The disorders include problems such as obsessive or compulsive tendencies and anti-social behavior that can sometimes lead to violence.
The study also found that fewer than 25 percent of college-aged Americans with mental problems get treatment.
One expert said personality disorders may be overdiagnosed. But others said the results were not surprising since previous, less rigorous evidence has suggested mental problems are common on college campuses and elsewhere.
Experts praised the study's scope - face-to-face interviews about numerous disorders with more than 5,000 young people ages 19 to 25 - and said it spotlights a problem college administrators need to address.
Study co-author Dr. Mark Olfson of Columbia University and New York State Psychiatric Institute called the widespread lack of treatment particularly worrisome. He said it should alert not only "students and parents, but also deans and people who run college mental health services about the need to extend access to treatment."
Counting substance abuse, the study found that nearly half of young people surveyed have some sort of psychiatric condition, including students and non-students.
Personality disorders were the second most common problem behind drug or alcohol abuse as a single category. The disorders include obsessive, anti-social and paranoid behaviors that are not mere quirks but actually interfere with ordinary functioning.
The study authors noted that recent tragedies such as fatal shootings at Northern Illinois University and Virginia Tech have raised awareness about the prevalence of mental illness on college campuses.
They also suggest that this age group might be particularly vulnerable.
"For many, young adulthood is characterized by the pursuit of greater educational opportunities and employment prospects, development of personal relationships, and for some, parenthood," the authors said. These circumstances, they said, can result in stress that triggers the start or recurrence of psychiatric problems.
The study was released Monday in Archives of General Psychiatry. It was based on interviews with 5,092 young adults in 2001 and 2002.
Olfson said it took time to analyze the data, including weighting the results to extrapolate national numbers. But the authors said the results would probably hold true today.
Dr. Sharon Hirsch, a University of Chicago psychiatrist not involved in the study, praised it for raising awareness about the problem and the high numbers of affected people who don't get help.
Imagine if more than 75 percent of diabetic college students didn't get treatment, Hirsch said. "Just think about what would be happening on our college campuses."
The results highlight the need for mental health services to be housed with other medical services on college campuses, to erase the stigma and make it more likely that people will seek help, she said.
In the study, trained interviewers, but not psychiatrists, questioned participants about symptoms. They used an assessment tool similar to criteria doctors use to diagnose mental illness.
Dr. Jerald Kay, a psychiatry professor at Wright State University and chairman of the American Psychiatric Association's college mental health committee, said the assessment tool is considered valid and more rigorous than self-reports of mental illness. He was not involved in the study.
Personality disorders showed up in similar numbers among both students and non-students, including the most common one, obsessive compulsive personality disorder. About 8 percent of young adults in both groups had this illness, which can include an extreme preoccupation with details, rules, orderliness and perfectionism.
Kay said the prevalence of personality disorders was higher than he would expect and questioned whether the condition might be overdiagnosed.
All good students have a touch of "obsessional" personality that helps them work hard to achieve. But that's different from an obsessional disorder that makes people inflexible and controlling and interferes with their lives, he explained.
Obsessive compulsive personality disorder differs from the better known OCD, or obsessive-compulsive disorder, which features repetitive actions such as hand-washing to avoid germs.
OCD is thought to affect about 2 percent of the general population. The study didn't examine OCD separately but grouped it with all anxiety disorders, seen in about 12 percent of college-aged people in the survey.
The overall rate of other disorders was also pretty similar among college students and non-students.
Substance abuse, including drug addiction, alcoholism and other drinking that interferes with school or work, affected nearly one-third of those in both groups.
Slightly more college students than non-students were problem drinkers - 20 percent versus 17 percent. And slightly more non-students had drug problems - nearly 7 percent versus 5 percent.
In both groups, about 8 percent had phobias and 7 percent had depression.
Bipolar disorder was slightly more common in non-students, affecting almost 5 percent versus about 3 percent of students.