Pressure is mounting on Biden to take a cognitive test. It could be a 'slippery slope' for future presidents, experts say.

• Joe Biden is facing pressure to take a cognitive test after debate stumbles and verbal errors.

• Biden said he won't take one, adding in an interview: "I have a cognitive test every single day."

• Experts cautioned that a cognitive test could set a dangerous precedent for future campaigns.

President Joe Biden is facing mounting pressure to prove he's medically fit to run for a second term, but experts say a cognitive test could set a dangerous precedent.

Biden stumbled through his first presidential debate of the year against former President Donald Trump on June 27.

The 81-year-old made verbal errors and appeared to lose his train of thought on several occasions.

During an interview with ABC's George Stephanopoulos that aired Friday, Biden dismissed his mumbling and incoherent sentences as a "bad episode" and "no indication of any serious condition."

His campaign team also offered various explanations for his performance, including jet lag, poor preparation, and a cold.

That isn't convincing voters, however. In a poll by Reuters and Ipsos after the debate, one in three Democrats said Biden should quit the presidential race. Others have said he has to prove his fitness.

Fit for duty?

Speaking on CBS's "Face the Nation" on Sunday, Sen. Lindsey Graham said all presidential nominees, including Biden and Trump, should take a cognitive test.

"Here's what I worry about, that our allies see a compromised Joe Biden, that our enemies see a compromised Joe Biden, and I'm offended by the idea that he shouldn't take a competency test given all the evidence in front of us," he said.

Three former colleagues of Biden's personal physician, Kevin O'Connor, told The Washington Post that the president should undergo cognitive screening after his debate performance.

Cognitive tests, which are usually quick and simple, can check for functions such as concentration, abstract thinking, language, and memory.

"The test is fairly easy to do for someone with a high level of education and no or even mild impairment," Dr. Kenneth Boockvar, the director of the Division of Gerontology, Geriatrics and Palliative Care at the University of Alabama at Birmingham, told The Wall Street Journal.

"It's only a screening test. It's not really diagnostic," he added.

So far, Biden hasn't said he would take a cognitive assessment, telling Stephanopoulos: "I have a cognitive test every single day."

"Not only am I campaigning," Biden added, "I'm running the world."

Karine Jean-Pierre, the White House press secretary, said that after Biden's physical in February, the president's doctors decided he didn't need a cognitive test.

A slippery slope

Even as doctors debate the effectiveness of the tests, political experts are concerned that it sets a bad precedent.

"One could reasonably worry about slippery slopes," Thomas Gift, an associate professor of political science at University College London, told Business Insider.

"Today, it's basic cognitive tests. Tomorrow, it's IQ tests. The next thing you know, candidates are battling each other over who is a member of Mensa," he said.

Gift said it's a "legitimate question" to ask Biden to take a test, but it shouldn't be mandatory.

Andrew Payne, an an author and lecturer of foreign policy at City University, London, agrees. He told BI that Biden was in "desperate need of evidence that he is up to the task," but that evidence shouldn't necessarily be of a medical nature.

"Does it do any good to introduce the personal health of each candidate as another axis of competition in a presidential election? I'm not so sure, and I think that is exactly what the introduction of a mandatory test could do," he added.

One issue is that there isn't an accepted standard for doctors using the test results to decide whether someone is fit to lead a country.

"You have two challenges: One is to take these kinds of tests and say something meaningful with them," Dr. Charan Ranganath, director of the Dynamic Memory Lab at the University of California at Davis told the Journal.

"But the other challenge is what does it take to do the job? What do we expect our people to do?" he said.

For now, Biden is resisting pressure to take any type of cognitive exam or entertain the idea that they should be mandatory. While that may not prove damaging for his political career, it may be a wise move in the long term, Payne said.

He added: "Democracy would be better served by candidates debating their policy positions rather than their test scores and golf swings."

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