Matt:

My brother has this old autograph from Mick Jagger that his girlfriend gave him back in the 80s. He has it locked away and says it's valuable and will only get more valuable. I think he's dreaming. So what's the truth?

-- Doubt It, Mira Mesa

Does Mick's sig figure prominently in your brother's retirement plans? If not, we can let him have his little fantasy. The elves are still crunching the numbers, but so far they estimate there's maybe a 5% chance that the autograph is even real. Few collecting fields involve as many fakes as autographs. Show biz scribblings are especially questionable. Most are deliberate forgeries, but some are more innocent errors of identification by people not familiar enough with the fine points of the discipline. At the risk of bursting your brother's balloon full of dollars, I'd recommend that he take the signature to a qualified autograph dealer.

According to several dealers in L.A., Mick's John Hancock has no great historic value. To autograph dealers, it's like hundreds of other pop culture names. At least as long as Mick is still around. If it's in ink on a photo or a program, it's probably worth $100 or so. If your brother's girlfriend only had an eyebrow pencil and an old shopping list for Mick to sign, you'll be lucky to get $25 or $50 for it. Those are retail prices; a dealer will give you less. And every time Mick obliges another fan with an autograph, yours diminishes a fraction in value.

To show you how scarcity can affect value, take Marilyn Monroe and Joan Crawford. They're both big stars, both deceased, and their memorabilia is highly collectible. Monroe signed virtually nothing herself. Most of Marilyn's signatures that dealers see are fakes-- the work of secretaries or publicity people, or reproduced on fan photos or letters with a machine that automatically signs the star's name to make it look real. A genuine Monroe signature is worth a few thousand dollars.

There's a chance that your Jagger sig is more valuable than Joan Crawford's. Apparently, in addition to her other publicized excesses, she signed everything in sight. The value of Joan's signature on an ordinary piece of paper is less than $50. You'd even get more money for a genuine Cindy Crawford.