OK, I've heard of this, but the idea that men go for blonde women because they look child-like and therefore "pliable" rather flies in the face of this research that supposedly shows blondes are more assertive/ambitous/aggressive (which I guess is not so daft as it may sound, as it doesn't require a genetic basis: blondes could have been conditioned by being treated differently from childhood onwards, or may have bleached their hair so as to become blonde, rather than being born that way). Interesting, too, that blondness is purportedly linked with looking young; I mean, yeah, some people's hair gets darker as they age but plenty of people stay blonde their whole lives, at least until they go grey.
But it's kind of odd having this conversation, as my last girlfriend was blonde (I mean properly, golden blonde) and also petite and very young-looking - she's the same age as me and still occasionally gets ID'd when she buys a bottle of wine, and bear in mind the legal age to buy alcohol over here is 18. Though having said that, I typically find women with dark hair more attractive...of course, that doesn't necessarily make any given brunette better-looking than any given blonde.