Even those with minimal altruistic tendencies are likely to self-isolate when infected, according to a new study by the University of Warwick. This behavior suggests that self-isolation may be a natural survival strategy rather than purely an act of selflessness.
Reducing social contact is well-known to slow the spread of diseases. However, since self-isolation offers no direct health benefit to the individual, it has been assumed that a degree of concern for others is necessary. But how much altruism is required for someone to choose self-isolation when sick?
Published in the Proceedings of the National Academy of Sciences, the study employs a mathematical model of epidemic behavior. It finds that even individuals who value their own life equivalently to the lives of about 100,000 others would still see it as rational to significantly reduce social interactions when infected. This collective behavior can limit disease transmission and potentially prevent major outbreaks.
“You don’t have to care deeply about others to help stop the spread of an infectious disease,” says 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 Decisions During Outbreaks
The researchers utilized mathematical modeling and game theory to explore decision-making during an epidemic. Factors considered include infection status, concern for others, outbreak size, expected time to vaccination, transmission rate (Râ‚€), and the costs associated with both contracting the disease and social distancing.
Two distinct epidemic outcomes emerged from the analysis:
- In one scenario, infected individuals were altruistic enough to self-isolate aggressively, suppressing the disease long-term while allowing uninfected people to continue normal activities. This is referred to as indefinite suppression.
- In the other scenario, infected individuals chose not to isolate, leaving susceptible individuals to practice social distancing, resulting in the disease spreading until population immunity, or herd immunity, is achieved.
“In game theory, we call these outcomes Nash equilibria – stable states that populations settle into because individuals cannot improve their situation by changing their behavior alone,” explains co-author Dr. Simon Schnyder, Project Assistant Professor at the University of Tokyo.
What differentiates these two equilibria is the level of altruism. Remarkably, the threshold needed for disease suppression can be extremely low. The model indicates that even minimal concern from infected individuals can lead to indefinite suppression, potentially resulting in fewer infections, deaths, and social disruptions.
Implications for Public Health Policy
The study’s 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 foundation for understanding the effectiveness of such messages.
“Some members of society are reflexively suspicious of what governments tell them to do, asking themselves, ‘Is this really in my interest?'” Adds Professor Turner: “We show that for anyone with even a small level of altruism, self-isolation is truly a Nash equilibrium, meaning you can’t do better (even by cheating). It’s not easy for members of the public to know this. Indeed, until today, it wasn’t properly understood, even in the scientific community.”
According to Prof. Ryoichi Yamamoto of Kyoto University, “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.”
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, become less active, or leave group environments when infected, behaviors that may have evolved to unintentionally reduce disease transmission.
“Policy doesn’t need to invent new behavior,” says Prof. John Molina of Kyoto University. “Messages like ‘stay home to protect granny’ tapped into a natural altruistic tendency.”
Dr. Schnyder concludes, “What this work reveals is that you should self-isolate even if you don’t like granny very much!”
