When people give to charity, they rarely give to the charities that do the most good per dollar. Why is this? One possibility is that they do not know how to give effectively. Another possibility is that they are not motivated to give effectively. Across six tasks (Studies 1a, 1b), we find that there is truth to both explanations. People have multiple misconceptions that affect the effectiveness of their giving, including about disaster relief, overhead costs, donation splitting, and the relative effectiveness of local and foreign charities. Similarly, they are unfamiliar with the most effective charities (Studies 2a, 2b). Debunking these misconceptions and informing people about the most effective charities makes them donate more effectively. However, to a certain extent people continue to give ineffectively even when informed how to give effectively. This is because they have preferences for ineffective charities: they want to donate to a charity they feel emotionally connected to even when they know that it is less effective. By contrast, members of the effective altruism movement, who are committed to effective giving, give effectively across all tasks. They neither have misconceptions nor preferences for ineffective charities (Study 3). Taken together, our studies show that only when people are both correctly informed and motivated to donate effectively will they consistently give to effective charities.