If there is one Darwinian adaptation that the American people have made to modern times, it is the ability to sift through a wide variety of claims and to determine for themselves which are specious and which are accurate. We realize that the days during which we could trust any one media outlet or candidate to give us the full story are long over -- if they ever existed in the first place. We realize that truth is a synthesis of the various claims made by the left and the right, the Democrats and Republicans, and the incumbents and the challengers.

Voters see negative advertising as another form of information. They so distrust politicians that they want to see their opponents tear them down so they can get at the truth. In fact, voter attitudes toward politicians are akin to their opinions of criminal defendants (they could be forgiven for confusing the two). Just as juries want a prosecutor who tears the defendant apart and punches holes in his alibi, so they want a political candidate to run ads exposing his opponent.

Of course, negative ads do not always work. Sometimes they backfire big time. So when a candidate runs a negative ad, he takes his life, career, and reputation in his hands. If the ad turns out not to be true and an alert opponent jumps on him and runs a rebuttal ad exposing its inaccuracies, he can lose the election in a heartbeat.

Voters have a skilled baloney detector embedded in their consciousness. They know that politicians who have proclaimed their own honesty have ended up in prison, while others who say "read my lips, no new taxes" have broken their solemn vows and jacked up rates anyway. So they watch all television with suspicion. To succeed, nega­tive ads must work overtime to get in under the detector.

Negative ads must emphasize fairness and accuracy even at the price of having less overt impact. The best nega­tive ad I ever ran was for Jeff Bingaman in his 1982 race to unseat astronaut turned Sen. Harrison "Jack" Schmitt. The ad went as follows: "Do you think we should drill for oil in national parks and wilderness areas? The candidates for Senate disagree. Jack Schmitt says yes, we need the oil. Jeff Bingaman says no, we need to protect our national heritage more. Two good men run for Senate, but they disagree on oil drilling in parks and wilderness areas. So, on Election Day, vote for the o ne who agrees with you." The ad appeared so evenhanded -- and was so accurate -- that it overcame voter distrust and led to an upset victory for Bingaman.

To work, negative ads must be believable. To accuse an opponent of being soft on child molesters won't work. It lacks credibility. One cannot ask voters to believe such ill of an opponent that he deserves not just defeat but imprisonment. But to say that he puts his perception of constitutional rights ahead of convicting child molesters does work.

Paint a picture. Negatives must be thematic. John McCain, in the current campaign, is too scattershot, one day hitting Barack Obama for his Chicago political connections and then accusing him of vapid celebrity the next. It is only when the negative campaign paints a consistent picture that it can work.

Some political consultants, including most Republicans, treat positive advertisements like the overture before the show begins, marking time until the real campaign starts and the negatives begin to hit. That's wrong. Positive ads that explain a program, develop a theme, or spell out hot-button issues are still the most effective communications in politics. But negative ads work and have their place. They are how the voters find truth in a morass of claims and counterclaims. With much of the media oriented toward the left or the right, negative ads are often the only way voters can penetrate the claims of the various campaigns and get the facts.

Voters always tell pollsters that they hate negative ads, but politicians continue to run them. That's because the same polls show that they work. In a world with flawed politicians, we need negative ads; otherwise, we won't know candidates' defects until it's too late.