The conjunction fallacy is a formal fallacy that occurs when it is assumed that specific conditions are more probable than a single general one. The most often-cited example of this fallacy originated with Amos Tversky and Daniel Kahneman: "Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations. Which is more probable? 1. Linda is a bank teller. 2. Linda is a bank teller and is active in the feminist movement." The majority of those asked chose option 2. However, the probability of two events occurring together (in "conjunction") is always less than or equal to the probability of either one occurring alone [...]
Wikipedia: “Conjunction fallacy”