According to economic theory, when making decisions, we are taught to consider future consequences rather than past investments solely. However, both humans and animals tend to exhibit the "sunk cost fallacy,” which is the tendency to continue making decisions based on previous investments, even when there are better options. In the research article “Sensitivity to ‘Sunk Costs’ in Mice, Rats, and Humans,” Brian Sweis et al. explore a cross-species examination of this behavior, attempting to prove the bias is not exclusive to humans. Through carefully crafted foraging tasks, Sweis et al. showed that mice, rats, and humans are sensitive to sunk costs, but this sensitivity only appears after making an initial commitment. This finding suggests that initial decisions and subsequent persistence are governed by distinct decision-making systems, with sunk cost sensitivity originating from mechanisms distinct from deliberative reasoning.
At first glance, this finding may seem purely theoretical, but its applicability becomes further apparent when we consider contemporary challenges, such as substance use. Evidence from a study conducted by Michael J. Sofis et al., titled “Persisting on the Past: Cross-sectional and Prospective Associations Between Sunk Cost Propensity and Cannabis Use,” illustrates that there is a direct relationship between the sunk cost bias and a person's propensity to use cannabis. Through two experiments, Sofis and associates discovered that people with a higher tendency to exhibit sunk cost bias, meaning an unrewarding, expensive persistence, also reported higher levels of cannabis consumption. Crucially, this relationship remained even after accounting for factors, such as alcohol consumption, mental health symptoms, and tendencies toward delay discounting. These findings suggest that a rigid focus on past investments, as opposed to future results, may drive the persistent substance use observed in these participants.
The connection between the findings of Sofis et al. and Sweis et al. is quite fascinating. Each study highlights the fact that sensitivity to sunk costs is not simply a rational error, but rather reflects fundamental, evolutionary decision-making processes. Sweis et al. demonstrated that rodents, like humans, escalate their commitment to an endeavor the longer they have already invested, indicating that the susceptibility to sunk costs may have once served evolutionary purposes, such as nurturing persistence in uncertain conditions. Likewise, Sofis et al. propose that the increased propensity humans have toward sunk cost biases could result from an overgeneralization of heuristics, such as perseverance and loss avoidance. These cognitive strategies, when lacking flexibility, may increase susceptibility to substance use.
This relationship is critical in opening new avenues of research. If the same mechanisms that drive sensitivity to sunk costs also influence substantive use, interventions focusing on the flexibility of decision-making could be promising. For instance, Sofis et al. emphasize that interventions, such as mindfulness training, which shifts attention from past regrets to current conditions, have been revealed to reduce the sunk cost bias. Furthermore, since Sweis et al. examined the disparity in processing between initial decision-making and subsequent persistence, it may be possible to develop behavioral therapies aimed at post-commitment reevaluation, which can help people identify when it is better to give up a poor investment, whether it is time, money, or using substances.
Alongside the clinical implications, these findings challenge different societal assumptions surrounding rationality and choice. They propose that behaviors deemed “bad decisions,” such as waiting to obtain a poor reward or continuing to use substances, stem not from impulsivity or lack of knowledge, but rather from how different species’ brains process investment and reward. With further understanding of these mechanisms at a deeper level, we can develop more effective approaches to treating maladaptive behaviors.
Sweis et al.’s approach across species and Sofis et al.’s examination of cannabis use illustrate the impact of past investments on current behavior. As the intricacies of decision-making continue to be refined, one of the key future directions will be to design strategies to assist individuals and societies in drawing the line between healthy, vigorous perseverance and harmful persistence, and understanding when it is best to leave the situation altogether.
References
Sofis, M. J., Lemley, S. M., Budney, A. J., Stanger, C., & Jarmolowicz, D. P. (2020). Persisting on the past: Cross-sectional and prospective associations between sunk cost propensity and cannabis use. Experimental and clinical psychopharmacology, 28(2), 225–234. https://doi.org/10.1037/pha0000299
Sweis, B. M., Abram, S. V., Schmidt, B. J., Seeland, K. D., MacDonald, A. W., 3rd, Thomas, M. J., & Redish, A. D. (2018). Sensitivity to "sunk costs" in mice, rats, and humans. Science (New York, N.Y.), 361(6398), 178–181. https://doi.org/10.1126/science.aar8644
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