This report describes insights gleaned from the Data Fellows collaboration between PayPal and the Golub Capital Social Impact Lab at Stanford University’s Graduate School of Business. By embedding researchers in PayPal’s charitable giving team, we co-designed and executed a 14-month-long experiment on PayPal’s Giving Fund website with more than 400,000 donors to address the question: How can default donation amounts be chosen to help nonprofits achieve their fundraising objectives and donors reach their charitable giving goals? We learn that decreasing the lowest default amount attracts more donors, while increasing the highest default amount raises more money, leading to a trade-off in these objectives. Personalization through showing potential donors the “right” default donation amounts can reduce that trade-off. We discuss how nonprofits and platforms can use these findings to personalize their default donation amounts.