When Carrie Bradshaw told her then-boyfriend Jack Berger that she likes her money where she can see it — hanging in her closet — we didn't exactly take her revelation as financial advice.But as it turns out, Carrie might have been on to something. That is, if you have a closet full of Birkin bags. A recent study published by Baghunter finds that Birkin bags are a better investment than the S&P 500 or gold.There are actually two reasons why this is true, according to the study. The price of Birkin bags has skyrocketed over 500% since 1980, and Baghunter predicts the price will double again in the next decade.The second reason a Birkin bag might be smarter than stocks? Their value has never gone down. While all three investments — gold, the S&P, and a Birkin bag — have increasing average annual returns, only Birkin bags have never once gone down in value.So while, over a 35-year spread, the S&P shows an average annual return of 11.6%, in some years the value actually went down by 36.55%. In comparison, the lowest annual return for a Birkin bag was still positive — 2.1% in 2001.Why do Birkin bags reliably increase in value? There are a couple reasons — for one, demand far outstrips the limited supply. As Evelyn Fox, founder of Baghunter, explained to Luxury Daily , Birkin bags are also considered an “ultra-luxury” good. And there’s a difference between ultra-luxury and just plain luxury: While the market for luxury goods suffers during tough economic times, the market for ultra-luxury goods remains stable.A word of warning: If you're ready to invest, you won't have much luck picking a Birkin up at your local Hermès store — they're reserved for "special clients" of the brand. But you could always try one of these tricks . Just think, with a Birkin at home, you can watch your money grow in your closet, like Carrie Bradshaw dreamed.