Money worries temporarily make you less intelligent according to studies showing that people with less cash to spend do less well than richer people in IQ tests.
Scientists found that poorer people faced with a large, unexpected bill perform significantly worse than they would otherwise have done without the financial worry hanging over them.
The difference, according to the studies, was equivalent to as much as 13 IQ points on an average scale of 100, which is equivalent to the temporary mental impairment caused by the loss of an entire night's sleep, the scientists said.
"Our results suggest that when you're poor, money is not the only thing in short supply. Cognitive capacity is also stretched thin," said Sendhil Mullainathan, an economist at Harvard University who took part in the research published in the journal Science.
"What we show is that the same person experiencing poverty suffers a cognitive deficit as opposed to when they're not experiencing poverty….your effective capacity gets smaller because you have all these other things on your mind - you have less mind to give to everything else," Dr Mullainathan said.
One of the studies was carried out on about 400 shoppers in a New Jersey mall who were asked to carry out standard cognitive tasks. Half of them, however, were asked a "teaser" question about how they would deal with an expensive car repair.
The poorer shoppers in this group performed significantly worse than the richer shoppers. They also did more badly than the poorer members of the other group who did not have to contemplate the teaser question about the expensive car repair.
"For the poor, because of these monetary concerns are just below the surface, the question brings [these concerns] to the top. The result was, for that group, the gap between the rich and the poor goes up, in both IQ and impulse control. There was no gap in the other group," he said.
A second study was carried out on 464 sugarcane farmers in India who did the IQ tests before and after the annual harvest, when they receive most of their yearly income. The farmers performed far worse before the harvest, when they were technically poorer, than after the harvest.
"The month after the harvest they're pretty rich, but the month before, when the money has run out, they're pretty poor. What we did is look at the same people the month before an the month after the harvest, and what we see is that IQ goes up, cognitive control, or errors, goes way down, and response time goes way down," Dr Mullainathan said.
"The effect here is about two-thirds of the size of the effect found in the mall study. It's at least nine or ten IQ points, just between these months," he said.
Eldar Shafir of Princeton University, who part of the study, said: "Previous views of poverty have blamed poverty on personal failings, or an environment that is not conducive to success. We're arguing that the lack of financial resources itself can lead to impaired cognitive function. The very condition of not having enough can actually be a cause of poverty."