Even those with minimal altruistic tendencies often choose to self-isolate when infected, suggesting it may be an inherent survival strategy, according to a new study from the University of Warwick. The research highlights that reducing social contact is crucial in slowing disease spread, yet the personal health benefits of self-isolation are not immediately evident, raising questions about the motivations behind such behavior.
Published in the Proceedings of the National Academy of Sciences, the study utilized a mathematical model of epidemic behavior to demonstrate that individuals who value their own life as equivalent to the lives of approximately 100,000 others still find it rational to significantly reduce social interactions when infected. This behavior, when adopted by many, can limit disease transmission and potentially prevent major outbreaks altogether.
“You don’t have to care deeply about others to help stop the spread of an infectious disease,” said senior author Professor Matthew Turner from the University of Warwick. “Even a tiny amount of concern for others can be enough to change the course of an epidemic.”
Understanding Isolation Choices During Outbreaks
The researchers employed mathematical modeling and game theory to explore decision-making during an epidemic. Factors considered included infection status, concern for others, outbreak size, expected time to vaccination, transmission rate (R₀), and the costs associated with catching the disease and social distancing.
Through this analysis, two distinct epidemic outcomes emerged. In one scenario, infected individuals were sufficiently altruistic to choose aggressive self-isolation, leading to long-term disease suppression and allowing uninfected individuals to continue normal activities—a situation termed indefinite suppression. Conversely, in the other scenario, infected individuals did not isolate, leaving susceptible individuals to socially distance, leading to disease spread until population immunity, or herd immunity, was achieved.
“What separates these two equilibria is how altruistic people are—and remarkably, the threshold needed to stabilize disease suppression can be extremely low,” stated PhD student Mark Lynch from the University of Warwick.
The model suggests that even minimal concern from infected individuals can enable indefinite suppression as an alternative to herd immunity, potentially resulting in fewer infections, deaths, and social disruptions. This remains true even when many cases are asymptomatic, some individuals act selfishly, or when a vaccine is anticipated.
Implications for Public Health Policy
Professor Turner noted that some individuals are inherently skeptical of government directives, questioning whether such measures are in their best interest. However, the study reveals that for anyone with even a small degree of altruism, self-isolation represents a Nash equilibrium, meaning individuals cannot improve their situation by acting differently.
The findings have significant implications for public health policy. During recent pandemics, public messaging often appealed to empathy, urging people to “stay home to protect others.” This research provides a theoretical basis for understanding how and when such messages can be effective.
“We can strengthen public cooperation further by communicating early and making isolation both morally and rationally compelling. It is crucial to understand that the required altruism greatly increases with the size of the outbreak: the more people already infected, the harder it becomes to preserve indefinite suppression,” explained Prof. Ryoichi Yamamoto of Kyoto University.
The simplicity of the behavior predicted by the model, combined with the low levels of altruism required, suggests that this response may reflect a strategy in social animals that evolved to protect relatives. In other species, sick individuals often reduce social signaling or leave group environments when infected—behaviors that may have evolved to unintentionally reduce disease transmission.
“Policy doesn’t need to invent new behavior,” said Prof. John Molina of Kyoto University. “Messages like ‘stay home to protect granny’ tapped into a natural altruistic tendency.”
Dr. Schnyder concluded, “What this work reveals is that you should self-isolate even if you don’t like granny very much!”
