Betting on the team you support

You know more about them than about anybody else and you price them worse than you price anybody else. The extra knowledge is real and the bias is larger than the knowledge.

Fandom and forecasting draw on the same information and weight it differently. Following a team closely produces genuine insight and simultaneously produces systematic optimism, and the second is bigger.

It is not universal — a supporter betting against their own team is often unusually accurate, precisely because the bias is working the other way and being resisted rather than followed.

That is also the expensive part. The strongest opinions in anybody's betting are usually about the team they care about most, which puts the largest stakes on the worst-calibrated view.

What to do with it. Either exclude your own team entirely, or log those bets separately and read the record after a hundred of them.