First, I'm surprised at how many opinions have been listed under this subject without benefit (seemingly) of having read the book.
I have my own theories of beauty just like the next humanoid
I HAVE read the book...it is quite good IMO and worth the read.
Here is a pet theory which was not quite in the book which applies MOSTLY to males...females might help us out here for their "selection" program.
Problem...how do you keep a proven genetic sequence success from "spinning off" entropically while generating novelty thru sexual reproduction?
Solution...you preprogram a sensory "template" conveying "beauty" which forcefully directs mating selection to the Median of the genome. This would argu that beauty is "average", and there is experimental evidence for this.
You then design a program which prunes the tree of all faces etc which are lower than the optimum median face in the currently senseable population (ie a man walks into a bar...he instantaneously scans the room...his program heat seeks the best two or three genomes as conveyed by "faces" and "bodies" etc...then a more refined weighting occurs to select from the top two or three...resulting in one candidate remaining...this candidate is imprinted as an image in the mind/short term image memory...then, the image returns on a periodic basis as a driver to attempt to encounter the genome and join with it sexually (ie saunter over to the bar and say "hey - don't I know you"?)...
During actual intercourse these images are returned to foreground memory if
the current partner is more than one standard deviation from the sweet spot
of the gene body as represented by some distance from the Median face
(brooks shields for caucasions?) in order to motivate the sexual apparatus
to perform with genetically inferior partners as predicted by the "face" etc. Humans may face each other to have sex because the gene sequence is drastically more complex/critical in order to maintain the programming for a vastly more complex organism.
the template program adjusts to locally available genome expression (please don't tell me about exceptions...i know there's a ton of them)
the opposite sex exploits knowledge of the program to improve mating probability (makeup, padded bras)
result of program...extraordinary attention focus on the median/most robust genetic expression of an allele/gene sequence representing a species