So, you’ve decided to volunteer at your local soup kitchen. By all accounts, this should be a win-win. According to scores of studies, the mere act of volunteering—whether you’re feeding the hungry, reading to old people or cleaning beaches—is the best sort of medicine. Volunteering has been shown to lower stress levels, reduce feelings of loneliness and make you feel better about the world and your place in it. And that’s just how it helps you. There are also the people you’re helping, the receivers of both your soup and your largesse.

Unfortunately, there are times when volunteering isn’t purely positive. Voluntourism, which allows you to help out while on vacation, is perhaps the most problematic example. A typical project will see well-meaning voluntourists swoop down ...