Weight Decay Deep Dive: How Regularization Locks In Old Knowledge Instead of Erasing It

In my previous article, I noted some interesting behavior regarding Weight Decay; here, I examine it in detail.
It is generally accepted in the ML industry that if we take a pre-trained model and fine-tune it on a new task, the old weights are gradually overwritten. Furthermore, if we add Weight Decay (L2 regularization), the process of "forgetting" superfluous information should theoretically happen even faster.
I tested this claim experimentally. The results were counter-intuitive: under specific settings, Weight Decay works in the exact opposite way—it protects the old structure from destruction.
Below is a description of the experiment and conclusions for those involved in model training and AI safety.

















