Rung Lock: Skills Only Grow on the Right Skills

During training, a capability that is about to emerge can look exactly like one that will never emerge. Both remain at chance, with no visible sign of what will happen next. We find that some capabilities are locked in this state until several pieces are in place at the same time. They earn no partial credit: if any one piece is missing, the training signal for the capability is exactly zero. The pieces therefore cannot be learned independently and assembled later. Each must help bring the others into place. We tested this by removing from the training data a simpler skill built from the same pieces, while changing nothing else. The target capability did not merely emerge more slowly. It never started. Across nine runs (thirteen with a preregistered extension), its score remained at chance; in two runs, it stayed there for $45\times$ the training normally required for emergence. We then tried to replace the missing skill with an unrelated skill of the same difficulty, training it until it reached perfect accuracy. The target capability remained exactly where it had been without the helper. When we could control the underlying pieces directly, the result became even simpler: a helper's value depended only on how many pieces it shared with the target, not on which pieces they were. A missing rung is not a delay. It is a lock—and the wrong key does nothing at all.