A common failure mode in retail portfolio thinking is spending ninety percent of the effort on which stocks to own and ten percent on how much of each to own. The academic and practical evidence points in the opposite direction. Two investors holding the exact same list of stocks can produce results a decade apart in outcome purely because they weighted the positions differently. Portfolio construction — the sizing question — is the more important half of the discipline.
The concentration curve
The relationship between number of holdings and portfolio volatility is well-mapped. Moving from one stock to five reduces idiosyncratic volatility dramatically. Moving from five to twenty produces a smaller additional reduction. Beyond about thirty stocks in a diversified universe, additional names produce very little further volatility reduction. The curve is steep at the start and asymptotic afterwards.
This has a practical implication that surprises many investors: a well-constructed twenty-name portfolio captures most of the diversification benefit of a five-hundred-name index. What it does not capture is the certainty of tracking the index — which is a very different property from the diversification math.
Equal-weight versus market-cap-weight
Given a universe of stocks, the two obvious ways to weight them are equally or in proportion to their market capitalisation. Neither is obviously right. Market-cap weighting is what the S&P 500 does, and it has the appealing property that no rebalancing is required — the index self-adjusts as prices move. It has the less appealing property of increasing concentration into the recently-winning names, which is exactly the wrong direction for someone worried about a bubble in those same names.
Equal weighting removes the concentration bias but introduces a subtle one of its own: it systematically overweights smaller stocks relative to their share of the economy, which historically has produced higher volatility and, in some periods, higher returns.
Neither is universally correct. Both are choices with observable consequences that a serious investor should understand rather than accept by default.
The role of correlation
Weighting two positions equally at 25% each looks diversified until you notice that both are semiconductor stocks whose prices move together with a correlation above 0.8. Effective diversification is not about the count of positions but about the count of independent bets. Five uncorrelated positions produce more diversification than twenty highly correlated ones.
This is the single most common construction error in retail portfolios. Someone owns Apple, Microsoft, Google, Nvidia, and Meta and believes they hold a "diversified" portfolio because there are five names. In practice they hold one bet on US mega-cap technology, weighted five ways. The weighting decision is nominal; the effective exposure is concentrated.
Rebalancing as an implicit position rule
A portfolio rebalanced to fixed weights at set intervals sells winners and buys losers — mechanically, without judgement. Over time, this rebalancing captures a small but positive return premium relative to a portfolio left to drift, because it forces the discipline of trimming positions that have grown into an oversized share of the portfolio.
The corollary is that a portfolio never rebalanced is not stable — it is drifting toward whatever has performed best, which changes the risk profile of the portfolio over time in ways the investor may not have intended.
Position sizing as risk expression
The final and most important idea. Position size is not a preference. It is the mechanism through which conviction is expressed. A 15% position and a 3% position in the same stock are two very different statements about how much you trust the thesis and how much of the outcome you are willing to depend on it. The most disciplined investors reserve larger sizes for cases where they can defend both the analysis and the improbability of a catastrophic downside.
If everything in your portfolio is roughly the same size, you are making a statement — that every idea is equally strong — that is almost certainly false.
Educational content only. Not investment advice.