Research on new investor behavior — drawing on large datasets of actual retail brokerage account activity — has identified a fairly consistent, well-documented cluster of mistakes that disproportionately hurt beginning investors’ returns, and understanding them in advance is considerably cheaper than learning them through direct, costly experience.
Overtrading is the most consistently documented and costly mistake
Large-scale studies analyzing retail brokerage trading data have consistently found that more frequent trading correlates with worse investment returns, not better ones — a finding that holds even before accounting for trading fees and tax consequences, largely because frequent trading tends to reflect overconfidence and reactive decision-making to short-term price movements rather than a disciplined, evidence-based strategy, and each additional trade represents another opportunity to make a behaviorally driven, rather than analytically sound, decision.
Chasing recent performance is a well-documented, costly pattern
New investors show a strong, well-documented tendency to allocate new investment toward assets or funds that have recently performed well, a pattern researchers call performance chasing — but because strong recent performance doesn’t reliably predict continued future outperformance (a well-established finding across fund performance research), this behavior systematically results in buying into assets closer to a performance peak than a starting point, a pattern directly reflected in academic research showing the average investor’s actual realized returns trail the reported returns of the very funds they’re invested in, precisely because of this poorly timed buying and selling behavior.
The data consistently shows that a significant share of the return gap between what a fund reported and what its actual investors realized isn’t the fund’s fault — it’s investors buying in after strong performance and selling after weak performance, doing the opposite of what the strategy actually requires.
Underestimating the impact of fees compounds quietly over decades
New investors consistently underestimate how meaningfully even a seemingly small difference in annual fees compounds over a multi-decade investment horizon — the difference between a low-cost index fund and a higher-fee actively managed fund charging even a couple of percentage points more annually can result in a substantial difference in final accumulated wealth over several decades, a mathematical reality that’s genuinely counterintuitive because the annual fee difference looks small in any single year, and only becomes visually dramatic once compounded across a realistic long-term investment horizon.