The longer an individual waits for a reward, the less willing they will be to give up pursuit of the reward, a new study in mice, rats and humans reports. The findings suggest that many animals make decisions wrongly, based on irrecoverable investments, known as "sunk costs," rather than on expected future outcomes, given the potential gain. To explore animals' perceptions of sunk costs, Brian Sweis et al. set up an experiment with mice that were trained to enter different rooms to forage for different treats. The flavors of the treats - and the waiting period before each treat was awarded - varied. How long would a mouse wait for a given, flavored treat before moving on to the next room? The researchers found that even if the treat was a less preferred one, the mice would wait longer to receive their reward if they had already invested some time to wait for it - highlighting a sunk cost bias. Similar results were found in rats foraging for food, as well as in experiments with humans browsing the web (foraging not for food, but for entertainment); the human participants were rewarded if they stayed on a webpage for a sufficiently long time. Across all three species, this sunk cost bias grew stronger the longer the subjects waited. The researchers discuss three possible explanations for the bias. They say that because predicting the value of future outcomes can be complex and difficult, animals may have evolved processes in which valuation is measured from effort spent, which is easier to process. Other explanations implicate the physiological and psychological state of an animal after it has invested a lot of energy into one resource. Sarah Brosnan discusses this study in greater context, in a related Perspective, noting that "Understanding [the sunk cost bias phenomenon] can help us to better understand why we make the decisions that we do and, ultimately, suggest ways of improving our decision-making."