Persuasive design draws on our basic psychological makeup to build products that make our engagement with them habitual. It uses variable rewards, creates Fear of Missing Out, and leverages social approval to incrementally increase and maintain user engagement. Social media and networking platforms, video games, and slot machines are all examples of persuasive technologies. Recent attention has focused on the dangers of PD: It can deceptively prod users into forming habits that help the company’s bottom line but not the user’s (...) wellbeing. But PD is not inherently immoral. We can take advantage of our psychological biases to make beneficial changes in ways that enhance our agency rather than limit it. Knowing that a tool is persuasively designed is a consideration in favor of using it when we are trying to break bad habits, such as smoking. How are we to conceptually distinguish between ethical and unethical uses of PD? In this paper, I argue that unethical uses of PD betray or erode our trust. Annette Baier offers a moral test for trust: If gaining knowledge about what other parties do with our trust in them would lead us to stop trusting, then that trusting relationship is immoral. I apply this test to the case of PD. Using trust as a litmus test for ferreting out unethical PD has several advantages, one of which is that it reveals how the harm of unethical PD extends beyond the individual to her wider social network. I close the paper by investigating these cascading effects. (shrink)