Why do the two sides always total 100?

Because Yes and No aren’t two separate prices — they’re one opinion, written down twice.

A market here asks a single question. “How likely is Yes?” and “how likely is No?” are the same answer read from opposite ends, so once the room lands at 67 on Yes, it has already said 33 on No. There’s nothing left over to decide.

Total anything else and the room is arguing with itself. Add up to more than 100 and the two numbers are giving two different answers to the same question. Add up to less, and part of the answer has simply gone missing. Either way somebody’s number is off — and a number that’s off, in a room full of readers, doesn’t sit still for long.

That’s true on any prediction market, and it’s why the pair is lighter to carry than it looks: the two prices are really one price. The second number is just the first one’s reflection. Say 67, and the 33 comes free.

That one number has a name here — the middle — and a whole room’s worth of reasons behind it.