Benchmark Volatility & Index Concentration Distortion Workbench

Global benchmark indices are becoming increasingly dominated by a handful of mega‑cap stocks. This concentration can cause the headline index volatility to diverge from the broader market because shocks to a few large constituents are amplified.

Use the sliders below to adjust the concentration level (percentage of market cap held by the top 10 stocks) and the base market volatility. The chart shows the resulting effective index volatility calculated with a simple amplification model:

Effective Volatility = Base Volatility × (1 + k × (Concentration‑30%))
where k≈0.8 reflects how extra concentration magnifies swings.
40%
12%

Interpretation

The blue line shows how the effective index volatility rises as concentration increases. When concentration is near historical averages (~30%), the amplification is modest. Above ~45% the volatility spikes sharply, illustrating the risk of a concentrated benchmark.

The written amplification is linear

Read the explanation

Source volatility equals base times one plus point eight times concentration minus thirty divided by thirty. At base twelve, concentration thirty gives twelve and sixty gives twenty one point six. Bars use twenty pixels per illustrative volatility unit. This is a toy linear mapping, not a measured covariance model or investment forecast. At concentration forty, base twelve gives fifteen point two and base twenty four gives thirty point four. Bars use fifteen pixels per illustrative unit. All concentration points are recomputed using the base slider. No market prices, portfolio holdings, correlations or returns are loaded. The source plots fifty one labels from ten through sixty inclusive. Changing selected concentration updates its text while recomputing the same curve for a fixed base. It does not add a selected marker or change dataset values. Bars compare fifty intervals and fifty one points at eight pixels per count. Prose about sharp spikes is not a nonlinear computation in this code.

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