All animals signal information to other animals. Biologists debate whether a signal is honest or deceptive -- does a deep, resonant voice mean he's big, and therefore could probably kick our ass, or has a non-threatening creature found a way to fake us out? (See Maynard Smith & Harper's book Animal Signals or Oren Hasson's articles on signaling.)
When we use appearances to divine the hidden essence of someone's personality, we rely heavily on the face. Sure, there are other parts that tell us about their behavior, but the face is always visible and doesn't move inconveniently when we're talking.
There's one known example of deceptive signaling involving the face -- namely, that babyfaced men tend to have more dominant personalities than maturefaced men, and tend to excel more at whatever they do. For example, high-IQ babyfaced men achieve more in school than their maturefaced counterparts, while low-IQ babyfaced men commit more crime compared to their gangmates. (See Leslie Zebrowitz's book Reading Faces, or her articles.) People expect these guys to be innocent, harmless, and docile, but they've been fooled. *
Most men don't have babyfaces because, otherwise, it would be very clear to people that an innocent-looking face didn't mean innocent behavior, and the jig would be up. But there clearly is an open niche to be filled. So, there will be a small but persistent fraction of men who fool others this way. The cost to others isn't so great because they will rarely encounter such a man, and there would be huge costs to ignoring the face when it was accurate -- that is, in most cases. So, there's no selection pressure to ignore what the face claims lies inside.
I think the same is true of the Eternal Ingenue type. Whether this is true of the exact type Clio had in mind isn't important -- that's just a matter of terminology and classification. Here is what she looks like, according to Clio:
She has a superficial physical resemblance to the Waif, in that she tends to have big eyes, small features, and a rather boyish body, thin and not voluptuous. (This is not invariable: there are curvy ingenue heartbreakers.)
How can it be that a girl with such a super-feminine face can have so masculine of a body shape, assuming she's past puberty? Based only on personal experience, I also contend that these women have masculine digit ratios (ring finger of right hand noticeably longer than the index finger). ** It shouldn't be hard to check this for the women Clio mentions, though.
To me, this suggests that she has a higher-than-average level of testosterone, which is reflected in her body overall, but which is disguised by her facial features. Why just the face? Because that's primarily where people -- the suckers -- are going to look when predicting personality from appearance, not her waist-to-hip ratio, digit ratio, boob size, or whatever else.
This high-T view also explains why these women tend to be heartbreakers, going from one man to another without feeling very bad about it -- that's a guy's way. I'm sure they rationalize it in a girly way, like they're just in love with the ideal of being in love (as Clio says). But regardless of how they spin their roving eye and wandering ways, in the end they do show fairly masculine dating behavior. It's also no accident that these women do well in highly competitive jobs like acting or dancing -- you don't climb to the top of anything without a fair amount of balls.
If this is correct, then the genes of an Eternal Ingenue have found some way to allow testosterone to affect most of her body -- including parts of her brain -- but to cut it off before it reaches the skull proper, where it would masculinize her face and let everyone know what her personality is really like. It also seems to preserve the girliness of the parts of the brain responsible for giggliness and extraversion.
I know almost nothing about molecular biology, but there could be some gene or team of genes that get testosterone where it needs to go, or that are local receptors for androgens. These genes would be expressed everywhere where testosterone typically has an effect -- but they're switched off when they're in the skull.
If altering her biology incurs some cost, in the same way that it takes more resources to partially dam a river rather than let it run totally free, then only females with lots of bodily resources to invest could afford the cost and still get by. If her genes were so great that she had to deal far less with infection, stress, random insults, and so on, that would free up resources to block a high flow of testosterone from reaching her face. So, if the deception is costly, a further prediction is that Eternal Ingenue types will be better looking than the average female. Maybe not Louise Brooks, but still.
This type of woman gets the best of both worlds: she gets to fall deeply in love with person A, who because she seems so innocent will actually slow the courtship down, so much so that she moves on to person B before having to give it up, though she may be physical with him. And so on, always having a steady supply of new flavors of her love-drug, which she hardly has to pay anything for. And of course, when she wants kids, she gets her pick, and all the while many men will continue investing resources in her. This is the female version of Genghiz Khan or Wilt Chamberlain, who get to plow through thousands of nubile women with nary a rejection to slow them down, and without having to stick around either.
When a guy sees an Eternal Ingenue, he's going to get completely suckered, just like those who interact with babyfaced men. But as with the latter, being fooled by her won't cost you so much because you so rarely run into them -- unless you go where they tend to congregate, such as among ballet dancers or actresses. Just make sure not to go with your gut the next time you see a girl with a mega-cute face and bubbly mien -- check her for boyish hips and a masculine digit ratio, as well as other tells such as a lowish voice, and doing anything competitive (including performing, since she had to beat out many others to get that spot).
It's not that there really aren't girly girls out there -- but the most captivating, inside and out, will tend to be these wolves in sheep's clothing. If you're after one for the long-term, you'd do better to look for one who has a wallflower's job and who blushes or turns shy easily, although not one who's depressive like the Waif / Neurotic. She'll be less of a rush to be around -- girls are more boring than boys -- but then she won't quickly skip away in search of another fairy tale boyfriend.
* Needless to say, this means that if a guy is babyfaced, then he's more likely than not to have that personality -- not that if a guy has that personality, then he's more likely than not to have a babyface.
** I base this on the ones I've known, including this enchantress from the teen dance club. My 19 y.o. close friend, who I initially thought was an Amazonian Alpha (in Clio's terms), I know recognize as someone whose delightfully girly face only fools men into thinking she's a non-threatening girly girl. Her masculine digit ratio and waist-to-hip ratio say otherwise. And she is most definitely a heartbreaker. (Fortunately I never fell for her, since as such a close friend I saw early how competitive and dominant her personality is -- which I don't mind in friends, but is a turn-off in girlfriends.)
For an example to check yourselves, see this video. Everyone is falling for this girl on YouTube because of her bursting-open brown eyes, delicate features, and uber-girly demeanor. (Just as in the lonelygirl15 hoax.) But she has a masculine digit ratio (pause that video at 6:10), a low voice, and (in some other video) very boyish hips. Plus she performs in front of a global audience without cracking, something a true girly girl would be too shy and nervous to get through -- let alone time after time.