The Lesson of the Watertight Compartments
The engineers who designed the Titanic weren't fools. Below her decks, the hull was divided into sixteen watertight compartments — puncture one, seal it off, and the ship sails on. The design assumed any collision would breach one, maybe two compartments. The ship was "unsinkable" because her risks were compartmentalized.
Except they weren't. The bulkheads between compartments didn't extend all the way up. When the iceberg opened five compartments instead of two, water filled them, the bow dipped, and the sea simply spilled over each bulkhead into the next compartment, one after another, like an ice cube tray tilting. Sixteen compartments, one failure mode. The compartments were real. Their independence was an illusion.
This is the single most misunderstood idea in investing. Diversification is not owning many things. It is owning things that fail differently. An investor holding five funds — a tech fund, a growth fund, an S&P 500 fund, a Nasdaq fund, and shares of their tech employer — has sixteen compartments and connected bulkheads. In calm seas, the portfolio looks beautifully spread out. When the iceberg arrives, everything floods together, because everything was exposed to the same water.
The previous lesson showed why you must own thousands of stocks instead of fifteen. This lesson goes a level deeper: why owning thousands of stocks still isn't diversification — and what genuine, sealed-to-the-top compartments actually look like.