S&P 500 (SPX) Up or Down on July 31?

Platform comparison

Market context

The close is already in, and the S&P 500 finished higher on the day, so the market should resolve **Up** rather than **Down**. The official close on 31 July was 7,489.72, above Thursday’s 7,437.63 close, which makes the crowd-implied **100% YES** price look fully aligned with the realised outcome rather than a live handicap. [2][4]

For framing, this is a favourites-only spot: there is no meaningful underdog case left once the index has printed a higher settlement close. The earlier July backdrop still matters for context, though, because the S&P 500 had been under pressure over the month, with MarketWatch showing a 1-month change of -6.27% and a year-to-date return of -5.11% before the final session, which explains why a late-month rebound could still have offered a contrarian bounce case. [3]

The main catalysts that mattered into the close were the Federal Reserve’s decision to leave rates unchanged, which helped spark Thursday’s rebound, and the market’s sensitivity to the next leg of earnings and macro headlines. Zacks said U.S. stocks ended sharply higher on Thursday after the Fed decision, while intraday commentary on Friday noted the index had been trading in a tight short-term range around 7,425 to 7,550, with 7,450 treated as an important trigger level. [1][5]

Sources: 1 · 2 · 3 · 4 · 5

Methodology

Methodologically we separate two layers: the live probability (Polymarket mid-price) and the platform attributes (fee, KYC, settlement currency, payment rails). That keeps the comparison honest — a single canonical probability across the row, with the venue-by-venue trade-offs spelt out in the columns next to it.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Who Will Win. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.