Once upon a time, back in the dark days of nerdiness, it was not possible to buy a mass-produced T-shirt emblazoned with the directive, “Talk nerdy to me.” Back then, paperbacks that hit the market under the banner “The Nerd Series” would have been a tough sell even among romance-novel readers, a deeply nerdy segment of the population.

But these are good times for the nerd, at least for the grown-up variety. The T-shirt is on Amazon.com, and the nerd romances have hit the best-seller lists. Now along comes Benjamin Nugent’s “American Nerd: The Story of My People” (Scribner, 224 pages, $20), riding the wave of nerd chic — a term that not so long ago would have been an absurd oxymoron.

Thinking deeply about nerdiness, examining it in book-length detail, is a severely geekish thing to do, akin to analyzing a joke. Mr. Nugent is aware of this and, luckily, his sense of humor is fully intact. A former dweeb who once spent childhood lunch hours playing Dungeons & Dragons with other nerd boys, he has since emerged into the bright sunlight of coolness.

“I’m not writing a defense of nerds or a celebration of nerds or a polemic against the nerd stereotype,” he explains at the book’s outset. “There is a rationale, I think, for despising the young me. I empathize with nerds and antinerds alike.” And yet, as its subtitle suggests, “American Nerd” is Mr. Nugent’s way of reclaiming his tribe.

Nerds, as he defines them, come in two types. The first, “disproportionately male, is intellectual in ways that strike people as machinelike, and socially awkward in ways that strike people as machinelike.” Bill Gates, for example, belongs to this group, whose members, Mr. Nugent argues provocatively, exhibit some of the symptoms of Asperger’s syndrome. The other type “is a nerd by sheer force of social exclusion”; think Lisa Loopner and her boyfriend, Todd, the awkward outcasts Gilda Radner and Bill Murray brought into the mainstream on “Saturday Night Live.”

Mr. Nugent’s definition may be flawed: Marginalization is inherent to the genus, so doesn’t that second group encompass the full spectrum of nerds? Isn’t the machinelike nerd merely a significant subset? In any case, it is this kind of nerd — the kind he once was — to whom Mr. Nugent pays the majority of his attention.

The portrait he draws of his young self is a boy, physically graceless and socially ill at ease, who spends a lot of his time inside his own head, living a medieval fantasy life. Instead of playing sports, he and his friends put in more than the normal amount of time playing video games.

Like Mary Bennet, the bookish, priggish younger sister in Jane Austen’s “Pride and Prejudice,” and Gussie Fink-Nottle, Bertie Wooster’s newt-obsessed chum in P.G. Wodehouse’s Jeeves stories, these boys fit neatly into Mr. Nugent’s machine category. Their passions don’t “revolve around emotional confrontation, physical confrontation, sex, food, or beauty”; their speech is oddly formal; they favor “logic and rational communication”; they are rule-bound, code-bound life forms. Another literary ancestor of the modern-day nerd is Mary Shelley’s Victor Frankenstein, a brilliant man who creates a human being and then flees from it because, Mr. Nugent writes, he lacks empathy.

In tracing this social history, Mr. Nugent does a fine job of explicating how American culture came to hold thought and feeling as incompatible, unequal activities, and how the repercussions of that belief can be harmful to the brainy nerd. In fact, one of the things the book does best, which is perhaps the most helpful thing for people reading it in an attempt to understand nerds (parents may be among them; Mr. Nugent makes more than one reference to parental alarm at a child’s social ineptitude), is to acknowledge the emotional life of the machinelike nerd:

The pathos of being a nerd is to feel that because you are comfortable with rational thought, you are cut off from the experience of spontaneous feelings, of romance, of nonrational connection to other people. A nerd is so often self-loathing because he accepts the thinking/feeling rift, and he knows and cares that other people accept it, too. To be a nerd is often to live with a nagging feeling of one’s own incurable heartlessness.

Is it heartlessness then, a nerdy failure of empathy, that keeps Mr. Nugent from embracing nerds much beyond the Dungeons & Dragons, sci-fi, gamer crowd most familiar to him?

It’s peculiar that he prominently cites nerd characters (Lisa Loop ner and Todd, Mary Bennet, Dr. Frankenstein) created by women and yet is capable of blithely stating that “nerds, as everybody knows, tend to be male more often than female.” Anyone who grew up female would beg to differ.

It’s strange that, with his understanding of the importance of role-playing to nerds, drama nerds don’t rate a mention. Neither, for that matter, do band nerds. Librarians are nowhere to be seen; copy editors are MIA.

And while Mr. Nugent has obviously noticed that children will shun other children whose parents have less money than theirs, a fact often betrayed by pitiable wardrobes, he doesn’t make the connection between relative poverty and nerddom beyond his own early friendships.

Similarly, though he vividly remembers childhood bullies calling him gay, a taunt aimed at nearly every nerd child, he never goes anywhere near touching the gay-nerd connection. He does explore why nerd stereotypes are disproportionately equated with Asian culture, as with Jewish culture before it, but mainly this book is about straight white guys. Which is fine. Straight white guys are American nerds, too, and they, after all, are Benjamin Nugent’s people.