Cost-effective external interference for promoting the evolution of cooperation.

The problem of promoting the evolution of cooperative behaviour within populations of selfregarding
individuals has been intensively investigated across diverse fields of behavioural, social
and computational sciences. In most studies, cooperation is assumed to emerge from the
combined actions of participating individuals within the populations, without taking into account
the possibility of external interference and how it can be performed in a cost-efficient way.
Here, we bridge this gap by studying a cost-efficient interference model based on evolutionary
game theory, where an exogenous decision-maker aims to ensure high levels of cooperation
from a population of individuals playing the one-shot Prisoner’s Dilemma, at a minimal cost.
We derive analytical conditions for which an interference scheme or strategy can guarantee a
given level of cooperation while at the same time minimising the total cost of investment (for
rewarding cooperative behaviours), and show that the results are highly sensitive to the intensity
of selection by interference. Interestingly, we show that a simple class of interference that
makes investment decisions based on the population composition can lead to significantly more
cost-efficient outcomes than standard institutional incentive strategies, especially in the case of
weak selection.