Could your immune system be making you impulsive?

Being able to forego a reward now in favour of gaining something better later is known to be important in determining all kinds of desirable outcomes in life, including greater educational attainment, social functioning and health. 

However, choosing to delay gratification won't always be the best option. If you're in desperate circumstances – you badly need money to buy food, for example – taking the £10 today could be sensible. But this isn't necessarily an entirely conscious judgment – there may be biological systems that automatically shift your decision-making priorities according to what is most likely to enhance your survival.

A new open-access study published in Scientific Reports provides evidence that having raised levels of inflammation in your body, which is generally caused by the immune system's response to infection or injury, can skew your judgment to focus more on present rewards, and on instant gratification. If further research backs this up, there could be wide-ranging implications not only for understanding why some people are more impulsive than others, but even for treating substance abuse. 

Jeffrey Gassen and colleagues at the Texas Christian University, US, reasoned that inflammation should enhance a person's desire for immediately available resources, as the body's response to sickness requires extra energy, and, for a sick person, their future is less certain. To explore whether this is actually the case, they recruited 159 healthy, non-obese, young college students who abstained from behaviours that can cause an acute increase in inflammation, such as smoking, exercise, sex and drinking alcohol, for two days prior to the study. 

The participants completed a widely-used impulsiveness scale, an instant gratification inventory, as well as two behavioural assessments – one of which explored their preferences for smaller and immediate vs. larger, delayed rewards. The participants also reported their body mass index, physical activity levels, smoking, alcohol consumption, sleep quality, stress and other variables (all of which relate to having a greater focus on the present, to inflammation levels, or both). 

The participants then gave blood samples, which were checked for levels of three pro-inflammatory cytokines (proteins that indicate greater inflammation). The researchers found that participants with higher levels of inflammation also tended to have a style of decision-making characterised by impulsivity, a focus on the present, and an inability to delay gratification. 

This analysis alone of course cannot indicate the direction of the relationship. Might it rather be the case that harmful, present-focused behaviours were driving higher levels of inflammation? The researchers explored this possibility by looking to see whether particular behaviours – usual levels of smoking, alcohol intake and risky sexual behaviours, for instance – predicted greater inflammation. But none did. 

This might sound surprising (as smoking, for example, has been clearly associated with inflammation in past studies). But these participants were chosen on purpose to be healthy and young. They were also instructed to avoid such behaviours for 48 hours prior to the study, in theory to make it easier to isolate any effects of inflammation on decision-making. 

All in all, "these results suggest that the activities of the immune system may play an important role in shaping decision-making preferences," Gassen and his colleagues write. In doing so, they add "…to the growing body of research demonstrating that the internal, physiological condition of the body plays an important role in modulating decision-making and behaviour."

If further experimental work can firm up this link, there could be "exciting new possibilities" for treating substance abuse disorders, the researchers suggest, and there could be implications for people who have experienced early life stress, who are known to be more prone to inflammation. Drug-based or behavioural interventions targeted at reducing circulating levels of pro-inflammatory cytokines might possibly "sever the link between an individual's propensity to inflammation and undesirable behaviours, improving outcomes for those from disadvantaged backgrounds." 

As the researchers themselves point out, clearly a lot more work has to be done. But as they also note, this study was on healthy young people, with relatively low levels of inflammation. People with chronic illness or obesity have persistently high levels and the activity of their immune system may then have an even greater effect on the way that they behave and even think.

Further reading

—Inflammation Predicts Decision-Making Characterized by Impulsivity, Present Focus, and an Inability to Delay Gratification

About the author

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest