Ronald Reagan, whose alleged lack of mental agility was the basis for many jokes, especially outside the United States, was suffering from the early effects of Alzheimer's in his first term as President, according to his son.
In My Father at 100: A Memoir, Ron Reagan cites two examples. He recalls watching his father, then 73, debate with 1984 Democratic presidential nominee Walter Mondale.
"I began to experience the nausea of a bad dream coming true," he writes. "Some voters were beginning to imagine grandpa - who can never find his reading glasses - in charge of a bristling nuclear arsenal, and it was making them nervous.
"Worse, my father now seemed to be giving them legitimate reason for concern. My heart sank as he floundered his way through his responses, fumbling with his notes, uncharacteristically lost for words. He looked tired and bewildered."
Ron Reagan, the youngest of the former President's four children, says his father may have suspected the onset of Alzheimer's in 1986 when he was flying over familiar canyons north of Los Angeles and became alarmed he could not remember their names.
Reagan was diagnosed with the disease in 1994, five years after leaving office, at which point the jokes about his mental powers suddenly ceased to be funny.
The popular Republican President died in 2004 at 93 from complications of the disease.
Ron Reagan believes his father would have left office before his second term ended in 1989 had the disease been diagnosed then.
"I've seen no evidence that my father (or anyone else) was aware of his medical condition while he was in office."
But he said the issue of his father's health should not tarnish his legacy as the 40th President of the United States.
"Does this delegitimise his presidency? Only to the extent that President Kennedy's Addison's disease or Lincoln's clinical depression undermine theirs.
"Better, it seems to me, to judge our presidents by what they actually accomplish than what hidden factors may be weighing on them. That likely condition, though, serves as a reminder that when we elect presidents, we elect human beings with all their foibles and weaknesses, psychological and physiological."