Harry Potter and the Methods of Rationality

'magic' from the father, and half the time the child will get the father's paper saying 'not magic'. When a witch marries a Squib, the result won't be a lot of weak wizarding children. Half the children will be wizards and witches just as powerful as their mother, and half the children will be Squibs. Because if there's just one place in the recipe that makes you a wizard, then magic isn't like a glass of pebbles that can mix. It's like a single magical pebble, a sorcerer's stone."

Harry arranged three pairs of papers side by side. On one pair he wrote 'magic' and 'magic'. On another pair he wrote 'magic' on the top paper only. And the third pair he left blank.

"In which case," Harry said, "either you have two stones or you don't. Either you're a wizard or not. Powerful wizards would get that way by studying harder and practicing more. And if wizards get inherently less powerful, not because of spells being lost but because people can't cast them... then maybe they're eating the wrong foods or something. But if it's gotten steadily worse over eight hundred years, then that could mean magic itself is fading out of the world."

Harry arranged another two pairs of papers side by side, and took out a quill. Soon each pair had one piece of paper saying 'magic' and the other paper blank.

"And that brings me to the prediction," said Harry. "What happens when two Squibs marry. Flip a coin twice. It can come up heads and heads, heads and tails, tails and heads, or tails and tails. So one quarter of the time you'll get two heads, one quarter of the time you'll get two tails, and half the time you'll get one heads and one tail. Same thing if two Squibs marry. One quarter of the children would come up magic and magic, and be wizards. One quarter would come up not-magic and not-magic, and be Muggles. The other half would be Squibs. It's a very old and very classic pattern. It was discovered by Gregor Mendel who is not forgotten, and it was the first hint ever uncovered for how the recipe worked. Anyone who knows anything about blood science would recognize that pattern in an instant. It wouldn't be exact, any more than if you flip a coin twice forty times you'll always get exactly ten pairs of two heads. But if it's seven or thirteen wizards out of forty children that'll be a strong indicator. That's the test I had you do. Now let's see your data."

And before Draco could even think, Harry Potter had taken the parchment out of Draco's hand.

Draco's throat was very dry.

Twenty-eight children.

He wasn't sure of the exact number but he was pretty sure around a fourth had been wizards.

"Six wizards out of twenty-eight children," Harry Potter said after a moment. "Well, that's that, then. And first- years were casting the same spells at the same power level eight centuries ago, too. Your test and my test both came out the same way."

There was a long silence in the classroom.

"What now?" Draco whispered.

He'd never been so terrified.

"It's not definite yet," said Harry Potter. "My experiment failed, remember? I need you to design another test, Draco."

"I, I..." Draco said. His voice was breaking. "I can't do this Harry, it's too much for me."

Harry's look was fierce. "Yes you can, because you have to. I thought about it myself, too, after I found out about the Interdict of Merlin. Draco, is there any way of observing the strength of magic directly? Some way that doesn't have anything to do with wizards' blood or the spells we learn?"

Draco's mind was just blank.

"Anything that affects magic affects wizards," said Harry. "But then we can't tell if it's the wizards or the magic. What does magic affect that isn't a wizard?"

"Magical creatures, obviously," said Draco without even thinking about it.

Harry Potter slowly smiled. "Draco, that's brilliant."

It's the sort of dumb question you'd only ask in the first place if you'd been raised by Muggles.

Then the sickness in Draco's stomach got even worse as he realized what it would mean if magical creatures were getting weaker. They would know for certain then that magic was fading, and there was a part of Draco that was already sure that was exactly what they would find. He didn't want to see this, he didn't want to know...

Harry Potter was already halfway to the door. "Come on, Draco! There's a portrait not far from here, we'll just ask them to go get someone old and find out right away! We're cloaked, if someone sees us we can just run away! Let's go!"

It didn't take long after that.

It was a wide portrait, but the three people in it were looking rather crowded. There was a middle-aged man from the twelfth century, dressed in black swathes of cloth; who spoke to a sad-looking young woman from the fourteenth century, with hair that seemed to constantly frizz about her head as if she'd been charged up by a static spell; and she spoke to a dignified, wizened old man from the seventeenth century with a solid gold bowtie; and him they could understand.

They had asked about Dementors.

They had asked about phoenixes.

They had asked about dragons and trolls and house elves.

Harry had frowned, pointed out that creatures which needed the most magic could just be dying out entirely, and had asked for the most powerful magical creatures known.

There wasn't anything unfamiliar on the list, except for a species of Dark creature called mind flayers which the translator noted had finally been exterminated by Harold Shea, and those didn't sound half as scary as Dementors.

Magical creatures were as powerful now as they'd ever been, apparently.

The sickness in Draco's stomach was easing, and now he just felt confused.