Have you ever noticed that cars in other lanes were moving faster, yet felt compelled to stick in the lane you chose first? People are notoriously reluctant to let go of past decisions, especially after investing time or effort into them. This is referred to as the ‘sunk cost’ bias in psychology, but is not well understood at the level of the brain, despite the societal significance of the sunk cost fallacy.

A new study from the University of Oxford published in Nature Human Behaviour has found an area of the brain which causes these biases. The study combined brain imaging in healthy people with an investigation of patients with brain damage. All evidence converged to show that a region called the ventromedial prefrontal cortex (vmPFC) plays a critical role in causing these sunk cost biases.

People in the study played a carefully designed video game where they made decisions between persisting with a current goal, or abandoning for alternative goals which were often better. Healthy people showed strong sunk cost biases meaning they were very reluctant to abandon goals once they had started, even when they would win more points if they did. However, patients with damage to vmPFC were less susceptible to this bias, which lead to the surprising result that they played the game better and behaved more ‘rationally’ than people without this damage.

The article is one of the first to combine brain imaging of healthy people with an investigation of lesion patients in the very same study. Thirty healthy people took part in the magnetic resonance imaging (MRI) study, where they played the game inside a brain scanner. Among healthy people, more activity in vmPFC predicted they would show higher sunk cost biases. However, these imaging techniques on their own cannot show that a region is necessary for a behaviour. To show this region was necessary for sunk cost biases, twenty-three patients with brain damage played the game. Damage to the very same vmPFC region found in the MRI study was linked to having lower sunk cost biases compared to patients with brain damage in other areas and healthy age-matched control participants.

Although patients with vmPFC damage did better than healthy people in this study, in the real world sunk cost biases are likely to be helpful as they help us stay committed to the goals we start. There is evidence from other studies that animals like rodents and birds also show sunk cost biases. The current study found that sunk cost biases were related to where people were paying attention. In people who were less willing to abandon current goals, their attention was fully captured by the goal they were pursuing. Lead author Eleanor Holton said, “It is important to remember that the ability to be fully immersed in goals is likely to be essential for us in the real world, even if this can lead to ‘irrational’ persistence with them at times.”

Senior author Dr Nils Kolling said, " Deciding whether to keep persisting with a difficult task or let go if better options come along is a difficult balance to get right. While our work gives us some good evidence how they brain might be able to strike the balance, it will be equally important to establish any clinical relevance for psychiatric conditions such as ADHD, where people are often more likely to ‘give up’ or OCD where patients cannot stop persisting once committed.”