This is Ellen, a single and bright woman. When she was a student —in high school and in college too— Ellen was deeply involved in environmental issues, and also participated in social justice protests.
Now it’s 2019, and Ellen is 31-year-old.
Question: Which of the following statements are more probable regarding Ellen’s occupation today, and in what order?
A) Ellen is a bank teller;
B) Ellen works as a TV reporter;
C) Ellen is a bank teller at a small community bank; she remains active fighting climate change.
Quick, what’s your answer? In what order would you rank those 3 options?
First of all, ignore how you ranked B), as it is not relevant to this brain teaser, it was there to distract you.
What matters is this: If you ranked C) as more probable than A), you are wrong … and in good company, as this is a common cognitive bias (also called “conjunction fallacy” as researched by Tversky and Kahneman).
By definition, it is more probable that Ellen is a bank teller than she is to meet these 3 conditions at the same time: 1) she is a bank teller AND 2) at a small community bank AND 3) she remains active in the environmental movement.