No one would ever expect the general who led the Union army to victory in the Civil War to have a debilitating fear of blood.

But Ulysses S. Grant was among the 49 percent of former U.S. presidents afflicted by mental illness, according to an article published recently by psychiatrists at the Duke University Medical Center.

Jonathan Davidson, professor of psychiatry and director of the Anxiety and Traumatic Stress Program, has a particular interest in history, especially U.S. presidents.

After culling data from presidential biographies, Davidson was joined by Kathryn Connor, associate professor of psychiatry, and Marvin Swartz, professor and head of the social and community division of psychiatry, to analyze the information. Together, they diagnosed the commander-in-chiefs from 1776 to 1974.

According to the study, published in January in the Journal of Nervous and Mental Disease, of the 37 presidents researched, 18 were found to suffer a mental illness of some form. Depression was the most prevalent disorder among presidents, occurring at a rate of 24 percent.

The researchers wrote that the 49-percent rate mirrored national mental illness statistics, but the rate of depression was high for a male population.

"A fairly high number of people have mental disease at some level, so it would be surprising if presidents didn't," said John Aldrich, professor of political science. "Certain things, like depression, are associated with artistic accomplishment."

Other diagnoses included anxiety, alcohol abuse, bipolar disorder and social phobia. Howard Taft apparently suffered from sleep apnea.

At least 10 presidents were affected by episodes while in office, and the study found evidence that symptoms interfered with their performance in almost all cases.

To make their diagnoses, the researchers used the criteria of the DSM-IV, the Diagnostic Statistical Manual all psychiatrists use to treat patients. They examined the data to identify symptoms, determine if they were persistent and caused dysfunction and then establish their own levels of confidence that mental illness existed.

Such remote diagnosis through secondary research, however, can be problematic.

"Using biographical materials may be an imperfect way to gauge mental illness," Aldrich said.

Swartz explained that detailed analysis of primary sources, while ideal, was outside of the scope of the study but that the published article elaborated on its own relevance and weaknesses.

"You have to rely on what historians reported based on their research," he said. Still, Swartz estimated that their sources erred on the side of undercounting illness among presidents.

The troubles of certain presidents are already very well known. Abraham Lincoln famously suffered from symptoms of depression, though he triumphed politically more than Franklin Pierce, whose more modest legacy the study attributed greatly to his illness.

Having witnessed the violent death of his son in a railway accident just before he assumed office, Pierce suffered from symptoms indicating depression or post-traumatic stress during his term. The study noted that his associates accused Pierce of being a different person than the one who had energetically campaigned for office.

While personal tragedy and the weight of the presidency may have incited the problems of some presidents, others were apparently afflicted long before they moved into the White House.

According to the article, contemporaries of Grant, James Madison, Rutherford Hayes and Woodrow Wilson who watched them as young men would have thought that these men would do very little with their lives based on their seeming mental problems or deficiencies.

Whether they were suffering from an illness before they entered the White House or not, presidents' afflictions raise questions about their ability to do the executive job.

"The extensiveness of Richard Nixon's alcohol abuse was pretty remarkable and alarming, given the authority he had," Swartz said.

Though Calvin Coolidge's hypochondria may not have had the most profound effect on affairs of state, Coolidge, Grant and Thomas Jefferson were diagnosed with social phobia by Davidson and his associates.

"Social phobia is kind of remarkable in a president. It meant he was shy and avoided social circumstances, and yet he was president," Swartz said.

The study noted among its implications that no national calamities seem to have been a result of presidential mental illness.

It also considered the possibility that knowledge of these afflictions might lessen the stigma of psychological treatment. But there remains a question about the public's right, and need, to know the psychological state of the president, in an age of increased psychological vigilance.

"It's obviously about as stressful and physically demanding a job as there is for mature adults, so it has to at least exacerbate any [already existing] problems," Aldrich said. "You know, the president is not a person, he's an institution.... There are a lot of checks and redundancies to make sure he doesn't do anything foolish."