Here's what they found:
"We were able to determine that people did not react primarily with sadness; that they experienced a number of anxiety outbursts, but recovered quickly; and that they steadily became angrier."

When you look at the graph, you see tiny spikes of anxiety through the day, a small wave of sadness, and a monstrous, growing wave of anger.

I remember the day well, and the emotional tidal wave that came with it. I knew people working in the World Trade Center and on one of the planes that crashed into the towers. From the moment the first tower fell, I remember feeling ripped open, raw, and profoundly sad. As the second tower crumbled, I remember asking my sister on the phone, "Do you think people who aren't from New York will understand how devastating this is?" (As it turned out, they would, although at the time, it was impossible to see how much the unfolding event would shape the decade.) As the day progressed and we all learned more about what had happened, my sadness was joined by fear of a world shifting, and a new uncertainty.

But I don't remember being angry. Not that day, and not in the days that followed. I do remember being astonished by how quickly other people's grief became anger. And I remember how full of anger President Bush's messages to the American public were. His anger was a relief to most of my friends and family, even a source of strength as people came to terms with what had happened.

Although anger has historically been labeled a negative emotion, and lumped together with emotions like sadness, fear, self-doubt, and disgust, it has a lot in common with the so-called positive emotions. Like happiness, excitement, and love, anger is an "approach" emotion. Anger makes you want to do something. To act. To express yourself. It might be to attack, fight, or scream -- something that many of us find inherently satisfying, at least more so than the overwhelm and incapacitation of grief.

No other negative emotion so strongly motivates us toward action. Most negative emotions render you frozen, stuck, or weak. You withdraw. You shut down. Not anger. Anger actually makes you feel stronger. It floods you with energy. It makes you feel right. It makes you feel important. It helps you feel in control. And this is why most people's main strategy for dealing with any kind of sadness and anxiety is getting angry.

It took me longer than most Americans to "get over" that day. The grief stayed with me for a long time. People seemed surprised that I would still sometimes cry when thinking about the events and the lost lives -- not just days, but weeks and months afterward. Images haunted with me, like the parking lot full of abandoned cars at the commuter's train station near my childhood home, whose owners worked in the WTC and never came home. I was one of those people who screamed "Too soon!" when the coming attraction for the first movie about 9/11 screened in theaters.

I think now that my delayed emotional recovery had something to do with the lack of anger I felt. Getting angry helped a lot of people get on with their lives. It's hard to say that the national anger that led us into two wars was good for us, or the world. But I can appreciate why anger was the instinctive emotional response for so many. When an event is so sad that it is hard to comprehend, anger may be the only way to keep your head above water. I know that I felt like I was drowning for a good, long time.

It's hard to know how a nation in mourning -- one that fully felt the loss, without quickly shifting to anger quite so fast -- would have responded that day. But I wonder.