The Dirty Finger problem.

[King Author]

Here's the scenario -- I wash my hands, spic and span. On the way out of the bathroom and to somewhere, I use one of my hands to open a number of doors, flip a lightswitch or two and, just for kicks, handle a television remote. Thus, I now have one shiny clean hand and one super-germy hand.

Somehow, I forget which hand is which and suddenly, I get a super bad itch in my eye. And uh, let's say I'm a nudist, too, so I can't use my sleeve or something, and there's no tissues or anything handy. Since I can't remember which hand is clean, I essentially just pick one at random, hope for the best and scratch my eye.

A few minutes later, my eye itches really badly again. Is it more advantageous to use the same hand to scratch it, or to switch and use the other hand?

The way I see it is this -- one clean hand, one dirty hand, two scratches. This gives three possibilities -- two clean scratches, two dirty scratches, or one of each.

If I don't switch my hand, I'm either getting two clean or two dirty scratches; the best or the worst possibility respectively. I have no idea of knowing which hand is clean and which is dirty, so it's 50/50 which one I end up with.

If I do switch my hand, I split the difference -- yes, I ensure a dirty scratch and have no possibility of two clean scratches, but I also avoid the worst possible outcome of two dirty scratches.

...

I admit, I'm weird. Discuss anyhow.

[jobriath]

Strikes me that you should assign a hand to an eye and stick to it. Problem being, suppose you scratched your eye with the dirty hand, then switched. Now, not only is your eye going to fall out, but you no longer have a clean hand to scratch your remaining eye!

Actual answer is dunno. If one contact is sufficient to cause unpleasantness, clearly don't switch. If two contacts are required and sufficient to cause unpleasantness, do switch. If three, you're free to scratch how you like.

[Izawwlgood]

Assuming it matters (you walked through ZombieCorp and wiped every surface with one hand...), I would say it's better to 50/50 risk rubbing germs all up in your eye than guarantee rubbing germs all up in your eye once.

[King Author]

Strikes me that you should assign a hand to an eye and stick to it. Problem being, suppose you scratched your eye with the dirty hand, then switched. Now, not only is your eye going to fall out, but you no longer have a clean hand to scratch your remaining eye!

Actual answer is dunno. If one contact is sufficient to cause unpleasantness, clearly don't switch. If two contacts are required and sufficient to cause unpleasantness, do switch. If three, you're free to scratch how you like.

Ooh, good point! I hadn't thought of that -- each hand to its own eye. That solves the problem pratically, but we can still debate the problem theoretically, stipulating that you only have one eye.

(Though I never imagined that the germy hand would actually even cause discomfort, certainly not within the few minutes between itches. Just, if you get sick at the drop of a hat like me, scratching your eye with a dirty hand could mean a 24 hour bug and crapping or vomitting your guts out for a day. Which does actually happen to me more often than I'd like.)

[Izawwlgood]

But assigning a hand to an eye just means when you scratch your eyes, you are guaranteed to be using one dirty hand.

[LaserGuy]

If I don't switch my hand, I'm either getting two clean or two dirty scratches; the best or the worst possibility respectively. I have no idea of knowing which hand is clean and which is dirty, so it's 50/50 which one I end up with.

If I do switch my hand, I split the difference -- yes, I ensure a dirty scratch and have no possibility of two clean scratches, but I also avoid the worst possible outcome of two dirty scratches.

...

I admit, I'm weird. Discuss anyhow.

It basically depends on the weighting of the two dirty scratch outcome.

For example, if I were to weight 0 for two clean, 2 for dirty, and 3 for two-dirty, then it would be clearly better to scratch with the same hand twice, because the expectation value is lower. If two-dirty was worth, say, a 5, then you should scratch with each hand. If two-dirty was worth a 4, then it would make no difference, statistically speaking.

You'd be better off just washing your hands though.

[WarDaft]

Or not forgetting. You can only wash your hands so many times a day before it becomes unhealthy, and possibly indicative of other underlying problems. The first thing you do after washing your hands in a public washroom is open the door, do you actually trust every other single person who went through there to have washed properly? So you either have to use a napkin, glove, or your sleeves to interact with everything in public, or remember which hand you used and not touch your face with it.

[thoughtfully]

Excessive sterility itself can be unhealthy. Immune systems need exercise to develop in children and to maintain their capacity in adults. This is documented, although I don't have any references handy.

[Interactive Civilian]

One thing to note: unless you are keeping the "clean" hand in some kind of sealed chamber or at the very least a sealed glove (in which case it would be pretty easy to distinguish which hand is which), then even without it really touching stuff, it is picking up its fair share of germs anyway. Dust in the air, incidental contact with yourself or other things, etc.

Assuming you have a reasonably good immune system, it really doesn't matter which hand you use to scratch your eye, with one caveat: NEVER use the hand that you were using to hold peppers while cutting them.

[Diadem]

One thing to note: unless you are keeping the "clean" hand in some kind of sealed chamber or at the very least a sealed glove (in which case it would be pretty easy to distinguish which hand is which), then even without it really touching stuff, it is picking up its fair share of germs anyway. Dust in the air, incidental contact with yourself or other things, etc.

Assuming you have a reasonably good immune system, it really doesn't matter which hand you use to scratch your eye, with one caveat: NEVER use the hand that you were using to hold peppers while cutting them.

True. And in the pepper case it's easy. Pick one hand at random to scratch your eye, then if your eye exploded use the other hand the next time

[Sizik]

Touch everything with the palm side of your hands, and scratch your eyes with the backs of your hands.

[semicharmed]

(Though I never imagined that the germy hand would actually even cause discomfort, certainly not within the few minutes between itches. Just, if you get sick at the drop of a hat like me, scratching your eye with a dirty hand could mean a 24 hour bug and crapping or vomitting your guts out for a day. Which does actually happen to me more often than I'd like.)

That's... not how germs work. Scratching your eye, as in hard enough to cause bleeding - as opposed to just rubbing - could maybe give you a nasty localized infection. Or maybe blood poisoning. But a 24 hour stomach bug which gives you the pukes and/or the shits? No.

And rubbing your eye with a dirty hand can give you pink eye, but it's certainly not guaranteed.

[SU3SU2U1]

Just, if you get sick at the drop of a hat like me, scratching your eye with a dirty hand could mean a 24 hour bug and crapping or vomitting your guts out for a day.

Yea, a "24 hour bug" of the type you describe is generally food poisoning,mate. You need to worry less about scratching your eyes and more about cooking your chicken thoroughly.

[Dopefish]

Here's a question that recently crossed my mind, that's probably close enough related to this as to not need another thread:

If you're going to scratch your eye with a dirty finger (not obvious grime and not after dealling with crazy stuff in a chem lab or something, but generally germy things like general keyboard/mouse usage), would it be better or worse to 'clean' your finger in your mouth first?

It seems to me to be blatently worse, but then I feel like I've heard that your mouth is reasonably clean, and it could conceivably be better to take in the germs where they're headed towards to vat o stomach acid, as opposed to a more germ friendly route such as your eye. Although, I have no idea how effectively you could clean your finger in your mouth though in the first place, so it may not even help with that and just generally be a poor idea...