I recently had a conversation with one of our senior managers about our company’s new banking division; he told me that only 21% of our cardholders account for 80% of spending. That skewed situation worried him greatly, and he wondered what we could do to spread our lending book more evenly. I’ve had similar conversations with the fundraising manager of a nonprofit I chair: The bulk of the funding comes from some 20 donors, which she tells me is unsustainable. The way she sees it, the organization is heading toward a cliff. Both of these reactions reveal a common cognitive error that has profound implications for leadership.