The Yanomamo

Indians of the Amazon rain forest have long been known as one of the most violent societies on Earth.

According to data gathered over the last quarter century, 44 percent of Yanomamo

men over the age of 25 have killed someone, about 30 percent of adult male deaths are the result of violence and nearly 70 percent of the men and women over the age of 40 have lost a close relative to homicide. Most of the deaths occur in endless cycles of "blood revenge" attacks between warring villages, usually armed with arrows.

Now the anthropologist who has studied the Yanomamo

the most has advanced a new theory to explain both the violence and an element of human nature that he suspects may pervade all societies.

Yanomamo

men kill, according to the theory, because the act wins such high social status in this polygamous society that the killers enjoy more wives and children than men who have not killed.

In other words, sexual competition can be so powerful a force that men will engage in social behaviors, even highly disruptive behaviors, if that will win them advantages in their striving for wives. Yanomamo marriages are arranged by the woman's father, who usually prefers to marry her to a man who, in industrialized societies, would be called a war hero.

Although the Yanomamo

say they live in fear of the violence, the men who have killed are so highly regarded that they have, on the average, 2.5 times as many wives and three times as many children as those who have not.

"These numbers are extremely significant and really stunning," said Napoleon A. Chagnon, an anthropologist at the University of California at Santa Barbara who has studied the Yanomamo

for 23 years and described his new theory in last week's Science. "Violence is a prominent part of the life of these people but I never expected to find it linked to this kind of result."

The Yanomamo , numbering about 15,000 and clustered around the Venezuela-Brazil border, are the largest group of people in the world still living in such a primitive society that they have no written language, no precise number system, no formal laws and no institutionalized adjudicators of disputes such as chiefs or judges. The Indians live in small villages of a few score people scattered through the forest where they plant plantains and bananas and hunt for meat.

Chagnon's statistics are based on studies of 12 villages in Venezuela, largely the same groups described in his classic book, "Yanomamo : The Fierce People."

Although Chagnon's theory on the causes of social violence applies to all peoples, the effects may be quite different in societies where a strong police force and judicial system deter violence. Male sexual competition may then be channeled into other arenas such as the pursuit of wealth or power.

"In many societies," Chagnon said, "achieving cultural success appears to lead to biological {genetic} success."

Chagnon's theory also contributes to a long-running debate among anthropologists about the primary forces that drive societies to war.

One well-established school, led by Marvin Harris of the University of Florida, holds that wars are always the result of competition among large groups -- economic classes or whole societies -- for material resources. These range from nomadic clans battling for a waterhole, for example, to nation-states seeking more land or a seaport.

Chagnon's theory would expand the root causes of war to include such individual motives as sexual competition. In so doing, Chagnon argues that social evolution is guided by the same selective forces that Darwin saw guiding biological evolution.

Darwin argued that in addition to natural selection, which is analogous to the social competition for material resources, species also evolved under the influence of sexual selection. In other words, the more individuals are attractive to the opposite sex, the more likely they are to find mates and reproduce. The peacock's tail, for example, has become so showy because the more spectacular it is, the more likely a male can attract a mate and reproduce his genes.

Though not necessarily guided by specific genes, Chagnon argues that human social behavior can still evolve if a person's reproductive success depends on performing well at a given behavior.

Chagnon has found that most, if not all, Yanomamo

wars start when a man from one village abducts a woman from another village for a wife. The woman's kinsmen then seek revenge by raiding the abductor's village and trying to kill him or his kinsmen. Every successful attack prompts a larger retaliatory attack. Cycles of attacks often persist for many years.

Violence also pervades much of life within Yanomamo

villages. Boys are encouraged to be valiant and are rewarded for showing aggressive tendencies. Conflicts, which usually involve sexual issues such as infidelity or attempted seductions, are played out in a graded sequence of increasing seriousness from shouting matches to chest-pounding duels to club fights to shooting with bows and arrows.

If a killing occurs within a village, the people split into two groups of partisans and one moves away to start a new village. Cycles of reciprocal raids follow.

Although such frequent killing might seem counterproductive, hurting a village more than it helps, Chagnon found that if the reprisal raid is swift enough, it has a deterrent effect on counter-reprisals.

"The Yanomamo

explain that a group with a reputation for swift retaliation is attacked less frequently and thus suffers a lower rate of mortality," Chagnon wrote. Villages with a reputation for aggressiveness are also less likely to lose their women to abduction.

Although the social evolution of the Yanomamo

has, in effect, trapped them in a violent tradition, Chagnon said the people can be quick to see the benefit of the legal system that minimizes such violence in more developed societies. He told of a young Yanomamo

man who had been taught Spanish by missionaries and sent to the territorial capital for training as a nurse.

"There he discovered police and laws," Chagnon wrote. "He excitedly told me that he had visited the town's largest pata {the territorial governor} and urged him to make law and police available to his people so that they would not have to engage any longer in their wars of revenge and have to live in constant fear."