Men everywhere have probably wondered for thousands of years: What turns women on?

In the age of the Internet, it's possible to find out. And with countless genres of images, videos and erotic stories available online, women are both able and empowered to access arousing material, and figure out for themselves what they like. Some are watching porn, as our sex columnist Ian Kerner pointed out in a recent column.

But what women are viewing and reading is usually not what men are searching for, according to a new book on the subject. In "A Billion Wicked Thoughts," released Thursday, neuroscientists Ogi Ogas and Sai Gaddam combine web searches, personal search histories, websites, and classified ads with insights from brain science to discover precisely how different women are from men.

Just like we're all born with taste cues - sweet, salty, savory, spicy, bitter - men and women's brains are wired with sexual cues, Ogas said. For men, the cues are predominantly visual, and aimed at the partner; they often enjoy seeing women orgasm, which may be one of the reasons why so many women fake. But women are more complex; they place a high importance on feeling desired, for example, whereas it appears men generally don't need to feel desired at all in order to feel aroused.

"A woman wants to know that there’s going to be repeat action, that he’s committed and is going to be coming back," Ogas said.

Women are sexually complex in other important ways. If a man is physically turned on, he's also psychologically turned on, which is why medications for erectile dysfunction (i.e. Viagra) can deliver fairly straightforward results. But a woman can be physically turned on and mentally turned off at the very same time, making efforts toward a treatment for low female sexual desire all the more complicated. And as to what makes a woman go beyond mere arousal and have sex, you'll have to read this other article.

The female sexual brain is also like a “detective agency” that investigates a man’s many qualities before deciding whether he’s worth her attention, the authors said. Evolutionarily, that makes sense. In the earliest days of humans, females who mated with the first males they encountered would not have fared as well as those who took the time to investigate their partners a little more. The "detective agency" would make sure that the chosen male would not be cruel, unfaithful or sneaky, would protect the woman and her child.

More important than evolution, though, is the "software" of the sexual brain, Ogas and Gaddam said.

Again, men are simple: The male brain is designed so that any stimulus can trigger arousal. For some, a single cue is necessary and sufficient, which is what makes a fetish, well, a fetish. For instance, some men get turned on in the presence of attractive shoes or feet, and need to see that in order to feel stimulated. On the other hand, fetishes are extremely rare among women. The female brain usually doesn't respond to a single trigger every time; there can be lots of different combinations of things that can get them in the mood.

"The male sexual brain is like a single toggle switch, whereas the female sexual brain is like the cockpit of an F1 fighter jet," Gaddam said. "There are tons of dials and instruments, and there's sophisticated calibration going on."

To fully appreciate this, feast your eyes on these two real search histories from America Online users:

MAN: college cheerleaders; cheerleaders in Hawaii; pics of bikinis and girls; pretty girls in bikinis; girls suntanning in bikinis; college cheerleader pics in bikinis; noooooooo; christian advice on lust WOMAN: orlando bloom as vampire fanfiction; 321 sex chat; kingdom of heaven fanfiction; cinderella wedding dresses; gossip on orlando bloom; legolas erotica; legolas heterosexual erotica; evil orlando bloom dark fanfiction

As you can see, this woman tends to prefer stories ("fanfiction," or narratives based on existing movies/TV/literature) much more than the man, who searches for specific images. The majority of these stories are romantic and lightly erotic, but plenty of women like graphic erotic stories, too. In the search above you can see this one gal is particularly interested in Legolas from "Lord of the Rings" and the actor who portrays him: Orlando Bloom. And there's a lot of conversation that goes on among women about erotic stories about the inner feelings of the characters, whereas men consume porn alone and don't talk about the aesthetics, Gaddam said.

"Male erotica is a solitary enterprise, and female erotica is a social enterprise," Ogas said.

A minority of women do watch visual porn. Based on analyses of user profiles on a porn website, these ladies tend to be more socially aggressive, comfortable taking risks, and open to bisexual experiences. They also tend to have a higher sex drive.

And while many men do seek out porn involving young women, there's a substantial interest in seeing scantily clad (or not clad at all) older women too. Significant numbers of web searches and websites out there are devoted to women in their 40s, 50s, 60s. There's even a genre called "granny porn," with a consistent following, in which the male performer's age is highly variable. It's hard to know how old the men are who are seeking this class of erotica, but the authors speculate they come from a wide range of ages, since men's sexual interests are pretty solid by age 25. Women, on the other hand, have much more flexible and dynamic interests during a lifetime.

What about homosexuality? The authors compared gay and straight male sexuality, and found they were strikingly similar. The differences: gay men like men, and are more likely to be aroused by the submissive role in sex than straight men. Lesbians are far more complicated, and Ogas and Gaddam don't think they have enough information to make any definitive comparisons about it.

The authors don't take any moral positions on any of this, but they do point out that individual tastes and preferences are difficult or impossible to modify. Yet everyone assumes their own interests are the norm, and we quickly label anything else as weird or even dangerous, Gaddam said.

"We should all be more sexually tolerant," Ogas said.