Double Down on Blackjack?

Had a discussion last night with a friend who runs a casino. Topic somehow got to doubling down while holding blackjack instead of taking the immediate 3:2. Some casinos here allow it, his doesn't. He believes the advantage would go to the player. I disagree.

Does anyone know how to work the math out?

It is mind boggling how stupid someone can be and still get a job running a casino.

Been there...

Mr. Donktastic, I asked this same question a ways back, here is the link to all you need to know on it...

http://www.blackjackinfo.com/bb/showthread.php?t=15310

By the way, even though I've gotten away with it in the past, it is definitely against the player and will wreck your EV.

Thanks. Yeah I don't know how he could imagine it would hurt his edge to allow players to do this. This is a fairly large Caribbean casino too.

I was hoping someone might be able to calculate what effect this would have on the house edge overall, like what was done in the 'rule variation' chart on this page: http://wizardofodds.com/blackjack but I imagine it's fairly complicated to work out the math.

It wouldn't affect the house edge at all, as Basic Strategy would be to decline the option to double down your blackjack in all circumstances.

Now, what would really make things interesting, is if they allowed the player to receive the 3:2 payout still if you doubled down and received a 10.

That payout would happen 4 times out of 13. 5 times (a,2,3,4,5) you need the dealer to bust to get paid or you've given the house twice as much money (30% of the time or so). I'd estimate the last (6,7,8,9)4 cards you can get would give the house a 65/35 advantage, since they're weighted towards getting 20 and you're evenly distributed among 17-20.

so 4/13 3:2 on 2 bets
. 5/13 .3*2 bets, .7*loss
. 4/13 .35*2 bets, .65*loss

4/13*(1.5*2)+5/13*(.3*2)+4/13*(.35*2)
=1.4 bets return on your 2 bets.

If my math's right, i can't think of a scenario in which doubling down on a non-DoubleExposure game is a good idea.

Not being able to double down on a blackjack would not change the house edge at all! Because the house edge is caculated on a neutral deck and not being able to double on blackjack won't hurt you. In extremly high counts where there are only 10's left in the deck doubling could be correct.

On my recent first trip to Atlantic City, I noticed that some of the tables had signs prohibiting doubling down with a natural. Just one of many silly things I saw there.

In order for it to be correct to Double on a BJ (where permitted) the remaining cards would need to be almost all Face Cards and tens.

That is not a situation that you will ever find.

It is possible, but only theoretically.

Remember that you are not going to win TWICE as much as you have wagered, as is ordinarily the case when doubling.

You are already winning a guaranteed 150% of your wager.
IF successful your gain is 50% as opposed to losing 200%.
You need to be more than a 4 to 1 favorite to draw the TEN after doubling.

When you lose, you lose a doubled bet [−200%]; when you win you gain an additional 50% [+50% ]

If you bet $10 and forego the immediate $15 win and double and lose you lose $20.
You will either win an EXTRA $5 or lose $20.
That is odds of 4 to 1. The same as ⅘ to ⅕ or 80% to 20%.

Will 80% TENS remaining be sufficient for you to double ?
NO ! Why not ? Because you can still PUSH, and pushing
means that you have made a play that gave up a sure 50% winner !

"The long and the short of it" is that this play is never correct; or as we
say in New Jersey, (where you are not permitted to self-destruct with way)
… "Fuggedaboudit."

One way you have a 100% chance of winning 150% of your bet; versus a chance of winning 200% of your bet ~ OR ~ losing that 200%.

This is the way I think about it -- a 100% chance of winning 150 vs. a ~50% chance of winning 200. Take the money and run.