Resilience, tipping, and hydra effects in public health: emergent collective behavior in two agent-based models
Authored by Christopher Robert Keane
Date Published: 2016
DOI: 10.1186/s12889-016-2938-8
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Abstract
Background: Collective health behavior often demonstrates
counter-intuitive dynamics, sometimes resisting interventions designed
to produce change, or even producing effects that are in the opposite
direction than intended by the intervention, e.g. lowering infectivity
resulting in increased infections. At other times collective health
behavior exhibits sudden large-scale change in response to small
interventions or change in the environment, a phenomenon often called
``tipping.{''} I hypothesize that these seemingly very different
phenomena can all be explained by the same dynamic, a type of collective
resilience.
Methods: I compared two simple agent-based models of interactions in
networks: a public health behavior game, in which individuals decide
whether or not to adopt protective behavior, and a microbial-level game, in which three different strains of bacteria attack each other. I
examined the type of networks and other conditions that support a
dynamic balance, and determined what changes of conditions will tip the
balance.
Results: Both models show lasting dynamic equilibrium and resilience, resulting from negative feedback that supports oscillating coexistence
of diversity under a range of conditions. In the public health game, health protection is followed by free-riding defectors, followed by a
rise in infection, in long-lasting cycles. In the microbial game, each
of three strains takes turns dominating. In both games, the dynamic
balance is tipped by lowering the level of local clustering, changing
the level of benefit, or lowering infectivity or attack rate. Lowering
infectivity has the surprising effect of increasing the numbers of
infected individuals. We see parallel results in the microbial game of
three bacterial strains, where lowering one strain's attack rate
(analogous to lowering infectivity) increases the numbers of the
restrained attacker, a phenomenon captured by the phrase, ``the enemy of
my enemy is my friend.{''}
Conclusions: Collective behavior often shows a dynamic balance, resulting from negative feedback, supporting diversity and resisting
change. Above certain threshold conditions, the dynamic balance is
tipped towards uniformity of behavior. Under a certain range of
conditions we see ``hydra effects{''} in which interventions to lower
attack rate or infectivity are self-defeating. Simple models of
collective behavior can explain these seemingly disparate dynamics.
Tags
Evolution
Dynamics
Social Network
transmission
Strategies
Game
Rock-paper-scissors
Transtheoretical model
Hand hygiene
Antiretroviral therapy