Sunk Cost Effects in Nonhuman Species

    As humans, when we invest time and energy into something, we are inclined to follow through with it. In many cases, we are even willing to withstand devastating losses to continue pursuing an activity that we previously put effort into. Examples of this include continuing to watch a terrible movie because money was spent on a movie ticket, or staying in a toxic relationship because time and energy were invested in it. This common phenomenon is referred to as the sunk cost effect. While the sunk cost effect is widely documented in humans, little is known about the psychological mechanisms underlying this phenomenon. One way to tease apart potential explanations is to evaluate the susceptibility of sunk costs in other species besides humans. If sunk cost effects are not demonstrated in other animals, then it could be inferred that the sunk cost effect relies on features unique to humans such as human language, culture, or economic understandings. However, if sunk cost effects are illustrated across species, then it can be inferred that there may be other evolutionary explanations for this cognitive bias. 

    In the article, “Sensitivity to “sunk costs” in mice, rats, and humans” by Sweis et al., the researchers discuss the sunk cost effect as it relates to humans, mice, and rats. Using laboratory foraging tasks, they tested subjects’ willingness to abandon tasks they previously invested time in. The researchers found that mice, rats, and humans spent more time on tasks they previously invested time in, suggesting that the sunk cost phenomenon affects all three species.

    In a similar study, “Capuchin and rhesus monkeys show sunk cost effects in a psychomotor task” by Julia Watzek and Sarah F. Brosnan, the researchers built upon previous discoveries by investigating susceptibility to sunk costs in capuchin and rhesus monkeys. Within their study, the researchers analyzed monkeys’ willingness to opt-out of suboptimal tasks they previously invested time in. They found that both species persisted 5-7 times longer than was optimal when they previously invested time on the task. Furthermore, the longer monkeys spent on a trial, the more likely they were to persist to task completion. These results show that capuchin and rhesus monkeys are susceptible to sunk costs. 

    Both of these studies looked at the presence of sunk cost effects as they relate to nonhuman species. While Dr. Sweis and his fellow researchers demonstrate the effect of this bias in mice, rats, and humans, Dr. Watzek and Dr. Brosnan demonstrate the effect of sunk costs in nonhuman primates. The results of both studies suggest that the sunk cost phenomenon is preserved across species. These findings are significant because they provide insight into the underlying mechanisms of the sunk cost effect. Although more research is needed to understand specific explanations for this phenomenon, it can be faithfully assumed that sunk cost effects emerge as a result of decision-making processes shared across species and do not rely on processes unique to humans such as human language or culture. 

Citations

Sweis, B. M., Abram, S. V., Schmidt, B. J., Seeland, K. D., MacDonald, A. W., Thomas, M. J., & Redish, A. D. (2018, July 13). Sensitivity to "sunk costs" in mice, rats, and humans. https://science.sciencemag.org/content/361/6398/178.full.

Watzek, J., & Brosnan, S. F. (2020, November 23). Capuchin and rhesus monkeys show sunk cost effects in a psychomotor task. Nature News. https://www.nature.com/articles/s41598-020-77301-w.