Everyone remembers the whitewashing scene in "The Adventures of Tom Sawyer." But how many recall the scene that precedes it? Having escaped from Aunt Polly, Tom is "playing hooky" and teaching himself to whistle when he spies a "newcomer" in his village -- a newcomer with "a citified air." Their conversation unfolds like this:

"I can lick you!"

"I'd like to see you try it."

"Well, I can do it."

"No you can't, either."

After that, the encounter deteriorates further ("Can! Can't!") until finally the two boys are wrestling in the dirt. Tom wins the battle -- the citified newcomer is made to shout "Nuff!" -- but returns home late and is thus commanded to whitewash the famous fence.

After this incident, the reader's sympathies are meant to lie with Tom. But try, if you can, to strip away the haze of nostalgia and sentiment through which we generally perceive Mark Twain's world, and imagine how a boy like Tom Sawyer would be regarded today. As far as I can tell, that fight is not just "inappropriate behavior," to use current playground terminology, but is also one of the many symptoms of "oppositional defiant disorder" (ODD), a condition that Tom manifests throughout the book.

And Tom is not merely ODD: He clearly has attention deficit hyperactivity disorder (ADHD) as well, judging by his inability to concentrate in school. "The harder Tom tried to fasten his mind on his book, the more his mind wandered," Twain writes at one point. Unable to focus ("Tom's heart ached to be free") he starts playing with a tick. This behavior is part of a regular pattern: A few days earlier in church (where he had to sit "as far away from the open window and the seductive outside summer scenes as possible"), Tom had been unable to pay attention to the sermon and played with a pinch bug instead.

In fact, Tom manifests many disturbing behaviors. He blames his half-brother for his poor decisions, demonstrating an inability to take responsibility for his actions. He provokes his peers, often using aggression. He deliberately ignores rules and demonstrates defiance toward adults. He is frequently dishonest, at one point even pretending to be dead. Worst of all, he skips school -- behavior that might, in time, lead him to be diagnosed with conduct disorder (CD), from which his friend Huck Finn clearly suffers.

I am not being entirely sarcastic here: I have reread both "Tom Sawyer" and "The Adventures of Huckleberry Finn" several times in recent years, precisely because Twain draws such fascinating portraits of children whose behavior is familiar, even if we now describe it differently. As a mother of boys, I find this weirdly reassuring: Although ADHD and ODD are often dismissed as recently "invented" disorders, they describe personality types and traits that have always existed. A certain kind of boy has always had trouble paying attention in school. A certain kind of boy has always picked fights with friends, gone smoking in the woods and floated down the river on rafts.

In previous eras, such behavior was just as problematic for adults as it is today. Poor old Aunt Polly -- how many times does she "fall to crying and wringing her hands"? To cope with Tom, she seeks names for his disorder -- he is "full of the Old Scratch," meaning the devil -- and searches for ways to control him ("Spare the rod and spile the child," she tells herself).

But if the behavior or actions of the children and the parents are familiar, the society surrounding them is not. Tom Sawyer turns out fine in the end. In 19th-century Missouri, there were still many opportunities for impulsive kids who were bored and fidgety in school: The very qualities that made him so tiresome -- curiosity, hyperactivity, recklessness -- are precisely the ones that get him the girl, win him the treasure and make him a hero. Even Huck Finn is all right at the end of his story. Although he never learns to tolerate "sivilization," he knows he can head out to "Indian territory," to the empty West, where even the loose rules of Missouri life won't have to be followed.

Nothing like that is available to children who don't fit in today. Instead of striking out into the wilderness like Huck Finn, they get sent to psychologists and prescribed medication -- if they are lucky enough to have parents who can afford that sort of thing. Every effort will rightly be made to help them pay attention, listen to the teacher, stop picking fights in the playground. Nowadays, there aren't any other options.

applebaumletters@washpost.com