The conventional wisdom has Islamic radicalism as the root cause of suicide attacks. Such attacks have been widely used in Israel and are now being used in Iraq as well. But the numbers tell a different, and somewhat surprising, story.

Robert Pape, a political science professor at the University of Chicago, has compiled a database of all suicide attacks of the past 20 years. "The fact is, the world leader in suicide terrorism, the group that has done more suicide attacks than any other, is the Tamil Tigers in Sri Lanka. This group has done 75 of the 188 attacks in the entire universe. So the connection between Islamic fundamentalism and suicide terrorism is misleading," he says.

The Tamil Tigers is a secular separatist group with a Marxist ideology. The group is believed to have been the first to use the so-called "suicide vest," in which explosives are packed inside a vest worn by the attacker and detonated when at the target. A female Tamil Tiger suicide bomber, for instance, used that method to kill Indian Prime Minister Rajiv Gandhi in 1991.

Wherever you find suicide bombings, Mr. Pape says, there is a common thread. "It is not driven by religion, but by a clear strategic goal," he says. "What all the suicide terrorist campaigns have in common over the last 20 years is that the groups that pursue them are seeking to compel liberal democracies to withdraw military forces from territory that the terrorists view as their homeland, from Lebanon to Sri Lanka to Kashmir to the West Bank."

Even al-Qaida fits the pattern. The terrorist group is seeking to force the withdrawal of all U.S. troops from the Gulf region.

Mr. Pape says terrorist groups utilize both guerrilla tactics and suicide bombings in tandem. But he adds that they turn more to suicide bombings out of frustration when guerrilla tactics against a superior military force fail. That, Mr. Pape says, appears to be what is happening now in Iraq. "As the guerrilla attacks that mainly occurred in the first several months of the war are actually losing their sting because the American forces are hunkering down and being much more able to deal with ordinary guerrilla attacks, you are seeing that increasingly that the groups are moving more towards suicide attacks. This pattern is unfortunately very common," he says.

Suicide bombings spark revulsion and anger in the populations under attack, thus causing governments to take tough stands. But the impulse to bring overwhelming military force to bear against the terrorists does not always work. Mr. Pape says suicide bombing campaigns have in past won concessions from governments that find that sheer force cannot stop such attacks.

Terrorists, he said, have found that suicide attacks can pay political dividends. "Suicide terrorists sought to compel American and French military forces to abandon Lebanon in 1983, Israeli forces to leave Lebanon in 1985, Israeli forces to quit the Gaza Strip and the West Bank in '94 and '95 and the Sri Lankan government to create an independent Tamil state in 1990. In all those cases, the terrorists' political cause made more gains after the resort to suicide operations than it had before," he said.