this hypothesis using the simplest, most general dilemma form: the prisoner's dilemma (PD). We compare behavior when players are given the possibility of choosing between two different representations of the same PD, to behavior when players are externally assigned to play a specific game. We find that cooperation rates are significantly higher in the games that were chosen.Freedom of Choice, Self-governance, Social Dilemmas, Framing

The Role of Choice in Social Dilemma Experiments

On the basis of problems related to asymmetric information, self-governance has been proposed and often empirically found to be superior to the external imposition of rules in social dilemma situations. The present paper suggests and experimentally analyses a different line of argument, namely to what extent behavioral aspects can explain these findings. We study this hypothesis using the simplest, most general dilemma form: the prisoner’s dilemma (PD). We compare behavior when players are given the possibility of choosing between two different representations of the same PD, to behavior when players are externally assigned to play a specific game. We find that cooperation rates are significantly higher in the games that were chosen.Freedom of Choice, Self-governance, Social Dilemmas, Framing

The Role of Rivalry. Public Goods versus Common-Pool Resources

Despite a large theoretical and empirical literature on public goods and common-pool resources, a systematic comparison of these two types of social dilemmas is lacking. In fact, there is considerable confusion about these two types of dilemma situations. As a result, they are often treated alike. In this paper we argue that the degree of rivalry is the fundamental difference between the two games. We show that rivalry implies that both games cannot be represented by the same game theoretic structure. Fur-thermore, we experimentally study behavior in a quadratic public good and a quadratic common-pool resource game with identical Pareto opti-mum but divergent interior Nash equilibria. The results show that partici-pants clearly perceive the differences in rivalry. Aggregate behavior in both games starts relatively close to Pareto efficiency and converges to the respective Nash equilibrium.

Ostracism and the Provision of a Public Good, Experimental Evidence

We analyze the effects of ostracism on cooperation in a linear public good experiment. Our results show that introducing ostracism increases contributions. Despite reductions in group size due to ostracism, the net effect on earnings is positive and significant.Experiment, Public Good, Ostracism

Cooperation in Symmetric and Asymmetric Prisoner's Dilemma Games

We experimentally study the effect of asymmetry on cooperation in a 40 period prisoner's dilemma game in fixed partner design. We distinguish between a high and low payoff symmetric prisoner's dilemma and an asymmetric game combined out of both symmetric ones. Asymmetry significantly decreases cooperation, as low-type players are more likely to defect after mutual cooperation while high-type players initiate cooperation more often than the former. Asymmetry also has a significant negative effect on the stability of cooperation rendering long sequences of mutual cooperation extremely rare.Symmetry, Asymmetry, Prisoner's Dilemma, Experiments

Four Contributions to Experimental Economics

In the following four chapters, distinct but related experimental research analyzing strategic and non-strategic economic behavior is presented. Chapter 2, entitled "Cooperation in Symmetric and Asymmetric Prisoner's Dilemma Games" is a systematic study of behavior in symmetric and asymmetric prisoner's dilemma games. The prisoner's dilemma is one of the key models in many disciplines for now over five decades. Previous prisoner's dilemma experiments show that in contrast to theoretical predictions, cooperation rates are generally very high in the symmetric payoff variant of the game. Chapter 2 studies cooperation in the prisoner's dilemma in a more realistic scenario by systematically analyzing the effects of asymmetric payoffs. Already in the early nineties Murnighan et al. (1990) noted that "research has been inexplicably absent on the effects of asymmetry". The present study takes this concern into account and focuses on this much broader type of conflict expanding the limited and rather unsystematic research conducted in this area. It analyzes and discusses the effect of asymmetry on cooperation in a 40 period prisoner's dilemma game in fixed partner design. A distinction is made between a high and low payoff symmetric prisoner's dilemma on the one hand and an asymmetric game combined out of both symmetric ones on the other hand. Asymmetry significantly decreases cooperation, as low-type players are more likely to defect after mutual cooperation while high-type players initiate cooperation more often than the former. Asymmetry also has a significant negative effect on the stability of cooperation rendering long sequences of mutual cooperation extremely rare. These results are not only a valuable addition to the existing (mostly symmetric) prisoner's dilemma literature but are also of relevance for understanding reciprocity, equity and fairness especially in light of recent theoretical developments based exclusively on symmetric experimental games. Chapter 3, entitled "Assignment versus Choice in Prisoner's Dilemma Experiments'' compares behavior in a repeated prisoner's dilemma game when players can choose between two different representations of the same prisoner's dilemma, to behavior when players are assigned to play a specific game. The chapter is concerned with the methodological question of the external validity of experimental research based on the assignment of participants to experimental games or decision situations. Experimental findings may systematically misrepresent field outcomes if assigning participants to experiments has an impact on the decisions made by the participants in the experiment and if such an assignment does not occur in the field. The chapter therefore analyzes to what extent experimental deviations from actual situations due to the assignment of participants is based exclusively on the possibility of self-selection or sorting, or whether choice has an important behavioral effect in itself. The chapter extends the results obtained in the experimental psychology literature by analyzing whether choice effects are also found in strategic contexts, rendering them of particular interest to economic environments. Based on the idea that choice either via active modification of the strategic environment or by passive self-selection into a particular strategic environment may be an important property of many empirical problems studied using experimental methods, the research goal is to separate a choice effect from sorting or self-selection effects. The experimental results clearly indicate that the mere fact that participants can choose the game they want to play has a statistically significant impact on behavior. Cooperation rates are up to 60% higher in the games that were not assigned to but chosen by participants. These findings are consistent with the robust evidence of the psychology literature on non-strategic contexts that choice increases motivation, trust, and performance. Given that in many contexts agents choose the strategic situation they get involved in, assigning participants to experiments may affect the external validity of some experimental findings. Chapter 4, entitled "The Role of Rivalry - Public Goods versus Common-Pool Resources" moves from the 2 person prisoner's dilemma game structure to the analysis of behavior in a 4 person quadratic public good and a quadratic common-pool resource game. Despite a large theoretical and empirical literature on public goods and common-pool resources, a systematic comparison of these two types of social dilemmas is lacking. In fact, there is some confusion about these two types of dilemma situations. As a result, they are often treated alike. An explicit example of this is provided by Gintis (2000) who argues that while "common pool resource and public goods games are equivalent for Homo Oeconomicus, people treat them quite differently in practice. This is because the status quo in the public goods game is the individual keeping all the money in the private account, while the status quo in the common pool resource game is that the resource is not being used at all." In line with the theoretical literature, the chapter first establishes theoretically that public good and common-pool resource games as used in the experimental literature are two distinct types of social dilemmas, the fundamental difference between the two games being the degree of rivalry. It is shown that the distinguishing feature of these two types of games lies in the distributional factor that determines whether the good is rival or non-rival. This difference gives rise to two distinct strategic environments. Based on these theoretical differences an experiment is devised that tests whether the theoretical differences have an impact on behavior. The results show that participants clearly respond to the differences in rivalry. Aggregate behavior in both games starts relatively close to Pareto efficiency and converges quickly to the respective Nash equilibrium. This clearly indicates that the differences in rivalry affect behavior, strengthening the importance of differentiating between the two types of games. Despite this difference reflecting the structure of the two games, there appear to be some behavioral similarities. In both games, aggregate behavior starts in the neighborhood of the Pareto optimum and moves rather quickly to the respective aggregate Nash equilibrium. Chapter 5 entitled "Purchase Decisions with Non-linear Pricing Options under Risk" moves away from a strategic game setting to an analysis of decisions under risk. The chapter reports on an experimental investigation of purchase decisions with linear and non-linear pricing under risk. Standard economic theory suggests that customers should be indifferent to the format of a price reduction. In particular this implies that one would expect a customer to switch from one pricing scheme to another (one supplier to another) as long as there is at least an expected reduction in the effective purchase price. The recent surge in the use of rebates, discounts, bonus and point schemes implemented by retailers but also observed in other levels of the production chain begs the question of whether traditional economic explanations do fully account for the increased usage of non-linear pricing methods. An understanding of potential behavioral reasons for using such pricing schemes - as presented in this chapter - may not only be relevant for their design, but also for wider policy considerations. The experiment presented is based on a single period stochastic inventory problem with endogenous cost. It extends classic binary lottery experiments to test standard decision theoretic predictions concerning purchasing behavior in a rebate and a discount scheme. The question to what extent customers continue to purchase under two mathematically isomorph formats of non-linear schemes even if switching to a linear pricing scheme is optimal is investigated. The results indicate that rebate and discount schemes exert a statistically significant attraction on customers. Given the increased role of non-linear pricing schemes, systematic deviations from optimal behavior are an important element in the design of such schemes and may raise consumer protection and competition policy issues. The chapter concludes with a discussion on how the results can be explained by decision heuristics.</p

The spread of parasitic sex factors in populations of Armadillidium vulgare Latr (Crustacea, Oniscidea): effects on sex ratio

International audienc

Le marais doux endigué de Bourgneuf-Machecoul (Pays de Loire) Premier éléments de connaissance du peuplement piscicole. Relation ichtyofaune-habitat et problèmes majeurs de gestion (Maroc)

Le peuplement ichtyologique d'un marais littoral endigué, géré en eau douce, a été étudié sur une zone de 2 700 ha, située au nord du marais Breton-Vendéen (Loire-Atlantique, France). Le réseau hydraulique, qui représente près de 16 % de cette surface, se compose, d'environ 91 m de linéaire de fossés par ha (en tout 234 km) et de bassins présents uniquement dans la partie d'origine salicole du marais. Les hauteurs d'eau, l'envasement et le recouvrement par la végétation aquatique dépendent de la gestion humaine et sont très variables (moyennes respectives : 42 cm; 43 cm; 70 %). Cela se traduit par un morcellement spatial de l'habitat pour les poissons. La stratégie d'échantillonnage adoptée, qui tient compte de cette hétérogénéité, a permis de décrire un peuplement comportant 21 espèces. Dominé par les poissons-chats et par les anguilles, ce dernier est caractéristique de la zone à brèmes des cours d'eau. Les abondances sont relativement élevées (en moyenne 315 kg/ha et 11 460 poissons/ha), mais elles sont très hétérogènes. L'évolution qualitative et quantitative de la répartition spatio-temporelle est décrite à l'aide d'une analyse factorielle des correspondances portant sur 74 échantillons prélevés par pêche électrique entre 1987 et 1989. Bien que l'approche de ce milieu soit complexe et les références bibliographiques relativement rares, l'analyse des premières données permet d'ores et déjà d'identifier quelques problèmes de gestion ayant des répercussions directes sur le peuplement piscicole de cette zone.The dammed up marshes of the French Atlantic coast cover about 200 000 ha between River Vilaine and the « Bassin d'Arcachon ». Eighty eight % are managed with freshwater. They constitute original environments initially created for agriculture or for salt production, and they are now threatened by land abandonment within the next decade. Concurrently to aquaculture (in created or existing ponds), exploitation of the fish stocks in the ditchweb is likely to encourage a diversification of agricultural activities. Unfortunately, bibliographic analysis reveals the relative scarceness of research about sampling methods and qualitative or quantitative characteristics of these fish communities. This is quite surprising considering the importance of the ditchwed of this kind of environment outlined by several authors. In the Netherlands, BELTMANN (1984) assessed that there is a total of 400 000 km of ditches. In France, the littoral dyked marshes of the Atlantic coast couid comprise 20 000 km of ditches and about 24 000 ha of open water. The present work provides for the first data on the fish community of Bourgneuf marsh.The northern part of the marsh of Bourgneuf, 2 700 ha provided whith f reshwater, contains nearly every kind of landscapes found throughout the whole Breton-Vendéen marsh. The pattern of the ditch network strongly changes from a zone to another (fig.1) : presence of former salt pans in the western part, regular geometric shapes in the recently created polders next to the River Falleron, irregular ditchweb pattern in the eastern part. The average density of the ditch network is 91 m of ditches per ha, totalizing 234 km in the study area. The total surface of open water, composed of ditches and basins (former salt pans), covers 411 ha (over 15 % of the study site). Diversity of ditch types occurs at fine scales (<1 000 m2), they vary according to their widths (0,3 to 7 m), depths (average, 42 cm; SD, 20,4), thickness of silt layer (average, 43 cm; SD, 42) and their hydrophyte vegetation cover (average, 70 %; SD, 60 %). As a consequence of this heterogeneity, available habitats are scattered over the marsh (mosaïc distribution). A nested sampling (FRONTIER, 1983) was carried out to take into account this high heterogeneity : 5 sampling areas were selected randomly. In each one, 3 to 5 ditches were chosen according to their characteristics (see above). Sampling stations were delimited by 2 stop nets (5 mm mesh) settled 30 m apart, in order to avoid fish migration. Field work was conducted using « Heron » electric fishing material (see LAMARQUE et al., 1978). In each ditch-section, we carried out as many successive catches as necessary to apply the maximum likelihood weighted estimation method of CARLE and STRUB (1978). Nine to 19 stations were sampled at 5 periods, between 1987 and 1989. A total of 74 samples were collected.The fish community was composed of 21 species (table 1) and corresponded to the bream zone of Verneaux's classification (1977). The densities and biomass were quite high (on average 315 kg/ha and 11 460 fishes/ha) but very variable (0 to 2 120 kg/ ha and 0 to 39 300 fishes/ha). The catfish, Ictalurus nebulosus (170 kg/ha), the eel, Anguilla anguilla (47 kg/ha) and the tench, Tinca tinca (28 kg/ha), represented on average 77,5 % of the standing crop, but their spatial distribution was very irregular. These estimates are assumed to be reliable considering that the data used for the calculations were provided by a sampling design which permits to respect the basic assumptions of the removal method. (f) The population size could only change because of the fishings (no migration because of the stop nets; no recruitment/death because of the short duration of the fishing sequences). (ii) The standard sampling design permitted to reduce the variations of the catch probabilities between the successive removals. Several studies have shown that this removal method under-estimates by about 20 % the true size of the fish populations (e. g. BOHLIN and SUNDSTROM, 1977; MAHON, 1980). But they were based on Zippin's method, and the estimator of CARLE and STRUB (1978), that we used, was shown to be more robust (COWX, 1983; GERDEAUX, 1987). Nevertheless, we assume that the values presented in this paper provide for an approached information on the sizes of the studied fish populations.To assess the fish-habitat relationship, a correspondence analysis (fig. 2) was performed on the 74 samples X 17 species matrix (excluding the sticklebacks, Gaslerosteus aculeatus and Pungitius pungitius, which population size estimations failed because of their low catchabilities). Four groups of samples were ordinated according to their specific richness and the species they contained. Several habitat parameters were projected on F1-F2 factorial map (fig. 3). Hydrophyte cover, thickness of silt layer, water depth (fig. 3 and 4), which are directly controlled by human maintenance, appeared to be the major structuring habitat parameters for the fish community. In the deepest and less silted stations, the communities were rich (on average 11 species; group 4, fig. 2). Predators such as pike-perch, Stizostedion lucioperca, and perch, Perca fluviatilis, occurred, and cold water species were found, such as minnow, Phoxinus phoxinus, or chub, Leuciscus cephalus. When the silt layer was thicker and the water level was intermediate, the specific richness decreased (average, 6,2 species) and the community was either dominated by the cattish (group 2, fig. 2) or by the rudd, Scardinius erythrophtalmus (group 3, fig. 2), according to the importance of the aquatic vegetation cover. Habitats with thickest silt layers, shallowest waters and maximum aquatic vegetation cover contained the poorest communities (average 3,9 species) dominated by eel (group 1, fig. 2). There is also evidence that the diversity of the community has progressively decreased since 1987 (fig. 5). The most stenothermous species disappeared, and the importance of the catfish increased : it doubled between May 1987 and September 1989 (fig. 6). Although the eel is the species most adapted to this environment, we emphasize the diminution of its biomass (fig. 7). These phenomena could be partly due to the climate (cold winter in 1987, important swelling in January 1988 and 2 droughts in summers 1988 and 1989). But they are mainly caused by the water management policy which is intended to favour agriculture by keeping stable water levels (evacuation of swellings) and by preventing the freshwater part from the marine influence (collective sluice gates). This does mot permit an optimal breeding of the species that have to spawn on flooded meadows, neither a proper colonisation of the marsh by elvers