How much money do I waste in a given month by doing most of my grocery shopping at the Whole Foods that's directly on my route home rather than taking the three-minute detour to Safeway? I have no idea. As a business writer, I'm aware that the Whole Foods markup is big on some items. But in my actual life, the main thing is that I prefer Safeway's tortillas, so I go there only if I want to buy some tortillas. Otherwise, convenience is king. All I need to know is that my grocery spending is within my budget. I'm probably wasting money, but it's not worth the time to think about it too much.

Such are the privileges of affluence. It's not just that you can afford nicer stuff than poor people. You get to take advantage of what is, in some ways, the greatest convenience of all - the convenience that comes from not having to sweat the small stuff.

A study published recently in the journal Science shows that the stress of worrying about finances can impair cognitive functions. The authors gathered evidence both from low-income Americans (at a New Jersey shopping mall) and the global poor (farmers in Tamil Nadu, India) and found that just contemplating a projected financial decision impacted performance on spatial and reasoning tests.

Among Americans, they found that low-income people asked to ponder an expensive car repair did worse on cognitive-function tests than low-income people asked to consider cheaper repairs or higher-income people faced with either scenario. To study the global poor, the researchers looked at performance on tests before and after the harvest among sugarcane farmers. Since it's a cash crop, the harvest signals a change in financial security but not a nutritional one. They found that the more secure post-harvest farmers performed better.

These findings complement the extensive literature on the negative physical impacts of low socioeconomic status, reinforcing that the harms of poverty extend beyond the direct consequences of material deprivation.

But the impact on cognitive skills is especially noteworthy for how it should influence our understanding of poverty. Poor people - like all people - make some bad choices. There is some evidence that poor people make more of these bad choices than the average person. This can easily lead to the blithe conclusion that bad choices, rather than economic conditions, are the cause of poverty. The new research shows that to some extent this is exactly backward. It's poverty itself that undermines judgment and leads to poor decision-making.

This effect may be an important psychological underpinning of recent economics research on the merits of unconditional cash transfers to the poor. Researchers have found that one-time grants of cash to poor Ugandans produced large observable gains in income as far as four years down the road. The easiest way to understand that is simply as a tangible return on investment. But perhaps the mental relief provided by the cash cushion allowed for sharper decision-making and problem-solving.

Most low-income Americans aren't poor at all by global standards. That's why it's so telling and fascinating that a study on the cognitive downsides of poverty would find identical results in New Jersey and Tamil Nadu. Much work on domestic poverty rightly emphasizes the idea of skills and "human capital" needed to navigate a complicated modern economy. This leads to a focus on education, whether in various school-reform crusades or the push to bring high-quality, affordable preschool to more households. But adults need help, too, and the perception that poor adults are irresponsible often leads to reluctance to see adults as being capable of deciding for themselves how best to use financial resources.

This paternalistic notion that we should be relatively stingy with help, and make sure to attach it to complicated eligibility requirements and tests, may itself be contributing to the problem of poverty. At home or abroad, the strain of constantly worrying about money is a substantial barrier to the smart decision-making that people in tough circumstances need to succeed. One of the best ways to help the poor help themselves, in other words, is to simply make them less poor.

Matthew Yglesias, author of "The Rent Is Too Damn High," is Slate's business and economics correspondent.