Let winners run, but know your exit conditions before you buy
"Let it run" is not a complete strategy. You need to know in advance what would actually change your mind.
TL;DR — Selling winners too early is one of the most common and costly mistakes beginners make. But "let it run" is not a complete strategy. You need to know in advance what would actually change your mind. Otherwise you hold forever, miss the turn, and give back all your gains.
Most beginner investors do the opposite of what they should. They hold losers (hoping they will recover) and sell winners (locking in the gain before it disappears). This is backwards. It feels safe but it destroys returns over time.
The right approach is to let winners run for as long as the thesis is intact, and trim only when the position grows meaningfully above its target size. Not because it went up, but because it grew too large relative to the rest of the portfolio.
But letting it run requires knowing in advance what would make you stop. Otherwise you are just holding indefinitely with no discipline.
Before you open any position, write down the exit conditions. These fall into two categories.
The thesis break conditions. What specific things would tell you the original reason to hold this no longer applies? For a country investment, it might be: the PMI drops below 50 for two consecutive months, or the central bank reverses policy direction, or the currency starts trending down after being the reason you entered. Pick specific, observable signals. Not vibes.
The valuation overrun condition. At what portfolio weight does the position become too large regardless of whether the thesis is intact? For most positions, this is roughly double the target. If your target is 10 percent and it runs to 20 percent, you trim back to target and keep the rest running.
The full exit happens only when a thesis break condition is met. Not when the price falls (that is noise unless the regime changed). Not when the news turns negative (news is always negative somewhere). When the underlying macro logic is no longer true.
One more rule: never let a single position stay above 10 percent of your total portfolio for a sustained period. It does not matter how good the thesis is. Concentration kills portfolios in ways that diversification prevents.
Example — An investor enters a copper mining ETF at 8 percent of the portfolio. Their thesis is: AI data center buildout is driving electricity grid expansion, and copper is critical for that infrastructure. Their exit conditions are: copper prices break below a level that makes new mine projects unprofitable, or the AI capex cycle shows visible deceleration. The position runs to 17 percent. They trim back to 8 percent, keeping the core. Two years later, the AI capex cycle starts slowing based on company earnings. The exit condition is met. They sell the rest fully, not partially.