Indexing is not ‘breaking’ markets simply because passive funds own a growing share of them. Prices are set by trading, not by ownership, and active investors still do the vast majority of price discovery. That means the key question is not how much of the market is indexed, but whether enough active capital remains to keep prices tied to fundamentals.

Even if passive investing did begin to create distortions, those mispricings would themselves attract active managers seeking profit. In that sense, the system is partly self-correcting. Rather than destroying market efficiency, the rise of indexing may simply be pushing out weaker active managers whilst leaving enough skilled investors in place to keep markets broadly efficient.

Recent debate about the US equity market has focused on two related facts: a small group of mega-cap stocks has driven a large share of recent index returns, and those same firms now make up a large share of the index itself. But those are not quite the same issue. Concentration tells us how the market is weighted. Valuation tells us what investors are paying for future cash flows. That distinction matters, because valuation is the more direct link to long-run expected returns.

The evidence suggests that concentration on its own is a weak guide to future market returns. Markets have been highly concentrated before, both in the US and abroad, without that reliably leading to poor outcomes for the market as a whole. The more persuasive concern is that dominant stocks often struggle to sustain exceptional outperformance once they have reached the top, and that elevated starting valuations have historically been associated with lower long-horizon returns. So the real question is not whether the market looks concentrated, but whether current prices already assume too much good news.

If you want UK inflation protection, the obvious route is index-linked gilts. The problem is that the linker market is relatively narrow and often pushes you into longer-duration exposure, so performance can end up being dominated by real-rate duration and price volatility rather than inflation protection. Dimensional’s design choice in the Sterling Short Duration Real Return Fund is to separate those moving parts: keep the bond portfolio short and diversified (including investment-grade credit), then attach inflation sensitivity via an overlay of UK RPI inflation swaps.

That is the key idea: a physical linker bundles inflation linkage with real-rate duration, whereas a swap overlay lets you target inflation exposure without being forced into long duration. The fund is not risk free. Credit spreads can widen, and implementation frictions matter, but the risk mix is different from a linker-heavy approach.