Jennifer Medina of The New York Times writes about a growing trend at American universities:

The debate has left many academics fuming, saying that professors should be trusted to use common sense and that being provocative is part of their mandate. Trigger warnings, they say, suggest a certain fragility of mind that higher learning is meant to challenge, not embrace. The warnings have been widely debated in intellectual circles and largely criticized in opinion magazines, newspaper editorials and academic email lists.