Six experiments were carried out with each monkey. Each human participant had one grape, which was placed on a platform midway between the human and the monkey. Sometimes one human faced away from the grape, and sometimes the eyes of one human were hidden by a sheet of cardboard, and sometimes both humans faced the grapes head on, but one had his or her eyes averted.
The idea was to determine if the monkeys studied the eyes of the human competitors closely enough to know whether the humans were really paying attention to what was going on, or were distracted enough that it might be possible to slip in there and grab that grape.
Each set of experiments involved around 20 monkeys, and all but three or four of them swiped the grape from the person who, based on the eyes, wasn't watching the grape. The researchers say the fact that even the person whose eyes were averted lost the grape is particularly significant because that shows it wasn't human intimidation but human inattention that determined the monkey's behavior. In other words, the monkey knew just what the human was thinking because the eyes were diverted upward and not locked on the grape.
That level of sophistication is probably surprising, even to monkey lovers, but Flombaum says the monkeys probably learned how to analyze a competitor's eyes because it's necessary for survival.
"When you live in a pretty large group of intelligent animals, especially where there's a lot of social behavior, it's a crucial ability," he says.
So is it something they've learned, or has evolution given them an ability that is so much like ours?
"I think it's a little of both," Flombaum says. "Evolution has equipped these animals to be able to quickly figure out how they need to behave, and how to figure out what others are thinking."
And evolution has probably "pre-wired" the monkeys' brains to "tell them that the way you know what others are thinking is to look at their eyes."
Humans do that, even on casual meetings with old friends, he says. A distracted gaze suggests the other person really isn't all that interested. Intense eye contact suggests genuine interest, and an inability to "attend to the eyes of others" may suggest some sort of mental impairment.
As the old saying goes, the eyes are the window into the soul, and as modern science adds, into the brain as well.
But another old saying might have to be revised. Now it will probably have to be something like this:
Hear no evil. Speak no evil. See if anyone's watching the grapes.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.