The United States has been the most generous country in the world over the past decade. That’s the conclusion of the World Giving Index, a ranking that measured how likely residents of 128 countries were to practice acts of generosity. The index, from the U.K.-based nonprofit Charities Aid Foundation, is based on Gallup’s World Poll surveys of 1.3 million people. Between 2009 and 2018, interviewers asked respondents whether they had done the following in the last month: helped a stranger or someone they didn’t know who needed help, donated money to charity, or volunteered their time to an organization. At...