Beyond making you cranky and impatient, it turns out that being hungry—or more specifically, having low blood sugar—may actually change the way that you make decisions. In a new study published in the journal Psychological Science, University of South Dakota researchers X.T. Wang and Robert D. Dvorak found that people with lower blood sugar levels were more likely to opt for immediate rewards, while people with higher blood sugar counts tended to opt for more distant, but larger, rewards.
Wang and Dvorak anticipated such results, hypothesizing that lower blood sugar levels would impact people’s tendency toward “future discounting”—or opting for less valuable present goods (smaller, immediate reward) over more valuable future goods (larger, later reward). As the authors point out, from an evolutionary psychology perspective, people with higher blood sugar—those who are full, or have a higher “body-energy budget”—should be more likely to opt for delayed rewards, as their immediate hunger is satisfied, making longer-term thinking more “affordable.”
To test the hypothesis, the authors recruited 65 college students, who were divided into two groups. Both groups were instructed not to eat the morning of the experiment, and blood glucose levels were monitored multiple times during the trial. After answering an initial series of questions such as, “Would you prefer $120 tomorrow or $450 in 31 days?”, where the amount and duration of time until reward varied slightly, but the central premise of instant or delayed reward was consistent, the students in the experimental group were given a sugary soda to drink, while those in the control group were given a diet soda, containing aspartame as a sweetener. After drinking the soda, they were again asked a series of questions about immediate or postponed rewards. To ensure that the answers accurately reflected their desires, the researchers told the students that, at the end of the experiment they would be allowed to roll the dice to determine which of the monetary prizes they had chosen would actually be rewarded.
The researchers found that, while, before drinking the soda both groups responded similarly to the questions, after a sugar boost, the experimental group became far more likely to choose delayed reward, while, as their blood sugar levels waned, the control group grew increasingly eager for immediate rewards. As the authors write:
“Our results in the lab showed that human preferences for future versus current rewards fluctuated from moment to moment based on blood glucose levels. As actually measured in the lab, increasing blood glucose levels via a soft drink containing sugar led to an increase in the value placed on future rewards.”
The findings, they say, suggest that our moment-to-moment decision-making may be influenced by “metabolic cues, indicating environmental scarcity on a micro level.” What’s more, they suggest that, instead of satisfying our craving for sugars, artificial sweeteners could potentially achieve the opposite effect, triggering a sense of alarm in the body by creating a perceived caloric crisis.
This latest research adds to previous work suggesting that people’s modern desire for money is rooted in our more primitive desire for food. As the authors highlight, a 2006 study found that hungry people were far less likely to give to charity than those who’d had a satisfying meal. If blood sugar levels hold such sway over our decision-making, the authors argue, perhaps techniques that target blood sugar fluctuations could possibly be incorporated into treatment regimes for people suffering from disorders such as compulsive gambling, anorexia and drug addiction.
Yet for those of us not battling more serious disorders, perhaps this study simply offers a clue to bringing out our most generous, responsible selves: make big decisions on a full stomach.