Two ETFs can track the same index and reach the same return by completely different routes. One buys the shares. The other signs a contract with a bank promising the index return. Both are legitimate; they carry different risks.

Physical replication

The fund buys the actual securities. Two flavours exist:

  • Full replication — owns every constituent at the right weight. Simple, transparent, works well for liquid indices.
  • Sampling (optimised) — owns a representative subset. Used where the index has thousands of members or illiquid corners, and buying everything would cost more than the tracking error it saves.

The appeal is that you can see what you own. If the provider fails, the securities exist and are held separately from the provider's own balance sheet.

Synthetic replication

The fund holds a basket of collateral and enters a swap with a counterparty — usually an investment bank — which agrees to pay the index return in exchange for the collateral basket's return. The fund does not own the index at all; it owns a promise plus security against that promise.

This sounds alarming and mostly is not. In Europe, UCITS rules cap uncollateralised swap exposure to a small percentage of net asset value, and in practice providers over-collateralise well beyond the requirement. Still, the risk is real and different in kind: if the counterparty fails, you depend on the collateral being sold at a fair price.

Why synthetics still exist

Because sometimes they track better. For certain indices — notably US equity exposure for non-US investors — a swap structure can avoid dividend withholding tax that a physical fund must pay. That tax advantage can exceed the entire TER, which is exactly why synthetic S&P 500 funds have often shown better tracking difference than physical ones.

Synthetics are also the practical route into markets that are difficult or expensive to hold directly — some commodities and some emerging markets.

Which should you pick?

For a core, long-term holding in a liquid market, physical is the simpler default. Choose synthetic deliberately, when the tracking or access advantage is real and you have checked the collateral policy — not by accident.

Summary

Physical means you own the assets. Synthetic means you own collateral plus a contract. Neither is a scandal; the mistake is not knowing which one you hold.