It’s well established that people with low economic status are the hardest hit by the current obesity pandemic, as well as related health problems such as diabetes. Poor healthcare, stress, unhealthy lifestyles, and a cornucopia of cheap junk food are all thought to play a role. But a new study suggests there’s a subconscious component, too.

When researchers merely prompted study volunteers to consider themselves low-class, they were more likely to prefer, choose, and eat larger amounts of food, as well as higher-calorie foods. The findings, published in the Proceedings of the National Academy of Sciences, echo what’s been seen in a variety of animals—from birds and rodents to nonhuman primates. Thus, the authors speculate that the mental glitch may be an evolutionary holdover intended to boost survival by compensating for a lack of social and material resources.

More important for humans, the findings suggest that we may not be able to tackle obesity by just improving access to healthier foods and promoting exercise.

For the study, psychology researchers at Nanyang Technological University in Singapore recruited nearly 500 healthy participants for a series of four experiments. In the first, the team had 101 participants complete a task in which they were shown a ladder with ten rungs and told to select which rung they were on relative to either a wealthy, well-educated, powerful person or a poor, uneducated, unimportant person. Participants were randomly assigned to the two comparisons. In keeping with past studies, they ranked their social status lower in the former scenario and higher in the latter.

Next, the participants got to pick foods from a hypothetical buffet. Taking into account things like each participants’ normal eating pattern, hunger, and gender, the researchers found that those who ranked their social status lower chose more food and more high-calorie foods than those that ranked themselves as having a higher social status.

In the second experiment, researchers gave 167 participants the same socioeconomic ranking task, then asked them to match high calorie foods (pizza, hamburgers, fried chicken) and low calories foods (vegetables and fruits) with either pleasant or unpleasant descriptors, such as tasty or nasty. Again, those who landed lower on the ladder were more apt to prefer the high-calorie foods.

In the last two experiments, researchers followed up the socioeconomic task with actual eating experiments. In this part of the study, 83 self-ranked participants got to watch a documentary while munching on their choice of three snacks: potato chips, M&M candies, or raisins. Again the low-ranked participants went for the chips and chocolate more than their higher-ranked counterparts. And finally, researchers gave 148 self-ranked participants a big bowl of noodles and then told them to eat until they were “comfortably full.” The lower-ranked participants ate an average of about 20 percent more calories' worth of noodles.

“These findings suggest that mindsets of deprivation and low social standing may be critically linked to obesity risk via increased intake of calories,” the authors conclude. As such, the subjective experience of low social standing may be another barrier to improving health.

PNAS, 2016. DOI: 10.1073/pnas.1607330114 (About DOIs).