A hundred million men in the United States and Canada sought and accessed online porn in 2008. That same year, 74.8 million people — more than 90 percent of them women, of course — read a romance novel.

In a nutshell, ladies and gents, that’s why they can’t cook up Viagra for women. We of the tender gender don’t crave those hunky-funky close-ups. Our sexual brain circuitry is built to handle exquisitely delicate, elaborate physical and psychological arousal cues — emotional, social and intuitive as well as sexual — almost all essentially directed at snagging a worthy daddy. The Democratic pol who tweets a pic of his bulgy drawers? He won’t turn a lady on at all.

Males, of course, are the ones who pay for porn. According to research at Duke University, even male monkeys, such as Rhesus macaques, gladly fork out their coin of the realm — treasured juicy treats — for peeps at photographs of a bright female monkey-part.

This new dirty book makes it all too clear that Alfred Kinsey’s efforts to sort out human sexuality back in the 1940s and ’50s only scratched the surface. Interviewing a mere 18,000 horny humans? Please. Compare that with the achievement of these two neurowonks, Ogi Ogas and Sai Gaddam, authors of “A Billion Wicked Thoughts.” Drs. Ogas and Gaddam tracked more than 1 billion Web searches by seekers of sexual excitement. Their exhaustive quest trawled more than 1 million Web sites and 5 million classified ads. Their book fills any gaps in the avalanche of results with hot new scientific findings on brain and behavioral research among the sexually savvy. (Of the book’s nearly 400 pages, almost a third are notes and bibliography — the best parts, actually.)

Some of their discoveries are startling, if not the kind you’ll share with your mother-in-law at the cookout. They say, for example, that while youth is still at a premium, on one metasearch engine, men and their mouses more hungrily sought out pictures of older naked ladies than they did of lithe 19-year-olds. And — maybe your mother-in-law would like this — secretly, many like them fat. The authors discover that 504 “adult” sites feature “Big, Beautiful” women, vs. only 182 for “skinny.” Furthermore, women should never imagine that they are over the hill as objects of desire. Searches for subjects such as “my friend’s hot mom” are big, understandably — but so are actual granny porn sites. Among the 42,337 sites on the Alexa Adult List of popular Web sites worldwide, we’re told, 313 boast unambiguous names such as “Granny Rides Again.” To our two bad lads, this makes particular sense in places such as Kenya and England: In the first, among the sweetly named Kisii people, it’s grandparents who dole out the sexual wisdom; in England, Ogas and Gaddam cite “a widespread boarding school culture involving strict rules, corporal punishment, and severe, elderly matrons.”

‘A Billion Wicked Thoughts: What the World's Largest Experiment Reveals about Human Desire’ by Ogi Ogas & Sai Gaddam. Dutton. 394 pp. $26.95 (Dutton/Dutton)

Still, there remain vast, inexplicable mysteries. What are we to make of the fact that Web sites on “cheating wives” (ranking at No. 5) are more popular than “cheerleaders”? That the Japanese are most aroused by a woman’s “absolute territory,” that bare zone between her thigh-high stockings and her mini-skirt? That a lap-dancer who is ovulating receives far, far higher tips than one who is not?

After one reads “A Billion Wicked Thoughts,” several oddities linger in the reluctant mind. There is the case of a successful businessman with three children, who, a couple of times a week, drives to a strange neighborhood, parks near a house with its lights on but no car in the driveway, creeps into a stranger’s coat closet and pleasures himself, with the risk of discovery by a housewife or returning mate heightening his excitement. Well, good luck in my closet, Buster. Try not to trample the hats.

The book’s tone is slightly strained, bobbling awkwardly between blokey-jokey and teachy-preachy. That’s understandable. The authors perforce use all the very rude words the online questers do, and so feel compelled to boomerang back to an almost comical gentility. They dub the female sexual mind, with its elaborate subtexts of desire, the “Miss Marple Detective Agency,” sniffing out via every possible clue the Mr. Wonderful who will respond to her long-term. The male mind, au contraire, is Elmer Fudd, in single-minded, one-shot-only pursuit of the elusive Wabbit.

Some of the mini-graphs they present showing an individual’s search subjects are weirdly touching. A man’s typical online search might alternate “mature oral movies,” for example, with “What is the optimum humidity for a house?” (“Honey, I’ll be up to catch ‘Bones’ as soon as I’ve nailed the moisture question!”)

On the minus side, scientifically speaking, there’s really no way to tell whether an online seeker is a rapt 12-year-old simmering with surmise in the Bible Belt, or Gramps manipulating his mouse in Maine while Nana’s off somewhere flouring the flounder.

I can’t help wondering how Ogas and Gaddam must feel after their endless wallows in the cyberswamps of desire. Me? Well, I think I’ll take my shower now.

Diana McLellan ’s most recent book is “The Girls: Sappho Goes to Hollywood.”