Being poor can sap a person’s mental resources, research published in the journal Science suggests.
The work, by an international team, demonstrates how poverty takes its toll on cognitive function, leaving less mental capacity for other tasks.
The evidence comes from two studies carried out in India and the US.
Previous data had shown a link between poverty and bad decision-making, but the root causes of this correlation were unclear.
The US, British and Canadian team tried to shed light on this chicken-and-egg puzzle by isolating the financial factor from others that might interfere with the results.
One part of study used the natural financial cycle (tied to the crop cycle) of sugarcane farmers in India.
The farmers go through three crucial stages in this cycle; before the harvest, when they have taken out loans to grow the crops and thus are extremely poor; after the harvest, but before being paid, when farmers are at the greatest extent of their poverty; and after being paid.
Dr Anandi Mani, from the Centre for Competitive Advantage in the Global Economy (Cage) at the University of Warwick, UK, set the farmers cognitive tests at each stage of the cycle and found that mental acuity varied with their income.
"With the sugarcane farmers, we are comparing the same person when he has less money to when he has more money. We’re finding that when he has more money he is more intelligent, as defined by IQ tests," said Dr Mani.
The study aimed to rule out other confounding factors like nutrition, health, physical exhaustion and family commitments.
The researchers also tried to limit the influence of factors related to stress, but measuring biomarkers such as blood pressure and heart rate.
To make sure the findings were not in some way unique to Indian sugarcane farmers, a control study was completed in the US.