Eriko Fukuda

Effect of Initial Fraction of Cooperators on Cooperative Behavior in Evolutionary Prisoner's Dilemma Game

We investigate the influence of initial fraction of cooperators on the evolution of cooperation in spatial prisoners dilemma games. Compared with the results of heterogeneous networks, we find that there is a relatively low initial fraction of cooperators to guarantee higher equilibrium cooperative level. While this interesting phenomenon is contrary to the commonly shared knowledge that higher initial fraction of cooperators can provide better environment for the evolution of cooperation. To support our outcome, we explore the time courses of cooperation and find that the whole course can be divided into two sequent stages: enduring (END) and expanding (EXP) periods. At the end of END period, thought there is a limited number of cooperator clusters left for the case of low initial setup, these clusters can smoothly expand to hold the whole system in the EXP period. However, for high initial fraction of cooperators, superfluous cooperator clusters hinder their effective expansion, which induces many remaining defectors surrounding the cooperator clusters. Moreover, through intensive analysis, we also demonstrate that when the tendency of three cooperation cluster characteristics (cluster size, cluster number and cluster shape) are consistent within END and EXP periods, the state that maximizes cooperation can be favored.

Spatial prisoner's dilemma games with zealous cooperators

The existence of a zealot who stays a cooperator irrespective of the result of an interaction has been reported to add social viscosity to a population and thereby helps increase the cooperation level in prisoners dilemma games, which premises the so-called well-mixed situation of a population. We found that this is not always true when a spatial structure, i.e., connecting agent, is introduced. Deploying zealots is counterproductive, especially when the underlying topology is homogenous, similar to that of a lattice. Our simulation reveals how the existence of never-converting cooperators destroys rather than boosts cooperation. We explain detailed mechanisms behind this interesting finding by referring to our previously presented concepts with respect to evolutionary dynamic processes for spatial games under the names enduring and expanding periods.

Effect of a large gaming neighborhood and a strategy adaptation neighborhood for bolstering network reciprocity in a prisoner's dilemma game

In 2xa0×xa02 prisoners dilemma (PD) games, network reciprocity is one mechanism for adding social viscosity, leading to a cooperative equilibrium. In this paper, we explain how gaming neighborhoods and strategy-adaptation neighborhoods affect network reciprocity independently in spatial PD games. We explore an appropriate range of strategy adaptation neighborhoods as opposed to the conventional method of making the gaming and strategy adaptation neighborhoods coincide to enhance the level of cooperation. In cases of expanding gaming neighborhoods, network reciprocity falls to a low level relative to the conventional setting. In the discussion below, which is based on the results of our simulation, we explore how these enhancements come about. Essentially, varying the range of the neighborhoods influences how cooperative clusters form and expand in the evolutionary process.

Effect of intermediate defense measures in voluntary vaccination games

We build a model to reproduce the decision-making process of getting a vaccination based on the evolutionary game theory dovetailed with the SIR model for epidemic spreading. Unlike the two extreme options of whether or not getting a vaccination leads to perfect immunity, we consider whether intermediate defense measures including masking, gargling, and hand-washing lead to imperfect effects of preventing infection. We consider introducing not only a third strategy as a discrete intermediate measure but also a continuous strategy space connecting the cases of getting and not getting a vaccination. Interestingly, our evolutionary analysis suggests that the introduction of intermediate measures makes no difference for the case of a 2-strategy system in which only either getting or not getting a vaccination is allowed, even does not ameliorate, or say, gets worse to prevent spreading a disease. This seems quite different from what was observed in 2-player and 2-strategy (2 × 2) prisoners dilemma (PD) games with relatively stronger chicken-type dilemma than the stag-hunt one in which the introduction of middle-course strategies significantly enhances cooperation.

Impact of Stubborn Individuals on a Spread of Infectious Disease under Voluntary Vaccination Policy

Achievement of the herd immunity is essential for preventing epidemics of vaccine-preventable diseases. However, an individual’s decision-making whether or not to be vaccinated depends on several factors, such as perceived risks of vaccination and infection, her self-interest, and response of others to vaccination under voluntary vaccination policies. In this study, we consider the case where “stubborn individuals” are presented in lattice populations, who consistently hold the vaccination strategy (stubborn vaccinated individuals) or the no-vaccination strategy (stubborn unvaccinated individuals). We investigate individuals’ decision-making process with vaccination by means of modeling the dynamics for epidemic spreading applied to evolutionary game theory. As a result, we find that the presence of stubborn ones, even if it accounts for a small fraction, significantly affect the epidemic spreading and vaccination behavior.

Large Strategy Adaptation Neighborhood Bolsters Network Reciprocity in Prisoner’s Dilemma Games

In 2 × 2 prisoner’s dilemma games, network reciprocity is one mechanism for adding social viscosity, which leads to cooperative equilibrium. Here we elucidate how strategy adaptation neighborhood affects on network reciprocity in spatial prisoner’s dilemma games. Presuming an appropriate range of strategy adaptation neighborhood, we can observe the evolution of cooperation than usual network reciprocity previously reported. In the discussion based on our simulation result, we explore why those enhancements are brought, which can be summarized that varying the neighborhood ranges influences on how cooperative clusters are successfully formed and expanded in evolutionary process.