A roofer shoulders a load of shingles and climbs onto the slippery rungs of a ladder. He knows he ought to double-check the ladder’s footing, but all he can think about is the cost of gas, and how he’s going to fill up his truck without another payday loan.

An exhausted CEO stares at her inbox, overwhelmed by hundreds of unopened emails. A promising proposal for a new business line sits on her desk, but she’s got back-to-back meetings scheduled until doomsday. She sticks a Post-it note on the proposal — “Read me!” — and adds it to a towering stack of folders on her credenza.

A man sits on the floor of an airport lounge, sipping a cup of overpriced coffee while his laptop charges. He’s been looking forward to vacation, but for some reason, he can’t get his head into it. All he sees are happy couples. They’re everywhere: kissing, passing a bagel back and forth, even just sitting there, side by side, looking slightly bored. That companionable boredom seems to him the pinnacle of happiness.

What do the roofer, the CEO, and Mr. Lonelyhearts have in common?

According to Harvard economist Sendhil Mullainathan and Princeton psychologist Eldar Shafir, they’re all suffering from the same problem: scarcity.

Professors Mullainathan and Shafir, who’ve collaborated on several projects at the intersection of economics and psychology, recently published Scarcity: Why Having Too Little Means So Much, which makes the case that scarcity — and its bedfellow, stress — impose a terrible tax on our ability to make good decisions.

Whether it’s scarcity of money, time, or companionship, the more acutely we feel the lack, the more we tend towards tunnel-vision.

This isn’t always a bad thing. There’s nothing like hunger, desire, or a deadline to sharpen the mind and get the creative juices flowing.

But there are limits. When need becomes chronic — as in the case of the roofer who spends all of his time worrying about money; or the CEO, whose horizons are sharply narrowed by prior commitments; or Mr. Lonelyhearts, whose desperation oozes from his pores, driving away potential mates — then it’s no longer productive, and actually a barrier to solving the core problem.

Drawing on decades of studies and innovative field work of their own, Mullainathan and Shafir argue that each of us is endowed with a certain capacity for making decisions — they call it “bandwidth” — and that scarcity actually causes that bandwidth to shrink.

The consequences of that shrinkage can be severe. Long-term planning is the first casualty when all we can think about is satisfying an acute short-term need. Impulse-control tends to fly out the window, too, leading a dieter to binge; a debtor to sink ever deeper in debt; and the harried executive to waste time massaging an impossible schedule.

Simply put: scarcity makes us dumber.

It turns out that the “bandwidth tax” imposed by chronic need can be scientifically quantified. One of the studies in the book involves sugar cane farmers in India who get paid once a year, after the harvest. The months before the harvest are full of money worries, as the farmers and their families try to stretch the last of the year’s rupees. The months after the harvest, on the other hand, are times of ease and plenty.

These farmers were given two sets of similar problems to solve, one in the lean time and the other when cash was plentiful. It turns out that money worries cost them, on average, ten IQ points!

And to demonstrate that the “bandwidth tax” isn’t just a problem in the developing world, Mullainathan and Shafir set up an experiment in a mall in New Jersey, tracking problem-solving ability against income. They asked a group of people from different walks of life to consider a thought experiment: your car needs a repair, and the repair will cost $300; do you get it repaired right away, or do you wait and hope the car doesn’t break down? How would you make this decision? they asked. Would it be easy or hard?

After the participants passed in their answers, they were then asked a series of standardized test questions.

The results of those tests showed very little difference in intelligence between well-off and poor participants.

Then the same thought experiment — with one important change — was run with a new group. This time, instead of proposing a car repair that cost $300, they were told that the repair would cost $3000.

The result? Low-income participants fared significantly worse on the intelligence test that followed.

The authors explain the difference in terms of “bandwidth.” The wealthier participants were able to absorb the idea of a $3000 car repair without too much trouble; the low-income participants, on the other hand, spent a lot more mental energy grappling with the challenge of coming up with 3000 imaginary dollars. By the time they got to the standardized test, their minds were worn out.

Of course, the problem with scarcity is that it’s real, and we’re all subject to it. Good luck telling someone living paycheck to paycheck, “Worry less about money!” or an executive working 80 hours a week, “Make better use of your time!”

But they say that the first step towards solving a problem is acknowledging it exists. The next time you feel hard pressed — before you go sneaking a doughnut, or putting a new pair of shoes on the credit card, or agreeing to dinner and a movie with the office schlub — try stepping back and asking yourself, “Is this really a good idea for the long run?”

The answer’s probably staring you in the face.