hese days at the National Air and Space
Museum in Washington, before visitors can see the Wright
Brothers' 1903 Flier, Charles A. Lindbergh's Spirit of St.
Louis, a touchable sample of lunar rock or any other souvenir
of humanity's bounce toward the heavens, they first have to
pass through a reminder of that morning the sky fell to
The family-friendly museum now has visitors pass through a
metal detector, open backpacks and diaper bags for inspection
and, if need be, step aside for a brief pat down or a few
waves of the beeping wand.
The reaction of the crowd outside to yet another gantlet of
Big Brothers and Bossy Older Sisters in uniform? "Mom, this is
so boring," a girl of about 6 grumbled.
After all, sanctioned nosiness and an almost aerosolized
military spirit have become the humdrum standard, as expected
as rush-hour traffic jams. Anybody who flies has the
opportunity to feel like a war correspondent, as one's effects
are searched and visibly armed officers wander airport
The National Institutes of Health in Bethesda, Md., used to
be an open complex, a symbol of the free spirit that is the
essence of the scientific enterprise. But with the temptations
the laboratories hold for potential bioterrorists, vehicles
are inspected from trunk to hood and even underneath with a
mirror on a stick. A lake that will be a protective moat
against car bombers is being built on the southern side of the
The Bush administration is talking about vaccinating
hundreds of thousands of Americans, if not more, against
smallpox, a menace thought to have been eradicated from the
And though a few people blink, tremble or grumble, many
find the signs of official vigilance comforting.
"I travel a lot, and when I'm waiting on line at the
airport I have a lot more patience than I used to," said Dr.
Dennis S. Charney, chief of the mood and anxiety disorder
research program at the National Institute of Mental Health.
"We're all doing what we have to do."
In sum, the abnormal has been normalized, integrated into
the bristling, blaring ecosystem through which Americans
navigate every day. And it is because human beings are so good
at adapting to change, including to stimuli that would under
many circumstances be considered negative and to updating
their world view as readily as they hyperlink from Web page to
Web page, that it sometimes looks as if the predictions of
last fall have failed to come true and that, gee, we have not
really changed at all.
We are still as fatuous and self-involved as ever, still
hankering after bigger and stronger Botox and S.U.V.'s, still
reading snarky "ironic" books like "Slander" and "Stupid White
Men," and Leonardo DiCaprio is still among us, starring in not
one but two movies this fall.
As scientists who study learning and memory see it, though,
we have changed in numerous, subtle ways, from a newfound
interest in world affairs and the inner workings of the
once-reviled United Nations, to a barely conscious but
unshakable expectation that the terrorists, whoever and
wherever they are, will surely strike again. Yet we hardly
notice these changes, any more than our skin expresses its
disturbance at being encased in long sleeves one day and short
The mechanism responsible for humanity's capacity to adjust
and, in former Mayor Rudolph W. Giuliani's phrase, to "get on
with life," is called habituation, and it is essential to the
functioning of the brain and the construction of a life.
"Habituation is really important and fascinating in its
details," said Dr. Thomas J. Carew, chairman of the department
of neurobiology and behavior at the University of California
at Irvine. "Typically what you habituate to are stimuli of no
proven consequence like the clothes on your body or the
ticking of the clock. It's adaptive to not pay attention to
The ease with which one can habituate to even the most
striking of stimuli became clear to him when he and his wife
were driving through the mountains of Canada.
"My wife coined the phrase, `Oh look, another breathtaking
view,' " Dr. Carew said. "That sums up how the bar needs to be
raised higher and higher for you to keep paying
Through studying laboratory animals and patients with brain
lesions, researchers have put together a rough model of how
the brain habituates, said Dr. Joseph E. LeDoux, a professor
at the Center for Neural Science at New York University. When
a rat is exposed to a sound followed by a nasty shock, for
example, the rat quickly learns to associate the sound with
pain, a reinforcement carried out by the amygdala, an
almond-shaped structure in the brain's limbic system, the seat
of emotions and emotional memories.
If over time, however, the rat hears the sound and is not
given a shock, another region of the brain, the frontal
cortex, sends signals to the amygdala to inhibit the fear
response. The frontal cortex, it seems, strives constantly to
shift as much information as possible from "amygdala alert" to
the polite yawn. Paying constant attention is
resource-intensive and exhausting.
Moreover, said Dr. Terrence J. Sejnowski, a theoretical
brain scientist at the Salk Institute in San Diego,
maintaining perpetual alertness, particularly of the anxious,
fearful variety, requires a chronic release of stress-related
hormones like the corticosteroids, which can eventually break
down muscle tissue, bone tissue and the neurons in some of the
more fragile parts of the brain like the hippocampus.
There is also a strong tendency in all biological systems
to return to a state of so-called homeostasis, or equilibrium.
The body has an array of mechanisms to maintain the blood in a
very narrow pH level, for example, as close to the ideal of
7.4 as possible, because that is the balance of acidity and
alkalinity at which hemoglobin is best at grabbing oxygen.
By the same token, said Dr. Steven R. Quartz, director of
the Social Cognitive Neuroscience Laboratory at the California
Institute of Technology in Pasadena, the brain has a deep need
to return to homeostasis, to its particular mix of signaling
molecules like serotonin and dopamine, whenever a powerful
event has thrown it out of biochemical whack. A tragedy like
Sept. 11, Dr. Quartz said, "triggers massive chemical
reactions in the emotional structures of the brain."
A result, he added, "is bereavement, the inability to focus
on everyday tasks or follow a routine and social
But within days, weeks or months, depending on the person,
the brain generally returns to its chemical set point.
"Thus," Dr. Quartz said, "paraplegics report themselves as
basically happy in a matter of months after their accident.
"Lottery winners also report themselves as being at their same
old level of happiness within a year after they hit the
A return to equilibrium is by no means guaranteed, and
great shocks can leave deep plangent scars on a subset of
people, particularly those who experienced the collapse of the
World Trade Center directly. Some have suffered from the
terrible syndrome called post-traumatic stress disorder, with
its symptoms of nightmares and flashbacks, chronic
hyperanxiety and a tendency to detach from the world.
Dr. Charney, who studies the syndrome, said various
interventions, including intense psychotherapy and the right
cocktail of antidepressants, had been shown to ease the
symptoms. And doctors emphasized that few of the many millions
who watched the attacks on television were likely to have
fallen into its vise.
As Dr. Quartz sees it, the nation has
been eased back to emotional homeostasis through the display
of exactly those symbols and actions that would in more
ordinary circumstances seem obtrusive and disturbing, the
armed guards, the metal detectors and the ID cards dangling
around every employee's neck. Large, intricate and impersonal
though contemporary society may appear to be, he said, it
holds elements of ancient human needs — for the comforts of
family, the strength of the tribe.
"We see familial metaphors all around us," Dr. Quartz said.
"The White House as the national family home, the first family
and so forth."
When disaster struck, laying waste the figment of our
invulnerability, we needed symbols that the family and tribe
still held, and we needed them quickly.
"It was imperative that protective symbols like armed
guards and heightened bag searches be put in place," Dr.
Quartz said. "Had they not, I wonder if there might have been
a major crisis of identity and statehood."
Even though many people may suspect that such measures are
largely useless — knives and guns still manage to pass through
airport security gates — they serve the same consolidating,
totemic purpose, Dr. Quartz said, "as the ritual spear a
tribal chief might carry."
Children, who have no choice but to be very good at
accepting change (their bodies are doing it to them every
day), likewise know the power of symbols, rituals, rules, the
Dr. Ellen R. DeVoe, an assistant professor of social work
at Columbia, who with Dr. Tovah Klein of Barnard has been
studying the reactions of young children in Lower Manhattan to
the trade center collapse, said children had a remarkable
capacity to create stories that reconstruct from the sorrow
they see around them a safe and bearable world. She described
two children who suggested that the towers be rebuilt, but
this time with a billboard on them saying, "No airplanes can
That sign would stop nothing, of course, except, perhaps, a