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It has long been suggested that human behavior can be understood as reflecting the contributions of multiple systems that cooperate or compete for the control of behavior. In the case of instrumental conditioning, action-selection has been proposed to be a product of two distinct controllers: a goal-directed system in which actions are selected with reference to the current incentive value of an associated outcome, and a habitual system in which actions are selected by antecedent stimuli based on prior reinforcement history. In observational learning, a similar dichotomy has been proposed between systems for emulation and imitation. Here, I will present evidence for the contribution of each of these systems to human behavior. I will focus on how it is possible to experimentally demonstrate the influence of these multiple systems at the behavioral level, as well as to identify the brain structures that implement them. I will then discuss at a theoretical level how the brain determines which system should control behavior at any one moment in time. I will argue that this is accomplished by keeping track of the precision of the predictions within each system, and by allocating control over behavior in a manner that is proportional to the relative reliability of those predictions. fMRI and neurostimulation studies suggest a specific contribution of the anterior prefrontal cortex in this arbitration process. Such a mechanism is necessary and sufficient to allocate control over behavior without necessitating explicit considerations about cognitive effort or computational cost. Our findings hint at the possibility that this mechanism may be domain general, exerting control over many different systems in order to produce sophisticated behavior.