Harry Potter and the Methods of Rationality

(And somewhere in the back of his mind was a small, small note of confusion, a sense of something wrong about that story; and it should have been a part of Harry's art to notice that tiny note, but he was distracted. For it is a sad rule that whenever you are most in need of your art as a rationalist, that is when you are most likely to forget it.)

Harry detached himself from Professor McGonagall's side. "I'll - have to think about this," he said, trying to keep his voice under control. He stared at his shoes. "Um. You can go ahead and call them my parents, if you want, you don't have to say 'genetic parents' or anything. I guess there's no reason I can't have two mothers and two fathers."

There was no sound from Professor McGonagall.

And they walked together in silence, until they came before a great white building with vast bronze doors, and carven words above saying Gringotts Bank.

Chapter 4: The Efficient Market Hypothesis

Disclaimer: J. K. Rowling is watching you from where she waits, eternally in the void between worlds.

A/N: As others have noted, the novels seem inconsistent in the apparent purchasing power of a Galleon; I'm picking a consistent value and sticking with it. Five pounds sterling to the Galleon doesn't square with seven Galleons for a wand and children using hand-me-down wands.

"World domination is such an ugly phrase. I prefer to call it world optimisation."

Heaps of gold Galleons. Stacks of silver Sickles. Piles of bronze Knuts.

Harry stood there, and stared with his mouth open at the family vault. He had so many questions he didn't know where to start.

From just outside the door of the vault, Professor McGonagall watched him, seeming to lean casually against the wall, but her eyes intent. Well, that made sense. Being plopped in front of a giant heap of gold coins was a test of character so pure it was archetypal.

"Are these coins the pure metal?" Harry said finally.

"What?" hissed the goblin Griphook, who was waiting near the door. "Are you questioning the integrity of Gringotts, Mr. Potter-Evans-Verres?"

"No," said Harry absently, "not at all, sorry if that came out wrong, sir. I just have no idea at all how your financial system works. I'm asking if Galleons in general are made of pure gold."

"Of course," said Griphook.

"And can anyone coin them, or are they issued by a monopoly that thereby collects seigniorage?"

"What?" said Professor McGonagall.

Griphook grinned, showing sharp teeth. "Only a fool would trust any but goblin coin!"

"In other words," Harry said, "the coins aren't supposed to be worth any more than the metal making them up?"

Griphook stared at Harry. Professor McGonagall looked bemused.

"I mean, suppose I came in here with a ton of silver. Could I get a ton of Sickles made from it?"

"For a fee, Mr. Potter-Evans-Verres." The goblin watched him with glittering eyes. "For a certain fee. Where would you find a ton of silver, I wonder?"

"I was speaking hypothetically," Harry said. For now, at any rate. "So... how much would you charge in fees, as a fraction of the whole weight?"

Griphook's eyes were intent. "I would have to consult my superiors..."

"Give me a wild guess. I won't hold Gringotts to it."

"A twentieth part of the metal would well pay for the coining."

Harry nodded. "Thank you very much, Mr. Griphook."

So not only is the wizarding economy almost completely decoupled from the Muggle economy, no one here has ever heard of arbitrage. The larger Muggle economy had a fluctuating trading range of gold to silver, so every time the Muggle gold-to-silver ratio got more than 5% away from the weight of seventeen Sickles to one Galleon, either gold or silver should have drained from the wizarding economy until it became impossible to maintain the exchange rate. Bring in a ton of silver, change to Sickles (and pay 5%), change the Sickles for Galleons, take the gold to the Muggle world, exchange it for more silver than you started with, and repeat.

Wasn't the Muggle gold to silver ratio somewhere around fifty to one? Harry didn't think it was seventeen, anyway. And it looked like the silver coins were actually smaller than the gold coins.

Then again, Harry was standing in a bank that literally stored your money in vaults full of gold coins guarded by dragons, where you had to go in and take coins out of your vault whenever you wanted to spend money. The finer points of arbitraging away market inefficiencies might well be lost on them. He'd been tempted to make snide remarks about the crudity of their financial system...

But the sad thing is, their way is probably better.