In my opinion, overfitting at its core is an attempt to scam the market out of returns and to collect the payment without carrying the risk the payment is for. Which means the whole question resolves into one distinction: is this risk compensated, or did I invent it? Compensated risk is the risk someone else wants off their book badly enough to pay you to hold it. Uncompensated risk is what you introduce yourself when you layer on a filter, tune a threshold, or carve the ugly stretch out of the backtest. So there’s very little fitting the average person can do before fucking things up, because each adjustment quietly moves you from the first category to the second (ask me how I know). The cure is, in my opinion, knowing what mechanism you’re trying to profit from and then accepting the full weight of the risk that comes attached. Including the part that hurts, especially the part that hurts. That’s the deal, that’s what you’re paid for, right? Just my uneducated opinion and how I like to view it.