Ten mistakes. Each one avoidable. All quietly expensive.
A quick story
Two investors start on the same day. Both put €10,000 into a broadly diversified global ETF.
One follows a simple plan. The other reacts to every headline, changes ETFs whenever a new theme trends, and worries about every crash.
Twenty years later, the gap between them isn't caused by intelligence — or even by ETF selection. It's caused by ten small, avoidable decisions.
Why this matters
Most investors don't lose money because they bought a bad ETF. They lose money because they repeatedly make small decisions that feel rational in the moment — and quietly compound against them for decades.
Recognising these mistakes is the single fastest way to improve your long-term results.
The 10 mistakes, grouped by cause
Tap any mistake to see a short explanation, one realistic example, and one takeaway. As you scroll, ask yourself quietly: have I done this?
Timing mistakes
The first family of mistakes rarely feels like a mistake — it feels like patience.
Portfolio mistakes
The second family looks like sophistication — more funds, more exposure, more research.
Emotional mistakes
The third family is the hardest to spot in yourself — because it always feels rational in the moment.
Planning mistakes
The final family is the quietest of all — small structural decisions that compound over decades.
Long-term investor returns are shaped more by behaviour than by ETF selection.
The same ETF can produce very different results for different investors, depending on when they buy, sell and add. The fund's return is only part of the story.
Why it matters: a modest ETF held with discipline usually beats an excellent ETF held with anxiety.
Myth: More ETFs means more diversification.
Reality: ten ETFs that each hold Apple, Microsoft and Nvidia don't reduce risk. A single broad global ETF often gives more genuine diversification than a shelf of overlapping funds.
A written plan is the single most reliable defence against these mistakes.
For most long-term investors, one page is enough: goal, horizon, monthly contribution, ETF, and what would make you change. The plan doesn't need to be sophisticated — it needs to exist before emotions take over.
Investors with a written plan are meaningfully more likely to stay invested through a downturn.
Your decision
You inherit €30,000.
Your existing portfolio holds a Global ETF, a Europe ETF and an AI ETF. Markets have fallen 22% over the last three months.
What do you do?
The most expensive investing mistakes rarely look like mistakes in the moment — they look like reasonable reactions to the news.
Test what you've learned
Three quick questions. Answers and explanations appear instantly.
Q1. Does buying more ETFs automatically increase diversification?
Q2. Is timing every major market move a reliable long-term strategy?
Q3. What matters most over decades?
Answered 0 of 3.
Grounded in landmark research.
This lesson draws on landmark academic research and evidence that has shaped modern investing.
Why this lesson matters
Each of the ten mistakes in this lesson is, at its core, an invitation to trade more than necessary — and to abandon a perfectly reasonable plan.
Last reviewed: July 2026
Explore the primary sources behind this lesson.
Lesson-specific sources: original research, regulatory texts, or index methodology — chosen to let you verify the claims in this lesson.
Barber & Odean (2000) — Trading Is Hazardous to Your Wealth
The foundational empirical study on individual-investor trading costs.
Journal of Finance 55(2)
Dichev (2007) — What Are Stock Investors' Actual Historical Returns?
Dollar-weighted vs. time-weighted returns and the investor timing gap.
American Economic Review 97(1)
Friesen & Sapp (2007) — Mutual Fund Flows and Investor Returns
Peer-reviewed evidence on the return cost of mistimed fund purchases.
Journal of Banking & Finance 31(9)
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Great investors aren't the ones who never make mistakes. They're the ones who make the same mistake less often.
Disclaimer
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