Home bias and global diversification: market cap, GDP weights and the growth puzzle

What you will learn

• How to measure home bias in your own portfolio in percentage points.
• Why market-cap weighting and GDP weighting give very different emerging-market (EM) shares.
• Why high economic growth does not automatically produce high equity returns (the growth-versus-returns puzzle).
• A simple rule for choosing an EM weight you can hold through a bad decade.

Explanation

Measuring home bias

Home bias is the tendency to hold far more of your own country's shares than their share of the world market. It is measured as the domestic weight in your equity portfolio minus the domestic weight in a global market-cap index. Germany's weight in a world index that includes emerging markets is only a few percent; a German investor with half of their equities in DAX and MDAX shares therefore has a home bias of more than 40 percentage points. Diversification and correlation are explained in the course "ETFs, Funds & Long-Term Investing"; home bias is a specific, measurable form of under-diversification. Its costs are concentration in one economy and tax regime and often a sector tilt (for Germany: industry and autos, little technology).

Some home bias is defensible (no currency risk on euro assets, familiarity); the point is to choose the number deliberately.

Market cap versus GDP weighting

Market-cap weighting gives each country the weight of its listed, free-float share value. As of 2026, emerging markets make up roughly 12 % of the MSCI ACWI index (All Country World Index). GDP weighting gives each country the weight of its economic output. Emerging and developing economies account for well over a third of world GDP at market exchange rates and more than half at purchasing-power parity. A GDP-weighted equity portfolio would therefore hold three to four times more EM than a market-cap portfolio.

Why do the two differ? Many EM companies are unlisted, state-owned or have small free floats; foreign ownership limits reduce investable weight; and large parts of EM output come from sectors with few listed firms. Market cap reflects what you can buy and is the only weighting all investors could hold at once; GDP reflects where production happens.

The growth-versus-returns puzzle

Long-run studies by Dimson, Marsh and Staunton and by Jay Ritter found little or even negative correlation between a country's real GDP growth and its real equity returns across countries and decades. The reasons are mechanical:

• Dilution. Growth is financed by new share issues and new companies; earnings per share grow slower than aggregate earnings.
• Valuation. Growth that everyone expects is already priced in. If the multiple paid for expected growth later falls, returns are poor even when growth arrives.
• Who captures the growth. Labour, the state and unlisted private firms capture a large share of GDP growth; listed companies may capture little.
• Currency. Growth with high inflation often comes with depreciation, which lowers returns for a euro investor.

The practical conclusion: do not choose your EM weight because "EM grows faster". Choose it for diversification and for valuation, and size it so that you can hold it after ten weak years.

Worked example

Hypothetical example: all figures are invented and rounded; fees, taxes and currency effects are excluded.

Step 1 – home bias. An investor in Germany holds 45 % of their equities in German shares. Germany's weight in the global index is assumed to be 2 %. Home bias = 45 − 2 = 43 percentage points.

Step 2 – EM weight under two schemes. Market cap gives EM 12 %; GDP at market rates gives EM 40 %. A 50/50 blend gives (12 + 40) ÷ 2 = 26 %. A middle path such as 15 to 20 % is a common compromise.

Step 3 – growth without returns. Country Y's economy grows 6 % per year for ten years, but new share issuance dilutes existing holders, so earnings per share grow only 3 % per year. The market's price-to-earnings ratio falls from 15 to 12 as the growth story cools. Price index after ten years = 1.03^10 × (12 ÷ 15) = 1.344 × 0.8 = 1.075, a total price gain of only 7.5 %, or about 0.7 % per year, plus dividends. A slow-growing developed country with 1 % earnings-per-share growth and a constant multiple returns 1.01^10 − 1 = 10.5 %, or 1.0 % per year, plus dividends, and beats the fast grower.

Common mistake

Practice activity

On paper: your equity holdings are €80,000, of which €60,000 is in German shares. Germany's assumed global index weight is 2 %. (a) Calculate your home bias in percentage points. (b) You decide to limit German shares to 10 % of equities. How much must you move into global funds? (c) Country Z grows 7 % per year; earnings per share grow 2 % per year because of dilution; the price-to-earnings ratio falls from 20 to 14 over ten years. What is the total price change before dividends?

Key takeaways

• Home bias = domestic weight in your portfolio minus domestic weight in the world index; write the number down.
• Market cap gives EM about 12 % of world equities as of 2026; GDP weighting gives three to four times more. Choose consciously between them.
• GDP growth is not equity return: dilution, valuation changes, currency and who captures the growth break the link.
• Size EM for diversification and valuation, at a weight you can hold through a weak decade.

Next step

Next we compare the actual EM ETFs available to German investors: index choice, ex-China and IMI variants, withholding-tax leakage and sampling.