The question has been raised frequently in recent days. Why has the public reaction to Katrina been so dramatically different from the response to Sept. 11?

Immediately after the 2001 attacks, President Bush’s approval rating shot up to 86 percent, the country appeared unified, partisanship was temporarily exiled and the “blame game” attracted fewer players than a typical round of three-card monty.

In the days since devastation washed up on the shores of the Gulf Coast, the president’s ratings have sunk, recriminations have risen and one head has already rolled.

Why the difference?

Some point to polarization. Every event, every issue, falls prey to the deep divisions wracking the country, the argument goes.

I’ve raised some doubts about the whole polarization hypothesis in the past, noting that “evenly” divided is not the same as “deeply” divided, while also recognizing that increased coherence has created the sense of greater polarization. But however one analyzes the state of polarization in the nation, it cannot explain the difference in reactions to these two catastrophes.

The country was as polarized on Sept. 10, 2001, as it was on Aug. 31, 2005. Indeed the president, then still seen as a somewhat accidental occupant of the White House, recorded his lowest approval rating to that point on Sept. 10, 2001 — 50 percent.

So, if the solution to this puzzle does not lie in differing states of public opinion before the event, we ought to search for the explanation in the varying characters of the events themselves. Americans responded differently because a murderous attack on our country by outsiders affects our emotions in radically different ways than does a natural disaster.

Presidential approval ratings always go up during foreign-policy crises. Bush’s ratings shot upward on Sept. 11 as they did the day we invaded Afghanistan and again as we invaded Iraq. So had President Jimmy Carter’s the day the hostages were seized in Iran. President John F. Kennedy reacted with bemused incredulity when his approval rating rose in the wake of the Bay of Pigs fiasco. (For those who have lost track of the details, the word “fiasco” should be a tip-off that it didn’t turn out so well.)

Crises involving foreigners raise approval ratings because they instantiate the “us” versus “them” dynamic that animates every social species. Whoever the “we” and “them” might be, whenever “we” feel threatened by “them,” “we” pull together to face the challenge from the outsider. That unity reflects itself in social solidarity and support for the leader. Such reactions are a fundamental tenet of group identity and behavior.

These sentiments create a mutually reinforcing set of feelings. By attacking “us,” “they” instilled a sense of national pride evident in responses to survey questions and in skyrocketing flag sales. Studies demonstrate that patriotism and national pride in turn increased trust in fellow citizens and in government.

In a natural disaster, there is no “them” and thus no impulse to the unified “we,” required to repulse the aggressor. With no greater sense of national pride, there is no increase in trust in government.

When ancient humans were attacked, they banded together to battle the invaders. When they were victims of a natural disaster, they wept, they helped their friends, and to appease angry gods they looked for one of their own to sacrifice. Little has changed.

Democratic Party leaders respond to these instincts and to one other. Activist Democrats have long complained about a party they believe is too capitulationist. Two years ago, Howard Dean was riding that wave of frustration to front-runner status in the contest for the nomination.

With no rally effect in evidence and failure obvious, Democrats feel free to respond to the pent up frustrations of their partisans.

Mellman is president of The Mellman Group and has worked for Democratic candidates and causes since 1982, including Sen. John Kerry (D-Mass.) last year.