Dear Dr. Eldritch,
My business partner has developed a split personality! We don't have time for therapy, how can I keep him on track until we go to market?

I've been best friends with Derek since grade school, about fifteen years. He's always been the quiet, nerdy genius type. When he had a breakthrough idea in the high-tech world (sorry, I'm sworn to secrecy about what it is), it seemed only natural that we'd build it together. Well, Derek is building the prototype, and I handle the business end. The other day, I was going to a meeting with some investors, and he's sitting outside a coffee shop, smoking! He's never smoked a day in his life, so when I confronted him, he acted like he didn't know me. When I reminded him that millions of dollars were riding on him finishing the prototype, he said he was just kidding, and he'd get right back to work. He took off (going the wrong direction), and I went to my meeting. When I asked him about it later, he insisted he was in the workshop all day.

Since then, when he's at work, he's his calm, normal self, focused on the tech side. When he shows up off hours, he's a foul-mouthed party animal, who keeps asking about earnings estimates and patent rights. I suspect he's got this Jekyll/Hyde thing going from all the strain. How can I keep him from going over the edge so he can finish our project?

-- Daryl, near Silicon Valley

Dear Daryl,

If this column ever appears daily, it will be with letters like yours. I get tons of "My friend/partner/child is acting weird/scary/Republican, how can I tell if it's really them/change them back/do them in?" questions. Unfortunately, there are scads of reasons that a person may suddenly act like they're someone else, and most people don't give me enough detail to make a reliable diagnosis. You've given me some good clues. For example, while it's possible that Derek is experiencing Jekyll & Hyde Syndrome, he wouldn't remember working all day if his Hyde personality took over and went out for latté and a smoke. I'd say you're dealing with an Evil Twin.

There's quite a bit of debate in the industry whether a twin raised separately becomes evil due to being given over for adoption, or parents give up one twin because it's evil. Few parents want to talk about it, even those who keep both twins, and spend a lifetime trying to bring the evil one around to good. It's undoubtedly devastating to realize that one of your babies is evil, and I'm not sure how I'd handle it if it happened to me, so let's not judge too harshly, OK?

Derek's evil twin may not be a biological one. Sometimes "twins" are born to different parents. Often referred to as "Doppelgangers," (although true Doppelgangers are malicious spirits that take the form of a living person) they are virtually identical in appearance, but often have radically different temperaments. Most non-biological twins, evil or not, never meet. As you can imagine, the trouble starts when the Evil Twin becomes aware of the Good Twin, and sets his or her mind to, well, evil. I believe this is the case for your Derek and Evil Derek.

This situation would be a lot easier if you had taken some simple precautions long ago. There are two things I recommend for everyone to do, because you never know when a friend or loved one may be replaced or possessed. First, select on code names for each other, e.g. "Buster" and "Ringo," and an identifying question like "Saint or Sinner?" (avoid common phrases like "Who goes there?" or "Is that you?" They're just too obvious). The identifying question is always answered with a code name, and the other must respond in kind. Failure to do so is a green light for inflicting an appropriate level of mayhem. This is incentive to follow protocols (just remember that he's entitled to do the same for you, so don't use it to embarrass him at formal dinners). Example: The two of you are fleeing a psychopath (like an Evil Twin) in abandoned subway tunnels. You get separated. As you slosh through the murky, dripping passageway, a shadowy figure appears in front of you. It looks like Derek, but in a dark tunnel, who knows? Releasing the safety on your Uzi (you wouldn't flee through an abandoned subway tunnel unarmed, would you?), you say "Saint or Sinner?" If he says "Ringo," then you know it's probably the real Derek. You say "Buster," and he knows it's the real you. (Of course, your code could be compromised if Evil Derek overhears this exchange and exploits it when the two of you are separated again. No system is perfect, so keep your voices down and the Uzi handy.)

Second, each of you have a uniquely identifying tattoo done with ink that is only visible under ultraviolet light. Include your name, social-security number and a simple but distinctive pattern. Obviously, don't tell anyone else about these tattoos, and put them on a body part normally covered so it won't glow when you wander past the black-light poster display at your local Paraphernalia Shoppe. Ideally, these should be done at two different tattoo locations in different states, pay in cash and destroy any drawings afterward. (And for Pete's sake, make sure they use a new needle and everything's sterilized in an autoclave. It would be tragic if your safety preparation yielded one or more of the almost two dozen blood-borne diseases transferable by tattoo equipment.) These tats have the grim side benefit of being useful for identifying remains if traditional ID methods fail, but as a regular reader of this column, this won't ever be necessary for you, right?

Between these two precautions, you should be able to identify each other through most scenarios of mind/body swapping, replacement, or duplication. Unfortunately, you probably haven't done the preparation in advance, so you need some ad hoc identifiers. Does Good Derek have any unique scars or tattoos? Any stories that only the two of you would know? You need to get him alone and use these to verify his identity. Then tell him the whole story. He may think you're crazy, but you've got to convince him. Set up some temporary code names, and send Good Derek home. Then get Evil Derek over to your house. I hope you've got some talent as an actor. You need to be calm, don't let on that anything's amiss, distract him for a moment, and hit him with your stun gun (you do have one, don't you?). Tie him up real well, and call Good Derek over. He'll need a few minutes to soak it in. It's not every day one comes face-to-face with one's double, after all. Especially an Evil double.

In a perfect world, you could just give Evil Derek a stern talking-to, make him promise to never trouble you again, and release him. If you doubt your powers of persuasion, turn the tables. I would never encourage my readers to commit a crime, so imagine the following scenario for Entertainment Purposes Only:

Good Derek commits a few high-visibility felonies, like calling in some bomb threats from Evil Derek's house, then attempting a bank robbery, fleeing in Evil Derek's car long before police can respond. Then, the two of you stun Evil Derek again, and ditch him with his car where the police will find him. His "Two guys kidnapped me and my double framed me for these crimes" defense will be laughed at, especially if Good Derek suddenly has a drastically different hair style and a good alibi. Of course, Evil Derek will bear quite a grudge when he gets out of prison, but you'll have some time to prepare. Not that you'd actually do these things, of course. Wear gloves.

Good luck, and let me know how it comes out!

-- Dr. Eldritch