The 2009 TIME 100

You know the old saw about half our advertising being wasted but we don't know which half? Well, now we do, thanks to Martin Lindstrom, a Danish brand consultant and the author of the book Buyology, who took a brave leap into neuroscience to figure out why we buy — or don't. Using functional MRI and other brain-scanning techniques, he went beyond the flimflam of the Mad Men and measured the minds of more than 2,000 consumers, all observed under the influence of marketing.

What Lindstrom, 39, found was that many ads are not only ineffective but also have a sort of reverse effect. Huge health warnings on cigarette packs may actually encourage smokers to light up because they trigger a mental echo of the desirable product. Ford spent $26 million sponsoring American Idol, yet Lindstrom found that consumers came to think less of the company, mostly because its ads interrupted the show.

It was in 2003 that Lindstrom started reading about brain-imaging tools and realized they could be applied to marketing. He raised research money, brought scientists on board and helped recruit subjects. He's one of the first brand experts to understand the biology of consumer desire.

When you look past what people say and measure what their brains say, you realize the subconscious controls purchasing. Pepsi, for example, always won the Pepsi Challenge, but Coke won in the marketplace, because it's not about which tastes better but about which we think tastes better. That's an emotional reaction, not a rational (or even gustatory) one, and the brain scans reveal how it happens.

As a generation grows up online, the tools of persuasion will have to be as measurable as the medium. Google does it with clicks and links, and Lindstrom does it with neurons and blood flow. Somewhere between the eye and the mouse finger is the secret to selling.

Anderson is editor in chief of Wired and author of The Long Tail