Among the points made in their 2003 book, "In the Wake of 9/11: The Psychology of Terror," was that modern people are much more exposed to "alternative meaning systems" than in the past.
"We have different versions of God, different versions of evil," Greenberg said. "It's one of the things that contributes to some of the large-scale conflicts that exist in the world. With exposure to other people and other belief systems, there's a threat to our own sense of meaning."
Some of the things Americans do, for example, violate the belief system common among residents of the Middle East, and vice versa. People in the Middle East tend to be less materialistic and more spiritual, Greenberg said. They think about death more consciously, and they consider people who die for their beliefs to be martyrs.
"And because we don't understand their worldview, we're angering them in a very deep way and we don't understand it sometimes," he said. "Our hope is that by understanding psychologically where people are coming from ... we can use education, communication and other diplomatic tools to try and change the way we're viewed."
In the United States, the threat of terrorism has made citizens feel more vulnerable and more anxious for security. A study Greenberg and his colleagues conducted before the 2004 presidential election found that college students were slightly in favor of Democrat John Kerry. But when the students were reminded of their mortality, a fear that terrorism provokes, the majority favored his Republican opponent, George W. Bush.
"It's the psychology of the soul in the sense of looking at the deepest things we rely on in our lives," he said. "It is a sense of inner being that helps us function and feel secure in what's really a scary world."
This idea also may help explain why some people living in an impoverished city neighborhood turn violent if they perceive their self-worth being disrespected or "dissed," Greenberg said.
Pelin Kesebir, a graduate student in social psychology at the University of Illinois at Urbana-Champaign, is conducting research in existential psychology. She found that when students were reminded of their mortality, they thought more about the meaning of life and they needed to think that the things they believed in are long-lasting.
They assert, for example, that people who represent American values (Kesebir named Oprah Winfrey as an example) will be remembered for a very long time -- twice as long as the time given by students not thinking about death.
"Famous people serve as an existential buffer," she said. "Their imperishability mollifies people's existential anxieties."
"We all want a sense of continuance, a sense that we're more than just these temporary creatures on this dirt ball," Greenberg said. "We want to feel we're significant beings in a meaningful world."