THE NATION’S pediatricians recently warned that children under two should not be allowed to watch television. All the science suggests that screen time is not good for their developing brains.

But there is new research showing that television is potentially dangerous for people of all ages. When someone watches a television drama that stretches the facts, a new study found, the misinformation gains power with time. Initially, we may be skeptical but weeks later, the false information seems to disconnect from its source in our mind. And at that point, we start believing it’s true.

The finding is important because most experimental work on the effects of media have focused on the immediate: expose a person to a message and see what happens. Sometimes, though, what we see or hear can have a more insidious effect, quietly worming its way into our mind and shaping our thinking later.

That is a bracing thought for a culture defined by its mass media. To live today in America is to swim in an ocean of information. It’s impossible to keep track of where everything comes from – was that something I heard on the news, or was it a tweet, or did I hear someone say it on a TV show last month? What we take in informs our reality, whether the sources are fiction, nonfiction, or somewhere in between. We have our critical faculties, but the research suggests that they are not as critical as we think.

“If you are consuming a bunch of junk, there is a chance that, later on, you will come to believe that junk,” said Jakob D. Jensen, an assistant professor of communication at the University of Utah who led the study.

In the 1940s, psychologists discovered the aptly named “sleeper effect.” When soldiers were shown a World War II propaganda film, their attitudes did not immediately change much. But later, the soldiers’ attitudes had shifted more. It was as if the piece of propaganda was a sleeper agent that parachuted into their mental landscape, only to be activated later – shades of the brainwashing in “The Manchurian Candidate”.

Historically, work on the sleeper effect has largely considered overt attempts to convince, like speeches or advertisements. Jensen wondered about the power of fictional entertainment, and particularly how people respond to the “facts” embedded in a television show.

To test this, he used a controversial episode of the ABC crime drama “Boston Legal.” In a 2007 show called “Nuts,” a student with a severe allergy has an attack in an elementary classroom. The teacher uses an EpiPen (a shot of epinephrine used to treat life-threatening allergic reactions) on the child, but the kid is dead in seconds.

There is much argument and counterargument – it is a legal drama, after all – but the audience is left with the distinct impression that EpiPens are unreliable. In fact, EpiPens are highly effective and dependable. The producer later apologized.

Jensen showed the episode to students who had never seen the show. He then followed up with a battery of questions, including a question about the reliability of EpiPens. He found that the students had more doubts about the devices two weeks later then they did immediately after seeing the show.

Works of fiction can do tremendous good, revealing the fundamental humanity we all share. But fiction can also be a source of misinformation, wittingly or not. In 2004, a controversy erupted when Michael Crichton published a thriller, “State of Fear,” with a climate-change denialist theme and plenty of factual stretches. Some fans argued that it was “only fiction,” and shouldn’t be taken so seriously, but that no longer looks like a defense.

It is difficult to overcome this quirk in our thinking. For most of our evolutionary history, we were confronted with what we saw or what others told us – not the overwhelming wave of vivid fiction that is the modern media. Jensen suggests that if you find yourself repeating something you heard “somewhere,” you stop and ask yourself “where.” And, as with food, you should choose your media diet carefully. You are what you eat.