"He was short and skinny with a big nose and he chain smoked. Ugh, I hate smoking!" recalls Ali, a TV producer in London. "But for some reason, I was completely blown away. Oh my God, he was just unbelievably sexy!"

Ali was drawn into a tempestuous affair with this foreign cameraman, though she didn't find him handsome, had little in common and didn't find him particularly likeable. Was it his smell? Did he remind her of someone else, or was she subconsciously seeking out a hot-tempered rebel? 

These are the kinds of questions psychiatrists and evolutionary psychologists have been trying to answer for years. The hypotheses are varied, but one thing is certain: If all you want to do when that sloppily dressed and somewhat obnoxious colleague walks by is jump into the nearest storeroom and get it on, you're not alone. 

Nearly all of us have found ourselves drawn to Mr. or Ms. I-don't-get-it-but-I-want-you. What fuels such desires? The answers are varied, of course, but according to Chicago psychiatrist Dr. Nada Stotland, "it could be that people remind you of someone, your dad or uncle or mom — some powerful experience you've had in the past." 

Before you cringe at the idea of being sexually drawn to someone that reminds you, even on a subconscious level, of your mom, take note: "The most powerful things happen when you're a child," explains Stotland. "Your mom may be 50-something now, but when you were a boy of 4 or 8, she was gorgeous, getting dressed up for a party or a special holiday. That snapshot remains in your mind for years." 

Perhaps this is what was operating when Jack, a graphic artist from San Diego, had a fling with a "woman that didn't fit the mold of the women I usually date." Although no oil painting, this person "had something very genuine about her," says Jack. "She peered right into my eye without any shyness or self-consciousness." Could it have been the all-knowing look only a mother could give? 

Either that, or Jack sensed this person was really strong, but was confused because she didn't "fit the traditional movie-star paradigm we think we're supposed to be attracted to," surmises Stotland. This kind of attraction happens a lot in teenage circles, she adds, when kids sense something deeper in someone, but are confused and afraid to act on it because they know the object of their desire will be labeled "uncool" at school. 

Despite his experience, Dr. Jack Porter, a clinical psychologist at West Chester University in Pennsylvania, thinks being sexually drawn to someone who doesn't fit your idea of attractive is more of a female phenomenon. 

"Women process more than men," says Porter. "Even if a woman doesn't think a man is handsome or appealing right at first — if there's any potential at all, she'll look further, she'll ask about him. She's just more curious." 

Maybe this is the kind of sexual curiosity that got the better of Gail — now a stay-at-home mom in New York — when she was in grad school a few years ago. While researching a paper with a man 15 years her senior, she recalls an intense moment of sexual tension. 

"I was so surprised," she says, "because he was not at all attractive. He certainly wasn't my ideal, or anyone's ideal. He was small and shrimpy — even mousy-looking with graying hair." 

Nonetheless, as they worked away in front of the computer in his suburban home, the tension grew. "He showed me around the house and when we came to the bedroom, I remember having weird thoughts about his bed," she recalls, a tad embarrassed. "It was right across the room and it made me really uncomfortable!" 

So much so, apparently, that she ditched him as a partner, even though "he was a perfectly nice guy." 

When asked about Gail's predicament and other mysterious instant attractions, people in online chat groups seem to understand, but offer more down-to-earth — and not surprisingly, cruder — explanations than the psychologists do. 

In a recent chat with AOL's Alone@home group, "Cloud9foru2baby" pitched in, saying "smell probably does it to guys." Then, on a more philosophical note, he summed it up: "It's called lust. But hey, it's part of what we are." 

That's one statement evolutionary psychologist Dr. David Buss can probably agree with. But as a teacher of the psychology of human mating at the University of Texas, Austin, he can offer more scientific explanations for these mysterious sexual urges. 

"First, you have to make a distinction between short-term and long-term mating," says Buss. "In the short term, there's evidence that women preferentially chose symmetrical men (as far as looks are concerned) who are more sensation seeking — the opposite of the reliable, dependable 9 to 5 type." 

Why might this be? Buss says it all comes down to the "sexy son hypothesis." The theory goes like this: Because women over the millennia have consistently been attracted to such "dangerous" testosterone-filled men, they will try to bear sons with the same sexy characteristics. The result? More "sexy sons" and in the end, more "sexy grandsons." 

"Basically, they are trying to out-replicate other women," explains Buss.