Instead, casual traders are bleeding cash while a small number of sophisticated pros—including trading firms with access to vast streams of data—eat their lunch, according to a Journal analysis of platform data and interviews with traders.

On Polymarket, the Journal found, 67% of profits go to just 0.1% of accounts. That means less than 2,000 accounts netted a total of nearly half a billion dollars. The Journal analyzed 1.6 million Polymarket accounts that have traded since November 2022. There are at least 2.3 million total accounts on the site.

On Kalshi, too, losers vastly outnumber winners. Spokeswoman Elisabeth Diana said there are 2.9 unprofitable users for each profitable one based on data from the past month. She said the number is subject to change as the exchange grows. The company doesn’t make public comprehensive data on users’ profits and doesn’t share its total number of users.

Total trading volume on both platforms has rocketed to $24.2 billion in April, up from $1.8 billion a year earlier, according to analytics firm The Block.