(FORTUNE Magazine) – Sy Syms, the discount clothier, has long maintained that "an educated consumer is our best customer." But is that true throughout the consumer landscape? Marketers, who usually prefer to control consumers' access to product information, probably don't think so, and they might have some unlikely allies: consumers themselves, most of whom would rather follow their hearts than their heads when making purchasing decisions. To demonstrate how a little knowledge can be a dangerous thing, let's examine the recent experiences of one consumer--me--as he wrestled with the issue of brand loyalty in the beer market.

First, some background: I'm a man of fairly simple drinking tastes--mainstream domestics like Budweiser and Miller are fine by me. For years, however, I always chose Miller when given the option, primarily because I preferred the Miller logo and packaging, generally disliked Bud's advertising, and often enjoy rooting for the underdog (in the beer biz, everyone's an underdog compared with Anheuser-Busch)--in other words, for reasons that had nothing to do with the actual liquid in the bottle.

We all know these are perfectly common reasons for buying something, especially in a product category like domestic beer, where all the liquids in all the bottles are essentially the same thing anyway. I've never believed Miller actually tastes better than Bud (frankly, I doubt I can even tell the difference). So, everything else being equal, why not stick with the brand whose logo pushes my buttons?

Ah, but what if everything else isn't so equal after all? That question confronted me when I came across Adams/Jobson's Beer Handbook, an industry reference guide that lists the alcohol percentage by volume for over 1,500 beers. Scanning the data, I noticed a disturbing factoid: Budweiser was listed as having an alcohol content of 4.8%, while Miller High Life and Miller Genuine Draft, which are the two Miller products I favor (and guess what: They taste the same), each clocked in at 3.7%.

Now, I'm not a big drinker, but I'd like to get full bang for my buck, if you get my drift. Having opened the Pandora's box of Adams/Jobson's data, I found myself facing a classic conflict between my heart and my head: If I stuck with Miller, I was getting an inferior product; if I switched, I was casting my lot with those annoying Bud frogs and sending myself on a long guilt trip for abandoning my old brand. Each beer purchase became an occasion for soul-searching. I began to dread stopping in at my favorite bar. Wistfully recalling the days when I didn't have to worry about any of this, I realized I'd become living proof that ignorance really is bliss.

If this all sounds a bit silly, here's the kicker: During a phone chat with Miller spokeswoman Kari McGrath, I discovered that the Adams/Jobson's numbers for Miller were wrong. Turns out they accidentally used figures for alcohol as measured by weight, instead of by volume, for the Miller products. The correct number for Miller is actually 5.0%, which is--surprise!--slightly higher than Bud's. A bit shaken by the thought that I'd put myself through an emotional wringer all because of a snafu, I called an Adams/Jobson's editor, who confirmed the error.

The good news is that I've gone back to my old brand. And as I try to forget everything I ever knew about alcohol percentages, it's comforting to know that the breweries themselves will probably never raise the issue in their marketing campaigns. As Beverage World editor Havis Dawson explained to me, "Anheuser-Busch and Miller have invested millions of dollars in cultivating brand images that have nothing to do with alcohol content." In other words, there's no need to clutter up your sales pitch with facts when you can just sell a feeling, a package, or a logo.

I already knew that, of course, but I hope I never have to learn it firsthand again. Meanwhile, if anyone ever gives me a reference guide with hard data about Cheerios, I'm not looking at it.

PAUL LUKAS, author of Inconspicuous Consumption, obsesses over the details of consumer culture so you don't have to.