You might want to send your children from the room. Put the cat out while you’re at it. And draw the blinds, in case the neighbors are getting nosy. The time has come, as it must every campaign season, to discuss negative ads.

It is a subject sure to upset anyone with delicate sensibilities, a category that includes many members of the American political community — especially reporters and commentators, who apparently sit by their computer terminals and reach for the beta blockers every time they force themselves to peep through parted fingers at each fresh outrage posted on the Drudge Report.

“Negative Ads Rule the 2006 Elections,” says Congressional Quarterly. This is “The Year of Playing Dirtier,” announces the Washington Post. “The Election Campaign Is Talking Trash,” says the Baltimore Sun.

Even those cold-eyed realists at the New York Times have succumbed to an attack of the election-year vapors. “Theme of Campaign: Don’t Be Nice” was the headline over a recent story about “the most toxic midterm campaign environment in memory.”

How toxic? “It is,” reported the Times, “a jarring blend of shadowy images, breathless announcers, jagged music and a dizzying array of statistics, counter-statistics and vote citations.”

Statistics? Vote citations? Call me thickheaded, but I remain unjarred. And I bet such outrages like “counter-statistics” don’t jar most voters either.

There’s a saying among political consultants popularized by the Republican ad man Mike Murphy: The difference between a positive ad and a negative ad is that the negative ad has a fact in it.

This bit of folk wisdom has recently found academic support. In an original and thoroughly refreshing book published early this year, “In Defense of Negativity,” the Vanderbilt University political scientist John Geer undertook a definitive survey of negative advertising.

The poor man viewed almost every presidential-campaign television commercial aired since 1964, positive and negative alike, and arrived at an unexpected conclusion: The negative ads were better.

Being an academic, Mr. Geer had to define “better” with some precision. He had four criteria to distinguish good ads from bad. The best ads discuss pertinent political issues, have a relatively high degree of specificity, rely on documentation to make their point, and raise questions that the public itself considers important.

By each measure, the negative ads scored higher than the positive ads. We have long known that negative ads usually “work” in the elemental sense of helping to elect the candidate who airs them. Now it turns out that negative ads work for self-government too.

“The demands of attack ads are different from positive ads,” Mr. Geer says. “The threshold is higher. You need documentation and support. If a candidate just attacks, without documentation to back it up, it rebounds against the attacker and he looks like a fool.”

Negative ads, to be effective, also have to be specific.

“If your opponent says, ‘I want to grow the economy. I want the best education for our children,’ you can’t just say, ‘No, he wants to shrink the economy. He wants a terrible education for our kids,” Mr. Geer says. “You have to get into policy — you have to say why his policies will hurt education or the economy. That forces negative ads to be more substantive.”

On the other hand, positive ads — so beloved by the schoolmarms and moralizers in the commentating community — are often much more slippery.

Leave aside the gauzy feel-good spots that show candidates walking through dewy meadows with their appallingly cute kids. Even the schoolmarms know this eye candy contributes nothing valuable to voters’ decision-making.

Instead, Mr. Geer likes to cite a classically “substantive” positive ad aired by the 1988 Democratic presidential candidate, Massachusetts Governor Dukakis, in which he bragged of his “fiscal conservatism.” For evidence, the ad said Mr. Dukakis had balanced 10 state budgets in a row.

Left unmentioned was the state constitution, which required the governor to balance the budget every year.

“If you or I had been governor, we’d have balanced the budget too,” Mr. Geer says. “That doesn’t make us fiscal conservatives.”

Besides, when negative ads go too far, a self-correcting mechanism kicks in, as it often doesn’t with misleading positive ads. One ad has been cited in almost every article about this year’s toxic environment: Written by the National Republican Congressional Committee, it accused a Democratic candidate in upstate New York of billing taxpayers for a call to a phone-sex service.

The ad was scurrilous and only minimally accurate (the phone call was made by mistake). Here’s the good news: According to the National Journal, it was shown once on a single TV station. Every other station refused to air it — at the request of the Republican candidate on whose behalf it was made.

All the fretting over negative ads — and the fretting seems to intensify every election year — betrays a lack of faith in the resiliency of selfgovernment and in the intelligence of the people that self-government relies on: voters.

“They’re not fools,” Mr. Geer says. “They can sort out good from bad, take what they need to make a real decision and hold candidates accountable. To think otherwise is really kind of an insult to voters. Democracy isn’t really as fragile as all that.”

Mr. Ferguson is a columnist for Bloomberg News.