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The host sanction (HS) theory aims to understand how cooperation evolves and is maintained in a mutualistic relationship between organisms. The host sanction model theorizes that the host organism of a mutualism is able to decrease the fitness of the other organism to prevent cheating.^[1]

The basis of the host sanction (HS) theory was first introduced by J.J. Bull and W.R. Rice in 1991 as the partner-choice model.^[2] The partner-choice model was proposed as a mechanism in which one organism is capable of rewarding or punishing the other organism based on its cooperation. Bull and Rice hypothesized the partner-choice model is evident in both fig tree-fig wasp and yucca-yucca moth (Parategeticula or Tegeticula) mutualisms.^[3] Further studies conclude these mutualisms align with the host sanction (HS) model given they both involve a host organism implementing sanctions on their symbiotic partners. The fig tree-fig wasp mutualism was observed by Axelrod and Hamilton in 1981.^[4] In this mutualism, the fig tree receives pollination from the fig wasps, and the fig wasps oviposit eggs into figs. This relationship represents a host sanction mechanism due to the fig tree's ability to prevent the fig wasp from ovipositing too many eggs by selectively aborting figs with too many wasp eggs. The yucca -yucca moth mutualism is similar; the oviposition of yucca moth eggs allows yucca pollination. The yucca plant is capable of enforcing sanctions on the moths by withholding nectar to prevent cheating.^[5] In both these situations, the yucca plant and fig tree are hosts to the insects and they administer sanctions to the insects. Host sanctions are commonly accepted as a mechanism that allows mutualisms to exist through evolving cooperation between organisms.