They all have the same simple quality - cave paintings, carved heads and masks from long-dead Stone Age people and artworks from existing tribes.

Anthropologists and other experts once dismissed the simplified faces, which lack detail and proper proportion, as evidence the artists were untrained or unsophisticated. But a Harvard University psychiatrist has another explanation.Stone Age tribes were just plain stressed out, says Dr. Anneliese Pontius of the Harvard Medical School. Constant fear - of dangerous animals, evil spirits and other tribes - changes the way the brain functions, she says.

Basically, the brain takes shortcuts under stress so it can work faster in dangerous situations, she told the American Association for the Advancement of Science. These shortcuts are reflected in art and may show up in other ways as well.

"They are not stupid errors or childlike errors or intellectual inferiority," Pontius said after testing people ranging from the Dani and Asmat hunter-gatherers in Indonesia to Canadian Inuits and Aucas in the Amazon.

"These hunter-gatherers live under very pervasive threat to their lives from other tribes and from animals - snakes and even insects," she told a news conference at the AAAS. "When the sun sets everybody flees into their huts out of fright."

She wanted to test brain function, but most tests reflect culture and require the subjects to be literate. So she used two tests that can check visual and spatial functions in the brain - the simple "draw-a-person-with-face-in-front-test" and one in which the volunteer uses four colored blocks.

"These tests are ideal for assessing such functions in large groups of intelligent but nonliterate peoples because elaborate verbal instructions are not required, the tasks are relatively simple and the data `yield' . . . is rich," she said.

Pontius found that people at a Stone-Age development level draw simpler versions of faces than literate people. Faces drawn by modern-day hunter-gatherers looked much like faces in art left by early humans.

"It's not a cultural difference because it exists in dyslexics," she said, referring to people with a fairly common reading disability. She also found that very young babies, who are primed to respond to faces, reacted more quickly to the simpler, Stone-Age style faces.

The differences in the tests are subtle. For instance, literate people usually draw the bridge of the nose narrower than the tip and include the brows above the eyes, while nonliterate and dyslexic people make fewer such distinctions.

What could be happening is that the brain is using a shortcut - in this case the subcortex - to process information. Brain processing in the subcortex is faster, to the tune of about 250 milliseconds - 250 thousandths of a second - Pontius said.

"If your life is at stake, 250 milliseconds may mean life or death," she told the news conference. One does not need subtle distinctions to flee from a snake or to recognize an enemy's face, she suggested, but this becomes a habit in continually strained people.

"Any processing goes first through the subcortex and then, if it is a more subtle type and has detail, it goes through the cortex," she told Reuters in an interview later. She said the subcortex evolved earlier and is a kind of pre-processor in the brain.

Pontius tested people with dyslexia and with brain damage that affects subcortical processes and found the same effect. She said her findings also vindicated the primitive groups, who some researchers had dismissed as intellectually inferior.

Pontius, who is also a child psychiatrist, believes her theory could explain why inner-city children fall behind in school. They are simply stressed out and afraid, she suggests. "It reflects the behavior of people in fear for their lives."