The S&P 500 is one of the cheapest, most liquid, most diversified equity products ever shipped. For most investors, owning SPY or VOO and never thinking about it again is the best decision they will ever make. Arithmos exists for the cases where that is not enough.
When the S&P 500 wins
- You want one product, low fee, no maintenance.
- You have no opinion on sectors, factors, geography or specific names — and that is a perfectly defensible position.
- You are comparing to active managers — over 20 years, the S&P 500 beats roughly 90% of US large-cap funds.
- You want maximum tax efficiency from a low-turnover ETF wrapper.
When a custom index wins
- You have an active view — "AI ex-China", "S&P 500 ex-tobacco", "the seven largest UK insurers". Off-the-shelf ETFs do not always exist; ETF issuers ship the index they think they can sell, not the one you want.
- You want a tax-loss-harvesting overlay on broad equity exposure (the historic core use case for direct indexing).
- You want to swap in a benchmark — "like SPY but tilted toward quality", "like SPY but capped at 4% per name" — and see the backtest.
- You want full holdings transparency: exact tickers, exact weights, exact rationale.
FAQ
Can I beat the S&P 500 with Arithmos?
Sometimes. Backtests will show some thematic indices beating SPY over 5–10Y, others underperforming. The benefit is alignment with your view, not a guaranteed alpha source — past performance does not guarantee future results.
Is Arithmos cheaper than buying an S&P 500 ETF?
ETFs have an expense ratio (≈3 bps for SPY-class products). Arithmos is a flat subscription. For a portfolio above roughly $25k, the subscription tends to be cheaper than the ETF expense ratio over time.
Can I just buy the S&P 500 stocks myself through Arithmos?
Yes — describe "the 500 largest US companies, market-cap weighted" and the agent builds it. Most users prefer to start with a thematic tilt, but the literal SPY replication is one prompt away.
