Gambling fallacies are irrational beliefs about how gambling works, which are common among disordered gamblers, and measured by questionnaires such as the Gambling Fallacies Measure (GFM). Less is known about the potentially rational cognitions of some skilled gamblers, such as professional poker players. The present research experimentally manipulated item 5 from the GFM, “A positive attitude or doing good deeds increases your likelihood of winning money when gambling”, by comparing two new versions focusing only on a “positive attitude” or “doing good deeds” to the original version (control). Item 5 is scored so that “disagree” is the non-fallacious correct answer, but it was hypothesized that the words “a positive attitude” might increase rates of poker players selecting “agree” in a non-fallacious manner. Online experiments were conducted on samples of professional poker players (N = 379), and a broad sample of poker players with no inclusion criteria (N = 1,510). Participants’ responses to item 5 were associated with the rest of their GFM scores (GFM-9). Participants in both samples were more likely to disagree with the good deeds version, and less likely to disagree with the positive attitude version, compared to control. In comparison to the other conditions, good deeds responses were most strongly associated with GFM-9 scores among professionals, while positive attitude responses were least strongly associated with GFM-9 scores among the broad sample. The good deeds version of item 5 has advantageous measurement properties among professional poker players. New approaches are needed to better understand the potentially rational cognitions of skilled gamblers.