A study of my own, and a growing body of research in psychology and political science, show that the political brain is an emotional brain. It is not a dispassionate calculating machine, objectively searching for the right facts, figures, and policies to make a reasoned decision. The reality is that our brains are vast networks of neurons (nerve cells) that work together to generate our experience of the world. Of particular importance are networks of associations, bundles of thoughts, feelings, images and ideas that have become connected over time.

Just how important networks are in understanding why candidates win and lose can be seen by contrasting two political advertisements: the first from Bill Clinton's campaign for the presidency in 1992, and the second from John Kerry's in 2004. Both men were running against an increasingly unpopular incumbent named Bush. Both ads were, for each man, his chance to introduce himself to the general electorate following the Democratic primary campaign and to tell the story he wanted to tell about himself to the American people. And both were a microcosm of the entire campaign.

The two ads seem very similar in their "surface structure". But looks can be deceiving. A clinical dissection of these ads makes clear that they couldn't have been more different in the networks they activated and the emotions they elicited.

Clinton's ad was deceptively simple, narrated exclusively (and with exquisitely moving emotion) by the young Arkansas governor. In the background was music evocative of small-town America, along with images and video clips that underscored the message.

Bill Clinton: "I was born in a little town called Hope, Arkansas [image of a small-town train station, with the name "Hope" on a small white sign against a brick background], three months after my father died. I remember that old two-storey house where I lived with my grandparents. They had very limited incomes. It was in 1963 [video clip of John F Kennedy, looking presidential, coming up to a podium] that I went to Washington and met President Kennedy at the Boy's Nation programme [video of the young Clinton and the youthful President Kennedy shaking hands]. And I remember [living-room video of a now adult Clinton, starry eyed and nostalgic thinking about the encounter with a man who was obviously his hero] just, uh, thinking what an incredible country this was, that somebody like me, you know, who had no money or anything, would be given the opportunity to meet the president [photo of their hands clasped, slowly and gradually expanding to show the connection between the two men].

"That's when I decided I could really do public service, because I cared so much about people. I worked my way through law school with part-time jobs - anything I could find. After I graduated, I really didn't care about making a lot of money [photos of poor and working-class houses in Arkansas]. I just wanted to go home and see if I could make a difference [photo of the young governor-elect raising his right hand to take the oath of office as governor of Arkansas].

"We've worked hard in education and healthcare [video clips of Clinton with children in a classroom, being hugged by a woman in her 70s or 80s, and talking with workers] to create jobs, and we've made real progress [photo of the governor hard at work late at night in his office]. Now it's exhilarating to me to think that as president I could help to change all our people's lives for the better [video of Clinton obviously at ease with a smiling young girl in his arms] and bring hope back to the American dream."

If you dissect this ad, you can readily see why it was one of the most effective television commercials in the history of American politics. Bill Clinton never shied away from policy debates, but this ad was not about policy. Its sole purpose was to begin creating a set of positive associations with him and a narrative about the Man from Hope - framed, from start to finish, in terms of hope and the American dream.

In his first sentence, Clinton vividly conveyed where he was coming from, literally and metaphorically - from a place of Hope. But he was not content to do this just with words. The ad created in viewers a vivid, multisensory network of associations - associations not just with the word hope but to the image of Hope in small-town America in an era gone by, captured by the image of the train station, and the sound of hope, captured in his voice. Clinton told his own life story, but he told it as a parable of what anyone can accomplish if just given the chance.

He tied the theme of hope to the well-established theme of the American dream, presenting himself not as a man of privilege descending (or condescending) to help those less fortunate, but as someone no different from anyone else, who grew up on Main Street in any town - indeed, as someone who had suffered more adversity than most, having been born after his own father's death. The theme of hope was reinforced by the final image of a young child, representing our collective hope for the future, and the hope of every parent.

Although you can't get much more "hopeful" than that, the final line of the ad actually included a subtle allusion to the Bush economy ("bring hope back to the American dream", implying that it had been lost), with an implicit negative message most voters would likely register only unconsciously.

The association with President Kennedy was instrumental to the emotional appeal of the ad. Kennedy was an American icon, whose brief tenure in the White House is widely remembered as a time in which America's hopes soared along with its space programme. Careful dissection of the sequence of visual images shows how brilliantly the ad was crafted.

The sequence began with Kennedy by himself, looking young, vibrant, serious and presidential - precisely the features the Clinton campaign wanted to associate with Clinton. Then came the video of a young Bill Clinton shaking hands with Kennedy, dramatically bringing the theme of the American dream to viewers' eyes - a poor boy from Arkansas without a father finding himself in the presence of his hero - while creating a sense of something uncanny, of "fate", of the chance meeting of once and future presidents that seemed too accidental not to be preordained. Then came a still photo of their hands tightly clasped, emphasising the connection between the two men. This image lasted far longer than any other in the ad and gradually expanded until the two hands panned out into an image of the two recognisable figures.

Clearly, a central goal of the ad was to establish Clinton as presidential, particularly in light of the rumours about his sexual escapades during the bruising primary season (which may actually have been turned to his advantage through the associations with Kennedy, who himself was linked with tales of infidelities but was none the less revered). In a race against an incumbent president, who needed only to stand in front of a podium with the seal of the presidency to appear presidential, the Clinton ad seized every opportunity to show what Bill Clinton would look like as president, with the image of him raising his right hand to accept the oath of office (as governor of Arkansas, but from a visual point of view, literally showing what Clinton would look like in his swearing-in ceremony as president) followed by a photo of him working tirelessly at his desk, signing bills (itself reminiscent of photos of Kennedy).

I do not know how much of this was consciously intended by Clinton and his consultants. I suspect that much of it was, although some of the emotional overtones and sequencing of images might well have simply reflected Clinton's extraordinary emotional intelligence and gut-level, implicit political horse sense.

Like Clinton's "Hope" ad, the first television advertisement run by the John Kerry campaign in the general election, in early May 2004, attempted to begin painting a picture - to tell a story - about John Kerry, the man and the potential president:

John Kerry [patriotic music, with prominent brass]: "I was born in Fitzsimmons Army Hospital in Colorado [initial video of candidate speaking, which returns throughout the ad]. My dad was serving in the Army Air Corps. Both of my parents taught me about public service [photos of the candidate's parents]. I enlisted because I believed in service to country [photo of the young soldier with his comrades in arms]. I thought it was important, if you had a lot of privileges as I had had, to go to a great university like Yale, to give something back to your country [video footage of a soldier, presumably Kerry, walking in the jungles of Vietnam]."

Del Sandusky: "The decisions that he made saved our lives."

Jim Rassman: "When he pulled me out of the river, he risked his life to save mine."

Announcer: "For more than 30 years, John Kerry has served America [photo of Kerry talking on the phone, with glasses hanging off his face]."

Vanessa Kerry: "If you look at my father's time in service to this country, whether it's as a veteran [photo of war service], prosecutor [photo of Kerry pointing toward a window in a setting that looks like a courtroom, which zooms quickly in to Kerry], or senator, he has shown an ability to fight for things that matter."

Teresa Heinz-Kerry [Kerry's wife]: "John is the face of someone who's hopeful [photo of the two, possibly as newlyweds, with Kerry smiling broadly], who's generous of spirit and of heart."

John Kerry: "We're a country of optimists. We're the can-do people. And we just need to believe in ourselves again [video of Kerry speaking again, followed by video of profile of Kerry waving in some political event]."

Announcer: "A lifetime of service and strength. John Kerry for president."

On the surface, the differences between this ad and Clinton's may be difficult to detect. Both begin with the candidate using his birthplace to drive home a central theme. For Kerry, the central theme was that he was born and bred in uniform, a theme central to a campaign trying to unseat an incumbent, George W Bush, widely seen as a strong leader in a perpetual "war on terror".

The ad began with moving, patriotic music that continued throughout, with an emphasis on muted brass tones, congruent with the military theme, and conveying both strength and majesty - precisely the tone Kerry needed to convey. The most moving moments of the ad came as Kerry's fellow soldiers told, with genuine emotion in their voices, how he had saved their lives. But that is where the similarity with the Clinton ad ends.

After Kerry's opening paragraph, in which he told the American people in his own words who he was and what he wanted them to know about him, the rest of the ad didn't matter. Kerry had already spent the first millions of his campaign dollars telling the story George W Bush wanted to tell about him, beginning to weave precisely the web of emotional associations in which the Bush campaign hoped to ensnare him: that he was not only privileged (a word Kerry, who was married to an heiress, introduced himself), but a north-eastern liberal intellectual.

The fact that he was from Massachusetts was well known - the Republicans were already emphasising that he was "Ted Kennedy's junior senator" - and the phrase "Massachusetts liberal" had become so successfully branded by the Republicans in the 1988 Bush-Dukakis campaign that either word readily evoked the other.

When Kerry added the reference to Yale, he fully activated the primary network that the conservative movement has worked for so many years to stamp into the American psyche to galvanise disdain and resentment toward Democrats: the liberal elite. Put together Massachusetts, liberal senator and Yale, and you have virtually the whole network activated. The only thing missing was a windsurfing outfit. That came later.

Whatever its intended goal, that first paragraph of the Kerry ad served to convey one primary message that would stick in the neural networks of voters for the remainder of the election: this guy isn't like me.

The references to Yale and privilege were the most glaring mistakes in that ad, but they were not the only ones. Perhaps most importantly, the ad did not, like Clinton's, tell a coherent story. Try to summarise it using the narrative structure of a good storyteller, and you'll see the problem.

In fact, it told two stories. The second had nothing to do with the first, and seemed as if it had come straight from the head of a consultant rather from the heart of the candidate. The first story, "John Kerry was born on a military base, served his country heroically because he believed it was his duty, fought bad guys as a prosecutor, and would be a strong commander-in-chief", was clear and effective. Then the ad introduced two related themes, using words associatively linked to military strength (service and fighting), which created two distracting subplots: one about a lifetime of service (not the same thing as being heroic in the face of attack), the other about fighting for things that matter (intended, I suspect, to smuggle in a populist theme under the banner of strength). Whereas the Clinton ad wove together and created an emotionally powerful network, the subthemes in the Kerry ad drew on existing associative links (the words military, service and fighting) but actually took them in diverging directions, essentially dismantling a network whose activation was the central goal of the ad.

Two-thirds of the way through the commercial, the plot shifted, with Teresa Heinz-Kerry introducing the theme of optimism. The insertion of this non sequitur no doubt reflected his consultants' belief that optimism is a "winner" for presidential candidates. The optimism theme seemed grafted on to both the message and the candidate.

Finally, the use of imagery in the Kerry ad stands in stark contrast to its effective use in the Clinton ad. The scenes of Vietnam, and particularly the faces and intonation of the men who served with Kerry, painted a clear and moving portrait of Kerry as a man and a potential leader. But after that, it seemed as if someone had just hastily rummaged through the Kerry family scrapbook. The photo of Kerry "serving" conveyed nothing about him, other than perhaps that he needed bifocals. And the image used to illustrate his service as a prosecutor and then as a senator was difficult even to decipher.

The difference between the Clinton ad and the Kerry ad - like the difference between the Clinton campaign and virtually every other Democratic presidential campaign of the last three decades - reflects the difference between understanding and misunderstanding mind, brain and emotion in American politics. If you think the failure to tell a coherent story, or to illustrate your words with evocative images, is just the "window dressing" of a campaign and makes little difference in the success or failure of a candidacy, you're missing something very important about the political brain. Political persuasion is about networks and narratives.

We can hear the whirring of the dispassionate mind in the following exchange on Medicare, which occurred during the first presidential debate between Gore and Bush in 2000:

Gore: "Under the governor's plan, if you kept the same fee for service that you have now under Medicare, your premiums would go up by between 18% and 47%, and that is the study of the Congressional plan that he's modelled his proposal on by the Medicare actuaries. Let me give you one quick example. There is a man here tonight named George McKinney from Milwaukee. He's 70 years old, has high blood pressure, his wife has heart trouble. They have an income of $25,000 a year. They can't pay for their prescription drugs. They're some of the ones that go to Canada regularly in order to get their prescription drugs. Under my plan, half of their costs would be paid right away. Under Governor Bush's plan, they would get not one penny for four to five years and then they would be forced to go into an HMO or to an insurance company and ask them for coverage, but there would be no limit on the premiums or the deductibles or any of the terms and conditions."

Bush: "I cannot let this go by, the old-style Washington politics, if we're going to scare you in the voting booth. Under my plan, the man gets immediate help with prescription drugs. It's called Immediate Helping Hand. Instead of squabbling and finger-pointing, he gets immediate help. Let me say something."

Moderator (Jim Lehrer, PBS): "You're -"

Gore: "They get $25,000 a year income: that makes them ineligible."

Bush: "Look, this is a man who has great numbers. He talks about numbers. I'm beginning to think not only did he invent the internet, but he invented the calculator. It's fuzzy math."

Now let's take a "clinical" look at this interchange. Note the expected utility model underlying Gore's approach. He saw his job as to convince the average senior citizen or ageing worker that Bush's plan would have a lower utility value than his own. Now there's nothing wrong with comparing and contrasting plans, although Gore's appeal would have been far more effective if he had simply reversed the order, reeling voters in with a personal story and then hooking them with a contrast between his plan and Bush's. And from the standpoint of the dispassionate mind, Bush clearly had few answers to Gore's charges, other than to play the Washington outsider and mumble some platitudes about helping hands.

After eight years as vice-president and months campaigning against George Bush, Gore clearly knew everything he needed to know about every "issue" in the campaign. The last thing he needed was a debate coach to quiz him on facts and figures. Yet precisely this kind of debate preparation set him up for the most memorable (and, for Gore, the most destructive) moment of the debate: Bush's line about Gore claiming to invent the calculator. Bush delivered this barbed one-liner with an affable style that stood in stark juxtaposition to Gore's nonverbal dismissiveness of Bush's arguments (and, by extension, of his intellect). The line was unfair, but the Gore team handed it to him, by attending to the facts and figures rather than to the stories Bush had been telling the public about Gore. Instead of getting voters to feel the difference between his concern for the welfare of seniors struggling to pay their medical bills and Bush's, Gore went to a level of numerical precision - premised on a model of expected utility, giving them every number they needed to make the appropriate calculations - that played right into Bush's strategy of portraying Gore as an emotionless policy wonk, "not a regular guy, like us".

Gore's statement, "Your premiums would go up by between 18% and 47%, and that is the study of the congressional plan that he's modelled his proposal on by the Medicare actuaries," may well have been accurate, and in rational terms, Gore had given Bush a beating. But in emotional terms, both the presentation of exact numbers (as opposed to "your premiums would go up by about a third") and the mention of actuaries undercut the story Gore most needed to tell the American people: that he cared about that 70-year-old man, and he would do something about it. Instead, his exacting reference to numbers and actuaries reinforced the story Bush wanted to tell about him: "Look, I'm like you, I don't care about all this fancy math. I care about people. They're just statistics to him."

In that single line about inventing the calculator, Bush killed three birds with one stone. He established himself as a guy with a sense of humour who would likely be fun to have around for the next four years. He reiterated themes about Gore's hubris and lack of trustworthiness that struck at the heart of his character. And most importantly, he disarmed Gore for the remaining debates - and the rest of the election - of the value of data. From that point forward, all references to numbers were just "fuzzy math".

It didn't help, of course, that the media did their postmodernism routine, turning Gore's claims about Bush's Medicare plan and tax cuts - which both turned out to be true - into a he said/she said contest of competing claims to a truth that somehow couldn't be adjudicated.

But it is the job of a campaign to get the media to convey its message, rather than the opponent's message, and in the past 30 years, with the exception of the Clinton years, Republicans have consistently outflanked Democrats in these manoeuvres, using the same emotional skill they have demonstrated with the electorate.

· Click here to read part two