Imagine this: you have an opportunity to make a little extra money and you can double your earnings if you lie. Would you do it?

If you’re like most people, your decision would depend on the circumstances. What would you be lying about? To whom? And how much money would be at stake? Yet for some, the decision would also depend on the time of day. According to recent research, some people are more likely to lie in the afternoon than in the morning.

When people stand to gain something rewarding from bad behavior, they have to exert self-control to keep themselves from acting unethically. Twenty years ago, psychologist Roy Baumeister proposed that humans have a limited ability to exert self-control. He and his colleagues have compared self-control to a muscle that can be overworked and grow fatigued. The more self-control you exert now, the less you’ll have when you need to make another decision five minutes from now. This will be a familiar concept to anyone who has had to work near a candy bowl or a plate of brownies that a coworker brought to share. You might be able to resist them for the first hour, or even the second, but eventually your self-control will crumble and you will succumb.

It turns out that many different kinds of decisions and actions draw upon the same limited reservoir of self-control. For instance, restraining yourself from eating a freshly baked cookie that’s placed in front of you will later cause you to give up sooner in trying to solve a tricky puzzle. Likewise, trying to suppress a forbidden thought saps your ability to follow directions and stifle your laughter at amusing videos later on. Sleep deprivation, low blood sugar, and time pressure compound the problem and effectively drain your reserves even more quickly, while sleep, quiet restfulness, or meditation can replenish it. Studies have also shown that people can increase their self-control resources with practice over time, either in the form of tailored self-control exercises or simply by sticking to a regular physical exercise regimen.

The notion of a limited quantity of self-control works well as a psychological model, but it is problematic both on a physiological and a practical level. Despite the ancient belief that thoughts and feelings are fluids circulating throughout the brain, it is now clear that everything people feel and think arises from the complex communication between brain cells. Analogies equating self-control to a reservoir that can be tapped or a muscle that can be fatigued don’t easily translate to how the brain actually works. Describing self-control as finite is also tricky on a practical level. Let’s say that you turn down cookies all day, effectively depleting your self-control. By the end of the day, you might lack the self-control to abide by the speed limit on your drive home, but you’ll almost certainly find enough to keep yourself from decking the police officer who pulls you over. In other words, your resources are finite – to a point – and the specifics of the situation matter.

These caveats raise important questions about how the model of finite self-control relates to human behavior in the real world. Since your daily life can be thought of as a string of self-control challenges, the model predicts that you’d have less self-control as the day progresses and would be less able to behave ethically in the afternoon than in the morning. But is this actually the case? Maryam Kouchaki and Isaac Smith recently addressed the question in a series of psychology experiments and published their findings in the January issue of Psychological Science.

Kouchaki and Smith ran initial experiments with undergraduate students that seemed to support the model’s prediction, but the experiments were flawed because the participants were allowed to select their own testing times. While the results showed that participants tested on a computerized task in the afternoon cheated more than those tested in the morning, it was impossible to say whether the difference was due to time of day or if those individuals who eschew morning appointments also tend to cheat. If that sounds far-fetched, consider that late-risers might rise late because they lack the self-control to get out of bed earlier. If a lack of self-control caused them to choose the afternoon session, it could also cause them to cheat for personal gain during the task.

The researchers addressed this issue by running a new experiment online that randomly assigned participants to either a morning or afternoon test session (8-11 a.m. or 3-6 p.m. local time). During these sessions, participants were instructed to select one of two messages to send to an individual the participants had never met. While the participants were told that the individual existed and would receive their message, this person was actually fictitious. The experimenters also told participants that this individual would determine how much they would be paid based on the message’s content. If the participant sent a truthful message, he or she would receive 25 cents. However, if the participant sent a clearly deceptive message to the anonymous individual, the pay would be double that. How did the time of day affect the participants’ choices? Far more people sent deceptive messages in the afternoon (65%) than in the morning (43%).

Overall, these results support the prediction that self-control depletion throughout the day makes people more willing to lie in the afternoon. Still, almost half of the participants in the morning condition were willing to lie. Meanwhile, the majority of morning participants and more than a third of the afternoon participants opted to tell the truth, even though their honesty came at a cost. Time of day aside, the willingness to lie clearly differs from one person to the next. As the study’s authors point out, people vary in how unethically they can behave without feeling bad. Those who can easily justify lying and cheating are said to have “high moral disengagement,” while those who wrestle with their own slightest transgressions have “low moral disengagement.”

The researchers ran a final experiment to test how people’s personal tendencies to morally disengage interact with time of day to influence bad behavior. First, they had participants rate their agreement with eight morally questionable statements like the following: “Considering the ways people grossly misrepresent themselves, it’s hardly a sin to inflate your own credentials a bit.” Based on the ratings for the eight statements, researchers computed a moral-disengagement score for each participant.

The following week, participants logged on at their assigned morning or afternoon times for the second part of the experiment. They were instructed to scan number arrays for pairs of numbers that add up to ten. They would only have fifteen seconds to search each array before it disappeared. If they indicated during this time that they saw a number pair that summed to ten, they would earn five cents. If they didn’t respond during this time, they earned nothing. Since participants weren’t asked to name the pairs they’d found, the task gave participants an incentive to say they’d found a pair, even when they hadn’t. Here’s the catch: ten of the twenty number arrays in the task contained no number pairs that summed to ten. When people reported finding such pairs in the ten unsolvable arrays, the researchers assumed that they were lying.

So what happened? Participants lied on more of the unsolvable arrays in the afternoon (4.48 on average) than in the morning (2.63 on average). When the researchers combined these results with the moral disengagement scores of the participants, they found an intriguing interaction. People who tend to morally disengage and justify their bad behavior were as likely to lie in the morning as in the afternoon. Rather, it was the people with high moral engagement – the ones who care about ethical behavior and feel bad when they behave badly – who were affected by the time of day. On average, individuals in this group lied more than twice as often in the afternoon than in the morning.

Before you despair that everyone around you is a liar and cheater (especially in the afternoon), keep in mind that these findings are based on artificial tests conducted online and with small amounts of money at stake. Fewer people would lie to your face, steal your wallet, or cheat on an exam than would lie in these contrived experiments. Still, the study demonstrates that people are more susceptible to the seduction of lying for personal gain later in the day – a finding that probably extends to real-world situations.

If you are morally engaged enough to be concerned about this daily decline, then you are very likely affected by it. You may be able to minimize its effects by making key decisions and doing sensitive activities like tax preparation early in the day. When possible, schedule yourself enough time to carry out important tasks without feeling rushed. And for heaven’s sake, move that plate of brownies out of sight!

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist and regular contributor to Gareth is also the series editor of Best American Infographics, and can be reached at garethideas AT or Twitter @garethideas.