New research provides some of the first experimental evidence that inflammatory processes influence our decisions. The findings, which appear in the journal Adaptive Human Behavior and Physiology, suggest that factors that promote inflammation may also contribute to impulsivity.

“We initially became interested in this topic after beginning to explore the role of the body’s condition (e.g., hunger, health, etc.) on decision-making in a variety of domains, like interpersonal processes, risk-taking, and impulsivity,” said study author Jeff Gassen, a doctoral candidate at Texas Christian University and member of Sarah Hill’s Evolutionary Social Psychology Lab.

“We developed a hypothesis that the immune system may play an important role in calibrating individuals’ behavior to their bodily state, given that the immune system both monitors the state of the body and can communicate with the brain. Specifically, inflammation increases when the body is threatened or in poor condition, a time when an individual needs to invest in what’s going on right now (whether it be rewards, opportunities, or taking steps to recover) and think less about the future.”

The researchers were specifically interested in the relationship between pro-inflammatory cytokines and delay discounting. Cytokines are proteins produced by the immune system, while delay discounting is the tendency to take a smaller reward that is available immediately rather than a larger reward that will be delivered in the future.

“So, we predicted that inflammation may play a mechanistic role in increasing present focus. We also have a recent paper published in Scientific Reports, that outlines our theoretical model in more detail,” Gassen said.

The study of 161 undergraduates experimentally-induced higher levels of pro-inflammatory cytokines by exposing the participants to either photographs of disease, threatening photographs, or sexually arousing photographs. To ensure that this manipulation worked, the researchers measured the participants’ levels of proinflammatory cytokines via saliva samples.

After being exposed to the images, the participants completed a 30-question survey designed to measure delay discounting.

The researchers found that higher levels of cytokines predicted greater temporal discounting. Participants who had higher levels of inflammatory markers after being exposed to the images were more likely to choose smaller, immediate rewards rather than larger, later rewards.

“Although we still have a lot of research to do in this domain, I think that there are two important takeaways for the average person,” Gassen told PsyPost.

“First, it’s that we need to abandon this idea that the brain and body/immune system are two separate entities. The immune system plays a key role in regulating the nervous system and behavior, not only when you’re sick, but also under normal day-to-day conditions.”

“A great example of this is with the phenomenon of sickness behavior. When you feel terrible because you have the flu, a bacterial infection, or whatever, those symptoms of fatigue, lack of motivation, and aches don’t come from the infectious agent itself, but from your immune system! Your immune system – primarily through changes in inflammation – makes you feel sick so that you stay home, rest, and recover. This is just one of the many examples of your immune system impacting your brain,” Gassen explained.

“The second takeaway follows from the first. Given that what goes on in your body and immune system can impact your behavior, what you put in your body and what you do to take care of your body may influence how you think, feel, and act. With impulsivity, things like poor diet, lack of exercise, stress, etc., may contribute to present-focused decision-making by increasing levels of inflammation.”

“Similarly, we may be able to help individuals with impulse control issues by improving their health. Although we have only just begun testing these possibilities in our recent work, they are interesting to consider,” Gassen said.

The researchers statistically controlled for factors known to influence inflammation, including age, gender, race, physical activity, sleep, body mass index (BMI), stress, recent illness, and socioeconomic status. But like all research, the study includes some limitations.

“One major caveat of the study in question is our lack of a true control group whose inflammation was not expected to increase after the experimental manipulation. This would have allowed us to see if those in the control group were behaving differently from the three other groups who saw an uptick in inflammation,” Gassen explained

“We are following up on this in multiple experiments right now, and also have longitudinal studies planned to help us really understand the causal direction of the relationship between inflammation and present-focused decision-making.”

The study, “Experimentally-Induced Inflammation Predicts Present Focus“, was authored by Jeffrey Gassen, Anastasia Makhanova, Jon K. Maner, E. Ashby Plant, Lisa A. Eckel, Larissa Nikonova, Marjorie L. Prokosch, Gary W. Boehm, and Sarah E. Hill.