Background: The scale and complexity of British gambling advertising has increased in recent years. "Live-odds" TV gambling adverts broadcast the odds on very specific, complex, gambles during sporting events (e.g., in soccer, "Wayne Rooney to score the first goal, 5-to-1," or, "Chelsea to win 2-1, 10-to-1"). These gambles were analyzed from a behavioral scientific perspective (the intersection of economics and psychology). Method: A mixed methods design combining observational and experimental data. A content analysis showed that live-odds adverts from two months of televised English Premier League matches were biased towards complex, rather than simple, gambles. Complex gambles were also associated with high bookmaker profit margins. A series of experiments then quantified the rationality of participants' forecasts across key gambles from the content analysis (Total N = 1,467 participants across 5 Experiments). Results: Soccer fans rarely formed rational probability judgments for the complex events dominating gambling advertising, but were much better at estimating simple events. Conclusions: British gambling advertising is concentrated on the complex products that mislead consumers the most.Behavioral scientific findings are relevant to the active public debate about gambling.