Previous studies have documented the effect in people who are unusually exhausted, but Smith's work with Maryam Kouchaki of Harvard University showed it can happen in just a day.
"We found the unremarkable experiences of everyday living can lead to this effect, which people might not suspect," he said.
They developed the theory as they watched subjects in other ethical studies, like the effect of speed or inspiration on morality, make more bad decisions later in the day.
The researchers tested the concept with four experiments involving a total of 265 people. In one, they found people who could earn more money by fibbing were 50 percent more likely to lie in a message to a virtual partner in the afternoon. In another experiment involving an unsolvable number-matching problem, the number of liars jumped from 2.5 in 10 in the morning to 4.5 in 10 in the afternoon.
The researchers also found people were more likely to pick up a celebrity magazine, People, over a more serious publication, the New York Review of Books, in the afternoon. While 41 percent of morning readers went for the gossip, that portion grew to 59 percent in the afternoon.
The effect seems to be more pronounced in people who are generally disposed to making good moral decisions. People who can "morally disengage," or avoid feeling guilty for unethical behavior, didn't tend to change based on time of day.
"Unfortunately, it might be that the most honest people … are most susceptible," Smith and Kouchaki wrote. "In other words, our findings suggest that mere time of day can lead to a systematic failure of good people to act morally."
For businesses, the takeaway might be to watch consumers or employees more carefully for bad moral decision-making in the afternoon. For regular people, it might be to plan a workout in the morning and hide the Reese's in the afternoon.