Even people who are only barely altruistic still choose to self-isolate when infected, suggesting it may be a natural survival strategy, finds new University of Warwick led study.
Reducing social contact is widely understood to slow disease spread, but because there is no personal health benefit gain from self-isolating, this would seem to require some concern for others. But how much do you have to care about others before you would choose to self-isolate when sick?
Published in the Proceedings of the National Academy of Sciences , researchers have used mathematical model of epidemic behaviour to find that even people who are only barely altruistic - valuing their own life as equivalent to the lives of around 100,000 others – would still see it as rational to drastically reduce their social contacts when infected. When many people behave this way, limit disease transmission, and even preventing major outbreaks from taking off at all.
"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."
How people choose to isolate during outbreak
Using mathematical modelling and game theory, the researchers explored how individuals make decisions during an epidemic based on multiple factors, including infection status, concern for others, outbreak size, expected time to vaccination, transmission rate (R₀), the costs of catching the disease and of social distancing to avoid it, as well as the proportion of symptomatic cases.
By analysing how these factors interact, the team showed that two different epidemic outcomes emerged.
"In game theory, we call these outcomes Nash equilibria - stable states that populations settle into because individuals cannot improve their situation by changing their behaviour alone," explains co-author Dr Simon Schnyder, Project Assistant Professor at the University of Tokyo.
In one Nash equilibrium, infected individuals were altruistic enough to choose to aggressively self-isolate, suppressing the disease long-term while allowing uninfected people to maintain normal activity - a scenario referred to as indefinite suppression. In the other, infected individuals choose to not isolate, leaving susceptible individuals to do all the social distancing to protect themselves, causing the disease to spread until population immunity builds up - the herd immunity outcome.
"What separates these two equilibria is how altruistic people are - and remarkably, the threshold needed to stabilise disease suppression can be extremely low." says PhD student Mark Lynch from the University of Warwick.
Under the model, only a small amount of concern from infected individuals is enough to allow for indefinite suppression as an alternative to herd immunity - potentially resulting in far fewer infections, deaths, and social disruption.
Strikingly, this holds even when many cases are asymptomatic, some individuals behave entirely selfishly, or when people expect a vaccine to become available.
What this means for public health policy
"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."
The findings have direct implications for public health policy. During recent pandemics, messaging often appealed to empathy, urging people to "stay home to protect others." This research now provides a theoretical foundation 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," says Prof. Ryoichi Yamamoto of Kyoto University.
The simplicity of the behaviour predicted by the model — combined with the low levels of altruism required — suggests that this response may reflect a strategy in social animals that could have evolved to protect relatives. In other animal species, sick individuals are known to reduce social signalling, become less active, or even leave group environments when infected — behaviours that may have evolved to unintentionally reduce disease transmission when infected.
"Policy doesn't need to invent new behaviour," says Prof. John Molina of Kyoto University. "Messages like 'stay home to protect granny' tapped into a natural altruistic tendency."
"What this work reveals is that you should self-isolate even if you don't like granny very much!" concludes Dr. Schnyder.
