And at 6, it seems, kids are still just as innocent, and just as sincere. That was the gaping flaw in the “Balloon Boy’’ hoax, Richard and Mayumi Heene’s alleged plot to win a reality TV deal by telling the world that their son was afloat in a Mylar balloon. A vast conspiracy - involving TV crews, Colorado law enforcement, crocodile tears, and untold amounts of public safety dollars - rested entirely on the honesty of a child.

Obvious caveat: I only know Falcon Heene, labeled now and perhaps forever as “Balloon Boy,’’ from the video footage floating around the Internet. There are the clips from his family’s turn on ABC’s “Wife Swap,’’ in which we learn that Falcon has learned and feels free to repeat certain swear words. There is the rap video, starring Falcon and his two older brothers, prepped to act as gangsta as three suburban kids can be.

Then there are the moments when he looks much more like an ordinary, stressed-out 6-year-old. There was his interview with CNN’s Wolf Blitzer, in which he let slip the damning statement that “we did this for the show.’’ Then came his more heart-wrenching turns on the morning shows. When Diane Sawyer questioned him about “the show’’ on “Good Morning America,’’ Falcon declared he had to vomit - and could be heard gagging offscreen. When Meredith Vieira pressed him on “Today,’’ his parents actually had a bowl ready.

That lying could make a child physically ill comes as no surprise to my friend Dr. Atilla Ceranoglu, a child psychiatrist at Massachusetts General Hospital. He knows no more about the Heenes than I do, but he does know how kids think. And he says secret-keeping is developmentally tough for kids under 11. The knowledge that they aren’t allowed to say something will be foremost in their minds. Their brains’ prefrontal cortex, which governs social filtering, isn’t fully developed. Even a benign secret is destined to slip out. Kids that young see morality in unambiguous terms, Ceranoglu says. They find it that much harder to tolerate a lie when they know what they’re hiding is wrong.

It’s stunning to think that the Heenes, for all of their experience onscreen, didn’t anticipate that reporters would ask detailed questions about their son’s behavior - wondering, for instance, why Falcon was hiding in a box when his parents called his name. Once the news anchors started raising doubts, those interviews must have felt like the ultimate visit to the principal’s office, albeit a principal with Sawyer’s sweetly husky voice.

It must have been a rude awakening for a kid who had been trained to think that cameras are always friendly. But then this was the first time the cameras had turned on the Heene parents, as well - not just feeding their sense of self-aggrandizement but exposing it to the world. Sooner or later, the tables turn on a lot of reality TV stars. But the prospect of fame is so intoxicating that nobody thinks to plan.

As parents, we all have had our mini-Heene moments, acting as our own paparazzi crews, finding cute kiddie shots to post on Facebook or YouTube. And there is some continuum - a long one, to be sure - between posting a YouTube video of your kid burping and inviting professional camera crews to document your children’s temper tantrums. Or claiming, as erstwhile TLC star Kate Gosselin has, that your children are upset when the camera crews leave.

Kids like to see themselves onscreen; my daughter can spend hours watching videos of her baby days. But Ceranoglu says they don’t understand that strangers might judge what they see, and not favorably. Eventually, though, they’ll grow up, which means it’s worth thinking twice about posting those cute-but-embarrassing baby videos on YouTube. And it means that someday, Falcon Heene will probably look back on his 6-year-old self getting sick on national TV and question whether the fame his parents sought was so rewarding after all.

Joanna Weiss can be reached at weiss@globe.com.