This is pilfered from a message that ZirconiaWolf posted in the thread about "Necessary Hacks" in the Discussions and Help section:

All I can gather from the Prima guide is that attraction is based on some (apparently very) complicated combinations of turn ons/offs, personality traits, aspiration type & of corse (the Maxis interpitaion of) their zodiac signs.

This- obviously- would "count" for more than 3 Bolts (which isn't even possible in the game) if the "counting method" made any sort of logical sense, so Lord only knows quite how Maxis programed the "attraction factor" in the game! (I still swear some of it's random! )

Also- aside from the "zodiac" signs- there is no real info (in the guide) as to exactly which trait conflicts with which! Turn ons/offs is pretty strait forword but just how "lazy" does a Sim need to be to be rendered un-attractive to a more "active" Sim (for example) isn't explained. (This is pretty wiered too, as those of you who are familuar with Prima guides will be VERY knowlegable about their almost cumpulsive use of charts/graphs/mathamatical calculations/etc for darn near everything else in the game! NOTE TO PRIMA: Yo guys, next time how about ditching the discriptions of EVERY piece of furniture in the game & tossing in an "Attraction Calculation Graph" or something???? )

Oh, just in case some of the folks out there don't have/want the Prima guide, here is how SIMS (1 & 2) determin the whole "ziodiac" attraction thing.........

-ZW

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Edit: Added color indicators
Mutual Attraction
Mutual Repulsion
Repulsed by someone who is attracted to me
Attracted to someone who is repulsed by me
Other person is neither attracted to nor repulsed by me.