Is it the eyes? The light? The sort-of smile? What is it that makes the Mona Lisa one of history's most memorable portraits--so compelling, even in its creation, that the young Raphael sat at Leonardo's knee just to watch him paint it?

Scientists, like most of the rest of mankind, do not have the answer, but they have noticed that Leonardo, like most portrait painters for the past 500 years, favored the left side of his subject's face and put her eye almost exactly on the canvas's centerline.

The reason for turning left, suggests Australian researcher Michael Nicholls, is that the right cerebral hemisphere, the side most responsible for emotion, controls the left side of the face. If someone wants to look warm, cuddly and personable--and most people do--there's only one cheek to turn.

At the same time, San Francisco visual expert Christopher W. Tyler says in a separate study, portraits by 165 masters from Van Eyck to Picasso show the artist focusing on one eye--not the mouth, not the chin, not the bridge of the nose--and putting it on the painting's north-south axis to draw the viewer's to a single point.

The effect of these twin techniques, one might say, is to bring viewer and subject into intimate contact so the subject's sunny left side can impart a feeling of friendliness and emotional kinship.

Done properly--and no one questions whether Leonardo, Rembrandt or Picasso could do it properly--the canvas has the impact of a thunderclap.

The curious thing about all this, however, is that neither the Nicholls team nor Tyler could find any evidence that artists studied these phenomena or deliberately set out to make use of them--even though Leonardo, for one, was as renowned a scientist as he was a painter.

"It's such an odd thing," said Nicholls, a psychology professor at the University of Melbourne.

"You've got portraits through history and they seem to have this remarkable [leftward] bias. You go across the generations and the styles of painting, trying to think of explanations."

In its article in the August edition of the British journal Proceedings: Biological Sciences of the Royal Society, the Nicholls team first explored the possibility that artists went left because most were right-handed. They preferred to paint left sides so they wouldn't have to peer at the subject over their painting arms.

Except that Holbein and Raphael, like Leonardo, were noted lefties--and they painted left sides. So did Picasso, who was also supposed to be left-handed, Nicholls said, even though "he's always right-handed in photographs."

But then there was Rembrandt: 57 self-portraits, most of them with the right cheek turned. Exception to the rule?

Probably not: "He used a mirror," Nicholls said. Like everyone else, he turned his left side to the mirror, but when he painted the mirror image, left was right.

In all, the Nicholls team found that 58 percent of 361 portraits it examined were left-sided. The team also noticed that the percentage of left cheeks rose sharply for portraits of women--to 78 percent.

"That's what made us start thinking about all this," Nicholls said, especially after one member of the team scanned a catalogue of portraits from the Royal Society, the British equivalent of the National Academy of Sciences, and found no leftward bias whatsoever.

"We came up with the thought that the side of the face was perhaps more closely tied to a desire to express or conceal emotion," Nicholls said.

Leonardo's Mona Lisa wanted to look nice for her merchant husband. Einstein wanted to look heavy-duty.

So the team devised an experiment to test the hypothesis: It was the subject, not the artist, who was responsible for turning one cheek or the other.

The experiment invited 165 Australian psychology students (122 women, 43 men) to sit for "portraits" in front of a video camera. Neither the subjects nor the proctor in charge of the experiment knew the purpose of the test.

Half the subjects received a script telling them they were "going overseas for a year" and wanted to leave a portrait for their "close-knit family" as a reminder "of how much you love them."

The others were told "you are a successful scientist at the pinnacle of your career." The Royal Society "has just accepted you for membership" and wanted a portrait. "You want to give the impression of an intelligent, clear-thinking person, but don't want to look at all smug or proud."

Then the students looked at the camera. Fifty-eight percent of the "close-knit family" women and 64 percent of the men turned their left cheeks.

Fifty-seven percent of both sexes turned right for their Royal Society portraits. Three subjects refused to turn their faces one way or the other.

"It was kind of neat," Nicholls said.

Tyler, associate director of the Smith-Kettlewell Eye Research Institute, had a problem similar to that encountered by the Australians. Nobody could tell him why artists put a single eye on a painting's centerline.

"If you read artists' discussions or manuals on how to compose, they just say 'choose a focus somewhere in the picture,' " Tyler said. "They also tend to say 'avoid the centerline.' "

So Tyler asked artists, and while "some did it consciously," many others "have no idea." Also, he added, "no one suggested it is a general principle."

In fact, artists "just don't like the whole idea very much," Tyler said. "They think they are in a domain of freedom where every picture is different."

But after drawing lines down the middle of hundreds of portraits, Tyler reported in the scientific journal Nature that one eye is within 5 percent of the centerline "at least two-thirds" of the time. The Mona Lisa's left eye is exactly on the axis. George Washington's right eye marks the centerline of the dollar bill.

Rembrandt's self-portraits have candlelight, shadows and all kinds of ghosting images chasing across the canvas, Tyler noted, but they "adhere very tightly" to the one-eyed principle.

So does Picasso, "even though he seemed to break nearly every canon of plastic art," he added.

"I think that it's intuitive--I got flak for using the word 'subconscious,' " Tyler said. "It's not a rule. A lot of artists will agree that there is a certain way to do it that's right, and certain ways that aren't right."

And when you're good, you just know.