Beauty brand loyalty is perhaps the fiercest of all types of devotion. When a girl finds her Holy Grail product, she buys enough of it to see her through the next decade, then prays it never, ever gets discontinued. Don't even think about telling her to try one of the generic dupes. No matter how many others say it's almost the same thing, good luck trying to convince a NARS Orgasm devotee that Milani Baked Blush in Luminoso is a worthy match.

Unless, of course, you've got science to back it up. In search of beauty truths, we sat down with two cosmetic chemists who revealed the biggest secrets behind the products women use every single day. Can you handle the truth?

The old saying, "Don't judge a book by its cover," applies here. "Like a house has curb appeal, a product has what's called 'shelf appeal,'" explains Sandy Alcide, a freelance cosmetic chemist and founder of her own organic skin care line, Motion Medica.

Because no woman wants to cover her face in something that looks like it was concocted in someone's basement, most consumers are willing to shell out a lot more for a product that looks fancy. But a pretty package doesn't mean the product will work any better.

"What's in the jar or bottle is more important, but people feel better about using a glass jar," says Alcide. Glass is more expensive than plastic, and the price gets passed along to the consumer.

Cosmetic chemist Don Frey, who has worked with beauty giants Avon, Proctor & Gamble and Jafra, agrees: "There's not necessarily a correlation in the price point of a product. If you're using Lancôme, go buy L'Oreal. It's so close to Lancôme -- the packaging is much more expensive, but understand that that's what you're paying for."

However, cosmetic companies dispute claims like these. Their position is that there are significant differences in formulas that affect performance, longevity of wear, etc., and that these more expensive Research and Development costs, as well as ingredients, are what affect product pricing.

But what about all those warnings about reusing plastic bottles? Doesn't that mean plastics contain ingredients that are bad for the products?

Not so, according to Alcide, who says that any plastic packaging used in skin care products is a high grade that's specifically used for cosmetics, so leeching of plastic into product isn't a concern.

... and not in a good way. Some companies add ingredients that aren't listed on the label. While the FDA does mandate that cosmetics cannot contain ingredients that can harm the user (i.e., known harmful substances like lead) or instruct the consumer to use the product in a harmful way (i.e., squirting alcohol into your eyeball), they don't actually regulate or test cosmetics. Testing is left up to the discretion of the manufacturer, so ask yourself: do you trust your favorite beauty brands to conduct rigorous tests of their own products?

Alcide calls out companies that slap "all-natural" on the label, but don't always live up to the claim, ingredient-wise. She sees a lot of labels that list botanical ingredients and distilled water, but they leave out any mention of preservatives. "If a product has water in it, it must have a preservative. Otherwise, bacteria will form," says Alcide. "Natural preservatives are expensive. When I tested one 'all-natural' water-based product with no preservative listed, I found methylparaben in it. That's a chemical preservative."

P.S. If you're scanning your labels a little more closely now, look for ingredients like aloe vera juice, honey, and any type of oil -- these are particularly susceptible to bacteria.

There's nothing wrong with glossing over the ugly parts -- except in your cosmetics. Alcide says many skin care lines use chemical names of ingredients to make them sound less scary or gross to consumers. Example: you'd probably be wary of slathering your face in formaldehyde (a known carcinogen), but you probably wouldn't bat an eye at "methanal" on the ingredient list. According to Alcide, it's the same thing, and you'll find this preservative in everything from hair-smoothing products to makeup remover. The same goes for common household bleach, which lives incognito in your beauty products under the name "sodium hypochlorite." This chemical has been linked to allergies and is classified as "toxic" by Canada, but it shows up in U.S. stores as an ingredient in disinfectant products meant for skin rashes.

If you're having a heart attack right now because you just discovered methanal listed in your favorite nail polish (and unless it's a "3 Free," it probably is), take Alcide's advice: do your research. "Because there are thousands of skin care products, it is up to the consumer to do their homework reading labels," she says. (A bit obvious, but do you? Ever?) Scour the ingredient list and look up anything you don't recognize. You can look up products and ingredients on the Environmental Working Group's website, or on California's new Safe Cosmetics Program Product Database. More buyer-beware tips from Alcide:

-Consider it a red flag when you buy a product online that only lists "key ingredients." Alcide says this is one way cosmetic companies try to highlight "marketable" ingredients: things that sounds like they're natural or good for you (like avocado oil, orchid extract, and coconut milk), but only make up a tiny percentage of the product. Check out the rest of the ingredients (the FDA's labeling act requires every ingredient to be published in descending order on the bottle or bottle's packaging) to find what the primary ingredients are.

-Be wary of products with an ingredient list printed on the box, versus printed on the bottle. Since you throw out the packaging once you open the product, Alcide says this is another way some cosmetic companies try to gloss over less-than-ideal ingredients. When you decided you want to look up the ingredients, it's too late -- the packaging with the list on it is in last month's trash.

-Be smart about products you buy outside the U.S. Harmful ingredients that are banned in products sold in the U.S. might not be banned elsewhere. Example: Mercury is banned in the U.S., but not in China. Europe has 1,373 ingredients on their banned list. The U.S. list of banned ingredients clocks in at ... eight.