arXiv subjects

Yutaka Nakai

Publications and source records attributed to Yutaka Nakai.

4 recordsLinked to original sources

arXiv2026

Integrating upstream and downstream reciprocity stabilizes cooperator-defector coexistence in others-only public goods games

Human cooperation persists among strangers in large, well-mixed populations despite theoretical predictions of difficulties, leaving a fundamental evolutionary puzzle. While upstream (pay-it-forward: helping others because you were helped) and downstream (rewarding-reputation: helping those with good reputations) indirect reciprocity have been independently considered as solutions, their joint dynamics in multiplayer contexts remain unexplored. We study public goods games without self-return (often called "others-only" PGGs) with benefit b and cost c and analyze evolutionary dynamics for three strategies: unconditional cooperation (ALLC), unconditional defection (ALLD), and an integrated reciprocity strategy combining unconditional forwarding with reputation-based discrimination. We show that integrating upstream and downstream reciprocity can yield a globally asymptotically stable mixed equilibrium of ALLD and integrated reciprocators when b/c > 2 in the absence of complexity costs. We analytically derive a critical threshold for complexity costs. If cognitive demands exceed this threshold, the stable equilibrium disappears via a saddle-node bifurcation. Otherwise, within the stable regime, complexity costs counterintuitively stabilize the equilibrium by preventing not only ALLC but also alternative conditional strategies from invading. Rather than requiring uniformity, our model reveals one pathway to stable cooperation through strategic diversity. ALLD serves as "evolutionary shields" preventing system collapse while integrated reciprocators flexibly combine open and discriminative responses. This framework demonstrates how pay-it-forward broadcasting and reputation systems can jointly maintain social polymorphism including cooperation despite cognitive limitations and group size challenges, offering a potential evolutionary foundation for behavioral diversity in human societies.

q-bio.PE↗

arXiv2018

Triadic Social Structure Facilitates Backing for Crowdfunding Projects

Crowdfunding is a new funding method through which founders request small amounts of funding from a large number of people through an online platform. Crowdfunding facilitates a new type of social capital and exhibits a unique form of social dynamics, thus attracting the interest of sociologists and other social scientists. Previous studies have focused on social relationships in crowdfunding such as direct reciprocity and consider how they contribute to the success of funding. The social structure of crowdfunding, however, involves more complex social relationships and it may contribute to the success of a new venture or project in many ways. In this study, we focus on a specific type of triadic social structure, the buddy relation, which can be described as a relationship through which project founder x, who previously backed another founder z's project, receives financial backing from the other backers of z's project. We found that the buddy relation occurs significantly more often than randomly, concluding that this structure facilitates the gathering of financial backing and may contribute to the success of a crowdfunded project.

cs.CY↗

arXiv2017

The evolution of conditional moral assessment in indirect reciprocity

Indirect reciprocity is a major mechanism in the maintenance of cooperation among unrelated individuals. Indirect reciprocity leads to conditional cooperation according to social norms that discriminate the good (those who deserve to be rewarded with help) and the bad (those who should be punished by refusal of help). Despite intensive research, however, there is no definitive consensus on what social norms best promote cooperation through indirect reciprocity, and it remains unclear even how those who refuse to help the bad should be assessed. Here, we propose a new simple norm called "Staying" that prescribes abstaining from assessment. Under the Staying norm, the image of the person who makes the decision to give help stays the same as in the last assessment if the person on the receiving end has a bad image. In this case, the choice about whether or not to give help to the potential receiver does not affect the image of the potential giver. We analyze the Staying norm in terms of evolutionary game theory and demonstrate that Staying is most effective in establishing cooperation compared to the prevailing social norms, which rely on constant monitoring and unconditional assessment. The application of Staying suggests that the strict application of moral judgment is limited.

q-bio.PE↗

arXiv2016

Indirect reciprocity can overcome free-rider problems on costly moral assessment

Indirect reciprocity is one of the major mechanisms of the evolution of cooperation. Because constant monitoring and accurate evaluation in moral assessments tend to be costly, indirect reciprocity can be exploited by cost evaders. A recent study crucially showed that a cooperative state achieved by indirect reciprocators is easily destabilized by cost evaders in the case with no supportive mechanism. Here, we present a simple and widely applicable solution that considers pre-assessment of cost evaders. In the pre-assessment, those who fail to pay for costly assessment systems are assigned a nasty image that leads to being rejected by discriminators. We demonstrate that considering the pre-assessment can crucially stabilize reciprocal cooperation for a broad range of indirect reciprocity models. In particular for the most leading social norms we analyse the conditions under which a prosocial state becomes locally stable.

q-bio.PE↗