Market-Cap Weighting, Fund Rebalancing, and Investor Ownership

Summary

The document raises questions about holding large and mid-cap equities alongside a small-cap tracker, with each fund initially allocated according to market capitalization. It asks whether an investor’s proportional ownership of publicly traded companies remains constant, and how dividends, reinvestment, share issuance, buybacks, and companies entering or leaving markets affect that share. It also contrasts accumulation and income share classes.

The central portfolio question is whether the fund allocation needs rebalancing to preserve market-cap weights, especially when dividends are reinvested. The text frames capital gains and income as economically interchangeable apart from taxes and trading costs, but provides no answers, calculations, or evidence resolving the questions. It is therefore useful as a set of issues for studying capitalization-weighted portfolios and fund mechanics, rather than as a prescribed rebalancing method. Actual outcomes depend on index rules, fund implementation, distributions, flows, and the definition of ownership being measured.

Key ideas

• A portfolio split across large-cap and small-cap trackers may drift from its initial market-cap allocation.
• Accumulation and income share classes handle distributions differently and can affect the investor’s holdings.
• Share issuance, buybacks, and changes in listed companies complicate the idea of fixed proportional ownership.
• The document poses rebalancing questions but does not provide a calculation or recommendation.

Tags

# Rebalancing market-cap-weighted investments


# Rebalancing market-cap-weighted investments












I've just (hypothetically) invested $1m in two market-cap-weighted global equities tracker funds: one large+mid-cap tracker and one small-cap tracker. Both are of an accumulation share class, and I balanced my investment by respective market caps.

I'm trying to understand the underlying theory and if/why I need to rebalance.

I think they should be insignificant, so I'm going to ignore: the indexing error of the global equity index by assuming it tracks the entire universe of publicly traded companies; tracking error of the funds; as well as tax & transaction costs.

By a back of napkin estimate I own approx 0.000001% of publicly-traded equities. i.e effectively 0.000001% of each and every publicly traded company. Is this right, and if I don't sell is this constant or does it grow? How is it affected by dividend income, share offerings/buybacks and companies entering/exiting stock markets? If I opted for the income share class rather than accumulation, would my 0.000001% stay constant or shrink?

Do I need to rebalance my allocation between the two funds to maintain their weighting by market-cap? Intuitively they'll be weighted by market-cap+income if I don't rebalance. But how does the relevancy of income reconcile with income and capital gains being perfect substitutes, tax and transaction costs aside?

Any links to relevant theories that help clarify this appreciated. Apologies if this isn't considered the right place for this sprawling question, but other answers here seemed more insightful than on other SE sites.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.