arXiv subjects

Yoh Iwasa

Publications and source records attributed to Yoh Iwasa.

4 recordsLinked to original sources

arXiv2026

Risk-mediated transmission dynamics govern epidemic trajectories beyond physical mobility

Standard epidemiological models rely on physical mobility and policy indicators, which fail when physical movement decouples from actual transmission risk. To address this, we establish a unified theoretical framework governed by risk-mediated transmission dynamics. Rather than treating societal responses as independent phenomenological proxies, we embed the underlying risk-avoidance tendency directly into the transmission mechanism. This is parsimoniously formulated via the Weber-Fechner law as a logarithmically scaled response to disease incidence, alongside behavioral fatigue. Analyzing multi-regional COVID-19 data, our risk-mediated model significantly outperforms traditional frameworks. While mobility metrics merely track movement volume, our approach directly captures unobserved qualitative contact changes, such as mask-wearing. By integrating this intrinsic behavioral principle, our framework provides a robust, mobility-independent baseline for predicting future epidemic trajectories.

physics.soc-ph↗

arXiv2023

Human movement decisions during Coronavirus Disease 2019

Modelling host behavioral change in response to epidemics is important to describe disease dynamics and many previous studies proposed mathematical models describing it. Indeed, the epidemic of COVID-19 clearly demonstrated that people changed their activity in response to the epidemic, which subsequently modified the disease dynamics. To predict the behavioral change relevant to the disease dynamics, we need to know the epidemic situation (e.g., the number of reported cases) at the moment of decision to change behavior. However, it is difficult to identify the timing of decision-making. In this study, we analyzed travel accommodation reservation data in four prefectures of Japan to observe decision-making timings and how it responded to the changing epidemic situation during Japan's Coronavirus Disease 2019 (eight waves until February 2023). To this end, we defined 'mobility avoidance index' to indicate people's decision of mobility avoidance and quantified it using the time-series of the accommodation booking/cancellation data. Our analysis revealed semi-quantitative rules for day-to-day decision-making of human mobility under a given epidemic situation. We observed matches of the peak dates of the index and the number of reported cases. Additionally, we found that mobility avoidance index increased/decreased linearly with the logarithmic number of reported cases during the first epidemic wave. This pattern agrees with Weber-Fechner law in psychophysics. We also found that the slope of the mobility avoidance index against the change of the logarithmic number of reported cases were similar among the waves, while the intercept of that was much reduced as the first epidemic wave passed by. It suggests that the people's response became weakened after the first experience, as if the number of reported cases were multiplied by a constant small factor.

physics.soc-ph↗

arXiv2015

Agent-based mapping of credit risk for sustainable microfinance

Inspired by recent ideas on how the analysis of complex financial risks can benefit from analogies with independent research areas, we propose an unorthodox framework for mapping microfinance credit risk---a major obstacle to the sustainability of lenders outreaching to the poor. Specifically, using the elements of network theory, we constructed an agent-based model that obeys the stylised rules of microfinance industry. We found that in a deteriorating economic environment confounded with adverse selection, a form of latent moral hazard may cause a regime shift from a high to a low loan repayment probability. An after-the-fact recovery, when possible, required the economic environment to improve beyond that which led to the shift in the first place. These findings suggest a small set of measurable quantities for mapping microfinance credit risk and, consequently, for balancing the requirements to reasonably price loans and to operate on a fully self-financed basis. We illustrate how the proposed mapping works using a 10-year monthly data set from one of the best-known microfinance representatives, Grameen Bank in Bangladesh. Finally, we discuss an entirely new perspective for managing microfinance credit risk based on enticing spontaneous cooperation by building social capital.

q-fin.RM↗

arXiv2010

The pace of evolution across fitness valleys

How fast does a population evolve from one fitness peak to another? We study the dynamics of evolving, asexually reproducing populations in which a certain number of mutations jointly confer a fitness advantage. We consider the time until a population has evolved from one fitness peak to another one with a higher fitness. The order of mutations can either be fixed or random. If the order of mutations is fixed, then the population follows a metaphorical ridge, a single path. If the order of mutations is arbitrary, then there are many ways to evolve to the higher fitness state. We address the time required for fixation in such scenarios and study how it is affected by the order of mutations, the population size, the fitness values and the mutation rate.

q-bio.PE↗