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As the nation marks the one-year anniversary of the Parkland mass shooting, hundreds of psychologists and other experts in child development have signed an open letter calling for major policy action on gun violence. The group, which includes child psychologist and bestselling author Alison Gopnik, said it was sounding the alarm about the negative effects that the “constant threat of violence is having on the children of our nation.” They cited connections between gun violence exposure and long-term stress and issues like depression, anxiety, and post-traumatic stress disorder.

Celeste Kidd, one of the University of California-Berkeley psychology professors who wrote the letter, notes that the wider impact of gun violence on children is lesser known outside the psychology community.

“I hope this is the first step in a larger discussion about ways in which we might be able to help advocate for better protections for our kids,” Kidd says.

A recent Washington Post report estimated that more than 221,000 students have been exposed to gun violence at school since the Columbine massacre in 1999.

The full text of the letter follows below; also see Mother Jones‘ recent video report showing how lockdown drills are impacting American schoolchildren, in their own words.