When you compare ETFs on the same index, you will often see two replication methods: physical and synthetic. Understanding physical vs synthetic ETFs matters because the choice affects costs, tax efficiency and, in a small way, risk. For most investors both are perfectly usable, but it helps to know what you are buying.
In short
- A physical ETF buys the shares in its index, or a representative sample.
- A synthetic ETF holds a basket of securities and uses a swap to receive the index return.
- UCITS rules limit exposure to any swap counterparty to 10% of the fund.
- Synthetic ETFs can be cheaper for US shares and are the only way to hold a world index in a French PEA.
How physical ETFs work
A physical ETF simply buys the securities in its index. A full replication fund holds every company in the index in the same proportions. A sampling fund, used for very large indices, holds a representative selection that behaves almost the same.
This is the most intuitive approach: if you buy an MSCI World ETF, the fund really owns shares of the companies in the MSCI World. Most popular UCITS ETFs, including the large all-world funds, are physical.
Many physical ETFs also lend some of their shares to other institutions for a fee, which slightly reduces the fund’s net cost. The borrower gives collateral, and the risk is small, but you can check the fund’s lending policy in its documents.
How synthetic ETFs work
A synthetic ETF does not necessarily own the companies in its index. Instead:
- The fund buys a basket of other securities, often large European or global shares.
- It signs a swap with a bank: the fund gives the bank the return of its basket, and the bank pays the fund the return of the index.
- You, as investor, get the index return, minus costs.
This sounds complicated, but swaps are standard instruments, and synthetic ETFs are run by the same large providers as physical ones.
The real risk: counterparty risk
The extra risk in a synthetic ETF is that the swap bank fails and cannot pay what it owes. If that happened, the fund would still own its basket of securities, but it would lose the money due under the swap.
UCITS rules limit this: a fund’s exposure to any single swap counterparty may not exceed 10% of its value, and in practice the swap is usually reset well before that. Many synthetic ETFs also use several banks. The realistic worst case is a small loss, not losing your investment.
It is also worth remembering that physical ETFs carry a similar, small risk through securities lending. Neither structure is risk-free, and neither is dangerous for a broad, large fund from a major provider.
Why choose a synthetic ETF?
Lower drag on US dividends
US shares pay dividends that are taxed at source. A physical Irish ETF holding US shares loses 15% of those dividends to US withholding tax. A synthetic S&P 500 ETF does not own the US shares, and under current US tax rules for swaps on broad indices, the swap can usually pass on the dividends without that withholding. That is why synthetic S&P 500 ETFs have often shown a slightly better tracking difference than physical ones. See the best S&P 500 UCITS ETFs for examples.
Access to restricted markets or wrappers
Some markets are hard or expensive to buy directly. Synthetic replication makes them easy to track. In France, a PEA can only hold European shares, so PEA-eligible world ETFs, such as the iShares MSCI World Swap PEA, are all synthetic. See PEA vs CTO vs assurance-vie.
Physical vs synthetic ETFs at a glance
| Physical | Synthetic | |
|---|---|---|
| What the fund holds | The index shares, or a sample | A basket of other securities plus a swap |
| Main extra risk | Securities lending (if used) | Swap counterparty, capped at 10% by UCITS |
| Typical use | Most world and bond ETFs | US equity, PEA world ETFs, hard-to-reach markets |
| Tracking | Very close for large indices | Very close, sometimes better for US shares |
Physical vs synthetic ETFs: which should you choose?
For a long-term investor, replication is a secondary choice. Focus first on the index, the total cost and the size of the fund. If two large ETFs on the same index are similar on all three, pick the one you feel comfortable with. For the full picture on costs, read ETF costs explained.
Sources
This article is for educational purposes only and is not financial or tax advice. Rules and prices change; check official sources or a qualified adviser before making decisions.