In risky choice situations, individuals are called risk-prone if they select an option with uncertain consequences, and risk-averse if they select an option with certain consequences. When the uncertain option involves either variability in the delay to reinforcement or a probabilistic reinforcer (one which might or might not occur), nonhuman subjects typically exhibit strongly risk-prone choices. This tendency can be explained by the hyperbolic-decay model, which describes how a reinforcer's effectiveness decreases with increasing delay. When the uncertain option involves variability in the amount of reinforcement, some studies have found that nonhumans are risk-averse when food sufficient to meet their needs and risk-prone when it is not. Studies with human subjects have typically found risk-averse choices when they choose between certain and uncertain reinforcers, but the procedures used in these studies differ in several ways from those used with nonhumans. In laboratory and real-world cases where humans experience a series of trials with real reinforcers, their choices tend to be more similar to those of nonhumans.