Weakness Turns Her On

"That whole pathetic, no self-esteem thing you have going on right now, I'm really vibing that. That's why I sleep with fat guys."

—Denise, Scrubs

Generally in older fiction, and in more 'traditional' stories today, women prefer men who are strong, independent, and willing to make the first move. However, in many forms of modern fiction (and in Real Life), some women prefer men who are seen as 'weak'. This trope is about the latter.

A woman who finds weakness or submissive behavior inherently attractive and a turn-on. Some women (many fanfic writers, in fact), find that having the dominant position in a relationship (pun possibly intended) is a turn-on, as the 'power' is theirs and can be quite addictive, while also stimulating their mothering instincts and the Florence Nightingale Effect. A lot of women are influenced into being afraid of any man with assertiveness or physical strength by "men are bastards" type movies . Also, some women feel a need to be reassured of their attractiveness, and since a 'weak' or 'submissive' boy is more likely to worship and adore a dominant girlfriend than a strongly independent guy is, this can also stroke her ego. If the relationship manages to last over longer time, expect the man to end up as a Henpecked Husband.

This is also why the 'Uke' in a yaoi pair is so popular. Many women like the idea of a submissive partner, and are therefore more likely to Squee over the 'uke'. In some cases, this can become Wimpification when the less sane ones derail a character to make him weaker in their Fanfic. There's some crossover between this trope and Moe, in that this trope is about women for whom Moe and Adorkable men are a turn-on.

To fit this trope, the desired or fetishized male must be seen as 'weak', either in temperament or body.

Temperament: Usually this involves a naturally submissive or sensitive personality on the man's part, or, if he's naturally aggressive and strong, a sense of vulnerability around the female which makes him 'weak' for her. Usually, the male will be younger than the female as well.
Body: Bishonen may fall under this category, with their slender builds and feminine faces (though many bishonen are still badass). Ill Boys or injured men also fall under this, as do adolescents, because their weaker bodies indicate a vulnerability on their parts. Not surprisingly this overlaps with Tiny Guy, Huge Girl on occasion.

Compare All Girls Want Bad Boys, which shares similarities to this trope except that personality failings and anti-social behaviour as / rather than weaknesses are what attracts the woman to the man. See also Beastess, Hot Amazon, and Amazonian Beauty, for the type of woman who a man in this scenario is likely to be attracted to.

Examples of Weakness Turns Her On include:

Anime and Manga

There's an imaginary one (or three, or seven) in Peacemaker Kurogane. Every time one of the older guys imagines Tetsu in a relationship, it's either with him as an extreme Uke, or with an older woman who is the dom in the relationship.
Shows up in Great Teacher Onizuka.
Brutally used in Welcome to The NHK. Misaki selects Sato for her "project" of curing him from being a Hikikomori, and the time they spend together eventually leads to there being quite a bit of romantic tension between the pair. It's later revealed that Misaki chose Sato, because she wanted to find someone who was even more "worthless" than she was and prove she could fix them because of her abusive stepfather telling her that she was a useless human being. As a broken recluse, Sato fit the bill.
In the Ranma ½ manga, Akane says this to Ranma to comfort him some after she finds out he's terrified of cats and becomes a frightened, submissive individual around them. Bonus points because she's tending to him at the time.

Akane: Don't let it bother you. It's cute to have a little weak spot.
Ranma: [traumatized] Get 'em off! Get 'em off!

Despite how much he's called a "Pussy" by most of Western fandom, Shinji Ikari in Neon Genesis Evangelion actually does have fans! (and is a perfect example of this trope)

Comic Books

In a world of overmuscled macho superheroes, meek, helpless Normalman finds he is irresistible to women partially because he's so pathetic that he activates some kind of motherly instinct and partially because he's the only man on Levram who isn't an oiled up thickhead.

Film

Truth in Television in Murderball, a documentary that stars a team of paraplegics who plays wheelchair rugby in the paralympics.
In Zombieland, Witchita actually commented that Colombus is like a scared bunny, or something along the lines. But he doesn't really get the girl until he goes all Monty Python and the Holy Grail KILLER BUNNY on some zombies for her.
Frank Sinatra seemed to begin his movie career playing characters like these. Lampshaded in Take Me Out to the Ball Game.

Literature

P. G. Wodehouse liked this trope. In one book it is a strong trait of a girl one character likes, that it is part of her ambitions to be a good influence on the man she loves (a sort of Love Martyr syndrome), and he wins her over when she mistakenly believes him to be an alcoholic, so she stays by his side to save him from himself. (Which seems to be working, considering that the guy was not actually a heavy drinker.)
- The phenomenon is further explained in Uncle Dynamite by the titular Uncle Fred whose nephew Pongo apparently elicits this reaction from girls (while not a strong or very heroic guy himself). (When Pongo's polar opposite Bill Oakshot, a large sturdy guy falls in love with the same girl he suggests that he'd be unable to use the same strategy and has to be the opposite, the "dominant male" - which eventually works for Bill.)
Miles Vorkosigan once lampshaded this trope himself. Being under five feet tall and, even after getting his bones replaced, delicate enough for most grown woman to readily overpower, he noted that he was singularly unthreatening and "it made them bold."
Glenda from Unseen Academicals seems to exist as an outlet for the audience's desire to comfort Nutt.
- OTOH, Nutt is a Badass who only looks weak. He's very troubled, but still.

Live-Action TV

Criminal Minds' Dr. Spencer Reid is practically the perfect poster boy for this trope—he's considered to be The Woobie, Distressed Dude, Uke, Bishonen, Adorkable, Moe etc—all in one deliciously sweet package.
While Buffy the Vampire Slayer tends to go for men who are much stronger than average, Dawn reveals in one episode, regarding a recently-injured Riley: "Oh, she just said you look even cuter when you're all weak and kitten-y, and she'd better go solo, or you'd get hurt."

Video Games

A good deal of the popularity of the Garrus and Thane romances in Mass Effect 2 compared to Jacob is the vulnerability behind the former two's romance scenes. The player is able to comfort both Garrus and Thane who are, with Thane even breaking down crying.
- However, while these two show a big soft side in their romance stories, neither of them is weak or submissive. the popularity is less about them being weak or submissive, and more about them being troubled, complex and, well, human(oid)
  - Which is somewhat ironic, considering they're both aliens, while Jacob is an actual human. A human burdened by an incredibly dull backstory.

Web Comics

In Something*Positive, Kim has a thing for guys who aren't completely conscious, even going so far as to chloroform a few. When she's left alone to look after Davan, who's zonked out on pain meds, well... you can guess what happens. It's never treated as okay, but it still comes up for humor, years later.

Web Original

Bones, the female bodybuilder in Cool Cat Studios, ends up dating the small, slender, and shy Michael.
Mercilessly lampooned here by Kate Beaton (along with Girls Need Role Models and Real Women Never Wear Dresses).
Subverted with The Nostalgia Chick. She has a thing for weak men, but what she loves about them is making them weaker.
A bit like Spencer Reid in Criminal Minds, the pitiful, subby, constantly puppy-eyed Nostalgia Critic induces this both in-universe and out.

Western Animation

This happens in Winx Club with those people who live underground.