The horrors that we’ve faced in the past 12 months are frightening, and they’re real. But they’re not new. And they’re not increasing. So our national reaction to them, while real and heartfelt, is curious.

There have been terrorists before, and there have been cells of enemies living among us. The saboteurs of World War II, the Communist spies of the Cold War, the radical leftists of the 1970s, the right-wing militia terrorists of the 1990s — all of these were real, and all were dangerous in their own ways. There have been pedophiles before, even among priests — the scandal over that goes back decades. There have been murderers who preyed on children.

But even though national figures show that all these crimes have decreased in number in the past decade — not increased — after September we’re jumpier than ever. We’re waiting for the next attack, the next time we realize the people we trusted have turned on us. We’ve been made jittery by the realization that the world can be a dangerous place in which evil people live.

When this happens to individuals, experts have names for it.

Heidi Resnick, a clinical psychologist at the National Crime Victims Research and Treatment Center, part of the Medical University of South Carolina, studies the effects of violence on victims. She says people who have been attacked go through a period of nervousness.

“You’re more vigilant,” she says. “You’re on the lookout for something that previously you never thought would happen.”

There are two main theories to explain this, Resnick goes on. The first emphasizes conditioning, of the Pavlov’s-dog variety, and says that “triggers,” or events that remind the victim of the earlier attack, send a shiver of terror through the victim. These triggers could be as benign as a setting sun, if the attack occurred at night.

The second theory is cognitive, and emphasizes our ability to reason. When a person is attacked, he or she suddenly awakens to the reality that attacks could come at any time. This, reasonably, makes the victim fear that an attack will happen again.

Whether you focus on either theory to explain the effects of victimhood, the results are obvious. People who have been attacked look over their shoulders. They feel anxious, jumpy. When faced with stress, or with situations that recall the earlier violence, they recoil with fear or turn belligerent to degrees far greater than before the attack.

This “hypervigilance” has its place, Resnick says; it can keep you from getting attacked again. As the old joke says, you’re not paranoid if someone really is out to get you. On the other hand, if you’re seeing monsters under every bed, being hypervigilant can distract you from real dangers.

Sound familiar?

“I do wonder,” Resnick muses. “The level of fear that we’ve seen in our society since September, the concern about our children, the [concern for] protection of our food and water supply. It may be that there is some connection there.”

But the study of victims also offers hope. Resnick studies victims of rape. She says that two weeks after rapes, the level of post-traumatic stress disorder is astronomical — 90 percent. Which is what one would expect. But she also has found that when she studied rape survivors three and a half months after the attacks, the level of PTSD had dropped — by half.

It seems that once people have a chance to see that the world may be dangerous, but it is not completely unpredictable, their anxiety recedes. In some cases, though, that may not happen. “Until and unless they learn that there is relative safety,” Resnick says, “they will continue to have that level of anxiety.”

We know the world can be a dangerous place, and after 9/11 no one can ignore that. But since September we also have been reminded that it’s a place where men and women will give their lives to save others, where miners can be pulled from the earth alive. Where there are fewer murders, fewer attacks on children, fewer horrors than before.

But until that sinks in so firmly that we believe it in our bones, until we understand that we live in what Resnick calls “relative safety,” we’ll keep watching the skies, waiting for the next plane to hit. S