Thanks to MacFarquhar’s curiosity and insight, and her embrace of complexity and ambiguity in storytelling, these portraits don’t read at all like a secular version of “Lives of the Saints.” Martyrdom doesn’t seem to be the point, not even for the man who donates his kidney to a complete stranger. Without exception, MacFarquhar’s do-gooders are as messed up and conflicted as the rest of us, if not more so. They long for connectedness and a sense of purpose. Most come from comfortable backgrounds, though Sue Hoag and Hector Badeau are an exception to this rule. They’re a working-class couple (Badeau is one of 16 children of a Quebec family) who realize after having two kids and adopting two more that they have a gift for raising children. In the same way other people take to the violin or gymnastics, parenting is the one thing they can do exceptionally well.

In MacFarquhar’s telling, Hoag and Badeau’s embrace of this vocation is an all-­consuming passion. They don’t like changing one dirty diaper or cooking one family meal after another; but they enjoy the satisfaction they feel when they’ve done those things well. Each new adoption offers a rush of adrenaline and a chance to fall in love all over again.

“It was like the feeling you had when you learned you were pregnant,” ­MacFarquhar writes, describing Hoag’s emotions as she prepares to add more children to her family. “You didn’t know the child, you couldn’t see or feel the child, the child barely existed; and yet you loved him.”

“Strangers Drowning” is filled with such moments. The reader may feel uplifted or disturbed by the things these altruists do and feel. MacFarquhar, however, offers their stories to the reader without commentary or judgment.

Often, this reader wished she had called out her subjects for their general weirdness. There is, for instance, the exasperating “Aaron Pitkin” (MacFarquhar doesn’t give us his real name), who tells his girlfriend that he can’t wash the dishes because it will detract from his crusade to reduce the pain suffered by poultry and other animals.

And there are the emotionally tone-deaf followers of the “effective altruism” movement, whose generosity often takes the form of bloodless, spreadsheet-style calculations. One gives away 60,000 British pounds and tells an interviewer that the polio vaccines his money paid for saved 180,000 lives: “Superman couldn’t even hope to do that,” he says. Another brags of saving “300 centuries of life” with his donations.

All of MacFarquhar’s altruists are ambitious. And thus all have their inevitable moment of hubris. Sue Hoag and Hector Badeau adopt one group of teenagers too many. The Methodist minister alienates her flock. Like many people who think they can change the world, there’s often a touch of ruthlessness to their behavior.