Monday, September 15, 2008

10,000 Different Smells? Enough Already.

Writing for the UK’s Daily Mail this week, journalist Josh Sims has a piece called “15 Things You Didn’t Know About Men’s Fragrances.” Many of his items are built around numbers—it’s a nice hook, with titles such as “30 Minutes - The Time It Takes for ‘Base Notes’ to Appear,” and “33 Percent of Men’s Perfumes Are Worn by Women.” Good fun and entertaining.

But then he offers this bit: “10,000 - The Number of Different Odours We Can Distinguish.” Here I have to blow the whistle.

Ever since I began working in the psychology of odor perception, I’ve seen references to 10,000 different smells. It shows up everywhere—from fashion magazines to scientific journals. It was even cited by the Nobel Foundation when it awarded the 2004 prize in Physiology or Medicine to Linda Buck and Richard Axel for discovering the olfactory receptor genes. So one can hardly blame Johs Sims for thinking it was a number he could bank on.

But, as I wrote in What the Nose Knows, “Something about it has always bothered me—why such a nice fat round number? Why was there no date of discovery? And, strangest of all, why did nobody take credit for it?”

I decided to discover the source of this mystery number for myself. After a lot of time in the library, and after following innumerable dead ends, I finally found it. Back in 1927, two American chemists—Ernest C. Crocker and Lloyd F. Henderson—were looking for an objective way to classify odors. They came up with a numerical coding system in which any smell could be assigned a four-digit identifier. According to the mathematics of the Crocker-Henderson system, it was theoretically possible to identify 6,561 different smells. Years later, Crocker rounded this up to 10,000, the number everyone has been citing ever since.

The trouble with these numbers—whether 6,561 or 10,000—is that they represent a theoretical upper limit based on a very specific set of starting assumptions, namely four odor standards and nine-point rating scales. These numbers are not based on experimental data. Crocker and Henderson never attempted to count all the different odors that humans can smell. Nor has anyone else as far as I know. And in any case, it turned out that people couldn’t rate smells reliably using four standards and nine-point scales. The system was eventually abandoned.

Don’t get me wrong—I think Ernest Crocker and Lloyd Henderson were innovators and optimists in the best “can do” tradition of American enterprise. But the figure of 10,000 different odors is, from a scientific perspective, utterly worthless. Now that my book is out and the story has been told, it’s time to lay the 10,000 number to rest.

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