Why TED talks are better than the last speech you sat through

February 25, 2014, 11:52 PM UTC

FORTUNE — Think about the last time you heard someone give a speech, or any formal presentation. Maybe it was so long that you were either overwhelmed with data, or you just tuned the speaker out. If PowerPoint was involved, each slide was probably loaded with at least 40 words or figures, and odds are that you don’t remember more than a tiny bit of what they were supposed to show.

Pretty uninspiring, huh? Talk Like TED: 9 Public-Speaking Secrets of The World’s Best Minds examines why in prose that’s as lively and appealing as, well, a TED talk. Timed to coincide with the 30th anniversary in March of those now-legendary TED conferences, the book draws on current brain science to explain what wins over, and fires up, an audience — and what doesn’t. Author Carmine Gallo also studied more than 500 of the most popular TED speeches (there have been about 1,500 so far) and interviewed scores of the people who gave them.

Much of what he found out is surprising. Consider, for instance, the fact that each TED talk is limited to 18 minutes. That might sound too short to convey much. Yet TED curator Chris Anderson imposed the time limit, he told Gallo, because it’s “long enough to be serious and short enough to hold people’s attention … By forcing speakers who are used to going on for 45 minutes to bring it down to 18, you get them to think about what they really want to say.” It’s also the perfect length if you want your message to go viral, Anderson says.

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Recent neuroscience shows why the time limit works so well: People listening to a presentation are storing data for retrieval in the future, and too much information leads to “cognitive overload,” which gives rise to elevated levels of anxiety — meaning that, if you go on and on, your audience will start to resist you. Even worse, they won’t recall a single point you were trying to make.

“Albert Einstein once said, ‘If you can’t explain it simply, you don’t understand it well enough,’” Gallo writes, adding that the physicist would have applauded astronomer David Christian who, at TED in 2011, narrated the complete history of the universe — and Earth’s place in it — in 17 minutes and 40 seconds.

Gallo offers some tips on how to boil a complex presentation down to 18 minutes or so, including what he calls the “rule of three,” or condensing a plethora of ideas into three main points, as many top TED talkers do. He also notes that, even if a speech just can’t be squeezed down that far, the effort alone is bound to improve it: “Your presentation will be far more creative and impactful simply by going through the exercise.”

Then there’s PowerPoint. “TED represents the end of PowerPoint as we know it,” writes Gallo. He hastens to add that there’s nothing wrong with PowerPoint as a tool, but that most speakers unwittingly make it work against them by cluttering up their slides with way too many words (40, on average) and numbers.

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The remedy for that, based on the most riveting TED talks: If you must use slides, fill them with a lot more images. Once again, research backs this up, with something academics call the Picture Superiority Effect: Three days after hearing or reading a set of facts, most people will remember about 10% of the information. Add a photo or a drawing, and recall jumps to 65%.

One study, by molecular biologist John Medina at the University of Washington School of Medicine, found that not only could people recall more than 2,500 pictures with at least 90% accuracy several days later, but accuracy a whole year afterward was still at about 63%.

That result “demolishes” print and speech, both of which were tested on the same group of subjects, Medina’s study indicated, which is something worth bearing in mind for anybody hoping that his or her ideas will be remembered.