University of Texas engineering student Charles Whitman had no problem purchasing guns and ammunition, even though several psychiatrists had diagnosed him as mentally ill. He would use these weapons to kill 14 people and wound 32 others in an attack from the university’s tower in 1966.

Fast forward to another mass murder committed by a student, this time at Virginia Polytechnic Institute in 2007. Although he had been diagnosed with such severe mental problems that a court had ordered him to undergo psychiatric treatment, Seung-Hui Cho had no difficulty buying semi-automatic guns. Comparing himself to Jesus, he took these weapons to campus, where he killed 32 people and wounded 17 others before committing suicide.

Although the Aurora, Colorado, massacre in 2012 may have taken place in a movie theater off-campus, it is probably no coincidence that the gunman, James Holmes, was a 24-year-old graduate student at the University of Colorado. Even though he had admitted fantasies about mass murder to a school psychiatrist, he was able to buy semi-automatic rifles legally at sporting goods stores and purchase 6,000 rounds of ammunition online. Holmes’ attorneys acknowledged that he was “in the throes of a psychotic episode” during the bloodbath.

Less spectacular deadly shootings are increasingly common on American campuses. And we have all encountered our share of scary students. But perhaps we should not be surprised. The age of the college population, the campus environment and lifestyle, and the easy access to guns all come together to create the perfect storm.

The age of onset of some severe psychiatric conditions is adolescence and early adulthood — the years typically spent at college. This is confirmed by the latest edition of The American Psychiatric Association’s “Diagnostic and Statistical Manual of Mental Disorders” (the DSM-5), the authoritative source for psychiatric diagnoses in the U.S. According to the DSM-5, especially for males, the psychotic features of schizophrenia start emerging in the late teens. The peak age of the onset of initial full-blown psychotic episodes, which may include hallucinations, delusions, incoherence and chaotic behavior, is the early to mid-twenties.

Bipolar disorder also typically emerges between the mid-teens and mid-twenties. Not the ideal time for those who are most vulnerable to leave the support of family, friends and community to go to college.

Drugs, sex and rock ‘n’ roll don’t help much either. University life may be especially conducive to mental illness since heavy drinking and partying, along with drugs, promiscuity and sleep deprivation, are common.

The turbulent behavior of some college students, which may be symptomatic of severe psychiatric disorders, is accepted — if not expected and encouraged — in permissive academic culture. In the “real world” and in the workplace, this conduct would be more readily recognized for what it is. And it would never be tolerated.

Pressure to do well academically and to fit in socially may also “trigger” mental illness for the vulnerable. And “trigger” is surely the appropriate word, given Americans’ easy access to guns. Supported by the Second Amendment to the United States Constitution, which protects the right of individuals “to keep and bear arms,” our gun culture contributes to the body count. Countries with restrictive access to guns experience fewer incidents of campus violence.

Of course, colleges and universities are not the only American settings conducive to mass shootings. So many post office workers have killed their managers and fellow employees that “going postal” has become slang for flipping out. Perhaps we should coin another phrase for American campuses: “going academic.”

Howard P. Segal is professor of history at the University of Maine. Deborah D. Rogers is professor of English at the University of Maine. This OpEd first appeared in London’s The Times Higher Education on April 24.