How do we learn what features of our multidimensional environment are relevant in a given task? To study the computational process underlying this type of "representation learning," we propose a novel method of causal model comparison. Participants played a probabilistic learning task that required them to identify one relevant feature among several irrelevant ones. To compare between two models of this learning process, we ran each model alongside the participant during task performance, making predictions regarding the values underlying the participant's choices in real time. To test the validity of each model's predictions, we used the predicted values to try to perturb the participant's learning process: We crafted stimuli to either facilitate or hinder comparison between the most highly valued features. A model whose predictions coincide with the learned values in the participant's mind is expected to be effective in perturbing learning in this way, whereas a model whose predictions stray from the true learning process should not. Indeed, we show that in our task a reinforcement-learning model could help or hurt participants' learning, whereas a Bayesian ideal observer model could not. Beyond informing us about the notably suboptimal (but computationally more tractable) substrates of human representation learning, our manipulation suggests a sensitive method for model comparison, which allows us to change the course of people's learning in real time.We live in a rich, complex environment, in which we are constantly bombarded with a wide variety of sensory input. Even an action as simple as walking down the street carries with it a large volume of low-quality information in the form of people we see, places we walk by, cars, colors, voices, noises, emotional content, etc. Intuitively, one would imagine that given sufficient resources, it is best to always represent every aspect of the environment so that any detail can potentially be acted upon. However, the "curse of dimensionality" (Bellman 1957) posits that task representations that involve unnecessary stimulus dimensions will not afford efficient learning and decision-making, where efficiency is measured in the number of examples needed to learn the task. In particular, an increase in the number of dimensions of the problem (in our case, the dimensions of the environment that the brain may represent) implies that the learner needs to collect exponentially larger quantities of data to learn to solve the problem. If we want learning to be feasible it is therefore both computationally optimal and a practical imperative to represent tasks with as compact a representation as possible.What are the computational strategies that humans use to learn a representation for a given task? To address this question, we tested participants on a multidimensional trial-and-error choice task, in which only one dimension was relevant to predicting reward (Wilson and Niv 2012;Niv et al. 2015). To test the explanatory power of different models of learning dynamics, we develope...