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If throwing good money after bad is generally a bad idea, is throwing more money after good Ok?
The principle behind the advice to not throw good money after bad is better restated in economics terms: sunk costs are sunk and irrelevant to today's decisions.   Money lost on a stock is sunk and should not affect our decisions today, one way or the other.  Similarly, the stock going up should not affect our decisions today, one way or the other.  Any advice other than this is assuming some kind of mispricing or predictability in the market.  Mispricings in general cannot be reliably identified and stock returns are not normally predictable. The only valid (efficient markets) reason I know of to allow money you have lost or made on a stock to affect your decision today is the tax implications (you may want to lock in gains if your tax rate is temporarily low or vice versa).