When people donate, they rarely give to the charities that do the most good perdollar. Why is this? One possibility is that they do not know how to giveeffectively. Another possibility is that they are not motivated to do so. Acrosssix tasks (Studies 1a, 1b), we found support for both explanations. Among laydonors, we observed multiple misconceptions—regarding disaster relief,overhead costs, donation splitting, and the relative effectiveness of local andforeign charities—that reduced the effectiveness of their giving.Similarly, we found that they were unfamiliar with the most effective charities(Studies 2a, 2b). Debunking these misconceptions and informing people abouteffectiveness boosted effective donations; however, a portion of lay donorscontinued to give ineffectively to satisfy their personal preferences. Bycontrast, a sample of self-identified effective altruists gave effectivelyacross all tasks. They exhibited none of the misconceptions that we observedamong lay donors and overwhelmingly favored the most effective option in theirchoice set (Study 3). Taken together, our studies imply that donors need to beboth informed and motivated to give effectively on a consistent basis.