Major League Baseball held its season openers this week under the shadow of a gambling scandal. Reports surfaced that the NBA is investigating a player over irregular bets. And college hoops fans await results from a review into unusual betting on a men’s basketball game.

The incidents have highlighted a tradeoff that U.S. professional sports leagues made when they embraced gambling.

Leagues have signed lucrative marketing deals with betting apps like FanDuel and DraftKings and use gambling to amp up fan engagement. But this new source of revenue has also opened the doors to a fundamental danger: that an explosion of sports betting could threaten the assumption of fairness at the core of athletic competitions.