Over de ratio van coöperatie : een speltheoretische benadering met een toepassing bij de mogelijke invloed van diergeneesmiddelen op voedselveiligheid = About reasons for cooperation : a game theoretical approach and an application to the potential influence of veterinary medicinal products on food safety

The central question in this study is: how can cooperation emerge in a non policed society? Due to the increasing availability of computers the answer has been sought by game theorists, too. In earlier times the search for the answer was characterised by trying to find simple patterns in a complex reality, but game theorists try to compose complex reality by evolutionary development starting with simple virtual individuals interacting according simple rules. Cooperation and its counterpart defection are studied by using the Prisoner's Dilemma (PD). In the 1980s the strategy TIT FOR TAT (TFT) has been discovered to use the right amount of cooperation to generate stable development. The essential elements of TFT are: always a cooperative start, immediate retaliation when confronted with defection and immediate reconciliation. The discovery of TFT has been inspirational to many scientists; the strategy has been analysed thoroughly and improvements have been proposed. Sociobiologists have found examples of the application of TFT in real life many times.In this study we pay attention to the history of TFT, the Prisoner's Dilemma and the game theoretical analysis.For cooperation to emerge direct reciprocity has been a conditio sine qua non , so counterparts had to meet over and over again. In 1998 the results of an experiment showed that indirect reciprocity could do the job as well. The experiment used a complex computer simulation model which was rebuilt by us. Surprisingly enough, the repetition of the experiment gave different results all the time, which questioned the conclusion. A hundredfold repetition showed the published result to be just one of many potential outcomes. We then have modified the model by introducing trust. A new series of one hundred experiments showed the original conclusion now to be right: indirect reciprocity is sufficient to make cooperation emerge, but trust is needed. This might be the first time that trust is no longer restricted to the field of belief, hope and love, and has become part of the rational answer to the central question. Fukuyama already established the importance of trust in (economical) life, but he considered trust to be part of the economically important phenomena which cannot be explained rationally.Next, the modified computer simulation model has been applied to an every day practical situation where a veterinary surgeon and a farmer must decide about medication of food producing animals. First the importance of veterinary medicinal products (VMPs) and especially antimicrobial agents is shown, not only as beneficial compounds for animal health and animal welfare but as risks as well, being potential sources of residues in food originating from animals. Part of the study is some work about residues caused by a widely used VMP for piglets containing penicillin. The aim of this work is to prove the correctness of legislation concerning residues. Farmers and veterinarians are prone to doubt this correctness. But MRLs do really matter. A survey of the results of an enquiry among 30 European veterinary surgeons regarding this subject is given; this survey shows the issue to exist among veterinarians.Using the modified model the problem to study was to find out how to guarantee that both farmer and veterinarian take the right decisions concerning food safety. Cooperation is respecting the waiting time and defection is not respecting it. The results of the application of the modified model are proof that payoff for the farmer is the most important factor. Educational ability of the veterinary surgeon does have some influence, too, but this influence is only moderate.Whereas the first part of the study increases our knowledge about cooperative science the second part illustrates a rather new approach to veterinary questions