As long as analysts presume that individuals cannot change such situations themselves, they do not ask what internal or external variables can enhance or impede the efforts of communities of individuals to deal creatively and constructively with perverse problems such as the tragedy of the commons.
—Elinor Ostrom, Governing the CommonsBastiat’s warning about the seen and
the unseen also applies to the external effects of behavior related to infectious diseases. We can easily observe behaviors that create negative and positive externalities, which lead to higher prevalence and mortality rates (Gersovitz and Hammer 2004). For negative externalities, people might engage in too much infectious behavior because they are unaware of the costs they impose on others. For positive externalities, people might engage in too little preventative behavior because they are unaware of the benefit such behaviors provide for others. These effects lead uncoordinated groups of people to be more infectious and less preventative. In the context of coronavirus disease 2019 (COVID-19), for example, we observed spring break parties, large church gatherings, and worrisome superspreader events such as the Sturgis bike rally and associated those events with the spread of COVID-19.1
What goes unnoticed, however, is that people are responsive to outbreaks and epidemics. As diseases spread throughout real-world populations, people are not automatons who act according to a predetermined set of parameters defined in a model; they can change the parameters they face. People can seek relevant information about diseases, how they spread and are prevented, and which people might be already infected or susceptible. With such information, they adjust their behavior so as to avoid infection or prevent a disease from spreading to others on the margin. For example, gay men became more cautious in their sexual behaviors as the prevalence of HIV/AIDS increased (Philipson and Posner 1993). In response to the H1N1 epidemics of 2008-9, similarly, people in Mexico voluntarily improved their hygiene (Aguero and Beleche 2017), and people voluntarily took fewer flights (Fenichel, Kuminoff, and Chowell 2013).
Ignoring responsiveness leads to a poor understanding of the role individuals play in epidemics, how they discover novel means of prevention, and how they partially internalize epidemic externalities. Just as we understate prevalence rates when we ignore large gatherings of people who spread a disease through air droplets, we overstate prevalence rates when we ignore individuals who are responsive. To avoid such errors, we should recognize that individuals are not automatons when confronted with epidemics.
The varied responses to COVID-19 by individuals and by governments offer added importance for understanding responsiveness as a way to influence disease transmission. In the early days and weeks of the COVID-19 epidemics, however, few scholars and policy makers recognized that people were more than automatons or individuals who are unable to change their behavior. Policies were enacted based on epidemiological models that assumed individuals might mix with others only at a predetermined rate or based on a predetermined set of differential equations with given parameters (Adam 2020).2 Such assumptions are striking in that they ignore voluntary behavioral changes in response to changing prevalence and mortality rates. Although some scholars did recognize the role of responsiveness (Gupta et al. 2020; Luther 2020; Maloney and Taskin 2020), the full range of individuals’ responsiveness remains underexplored.
To develop the logic of disease responsiveness and to make meaningful public-health improvements for infectious-disease prevention, this paper highlights the conditions under which individuals choose to be responsive. The primary condition of disease responsiveness is that individuals engage in preventative behavior, as they define preventative behavior, until the marginal cost of prevention outweighs the marginal benefit of prevention. Furthermore, individuals can alter the costs and benefits they face to encourage prevention by changing their behavior in novel ways, through innovation, and by adopting and changing rules. The ongoing COVID-19 epidemic clarifies and supports this framework, but
