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Evolution...The more difference between 2 mates genes the greater the chance of healthy offspring. You're not attracted to members of your own family as their genes are very similar...which causes problems with offspring.
This I doubt is a valid explaination. While we do prefer people of different genetic make-up (specifically the genes that cause the antigen-expression in our immune system), that doesn't mean that we want our partner to be as different as possible.
As my professor (an expert on smell & behavior) put it: That would mean that the african guy in the praerie is craving for the inuit woman ... would be kind of unpractical for mating.
Actually research shows that this smell attraction seems to be more of a screening ... that you DON'T sleep with people who are TOO similar. So it's not about maximizing the difference in your genetic make up, but about achieving a necessary difference.
In general people prefer mates of their own ethnicity.
So I'll go with the social conditioning explaination.
Actually their might be an explaination for a slight bias towards light colored skin in women and dark colored skin:
Women are more attractive in their fertile phase and one sign for fertility is that their skin lightens up.
Women prefer men with a darker skin tone, because they are perceived as more manly.
However, I doubt that this can explain a clear preference for someone several skintones lighter/darker which obviously does occur quite a bit.
In the end preferences are highly individual and their is still some mystery which makes it great
Plus one thing about evolution which you must always keep in mind, is that is creates DIVERSITY within society leading to many NICHES. So also in mating strategies and mate selection their may be a widespread general strategies but there always must be alternative strategies and individual differences.
In some cases, there is even a competitive equilibrium within a species for mating strategies - if all men acted the same, then women would adapt. If all people cheat, no one trusts anyone and new rules start to form, which make it harder for the cheaters - and the non-cheaters will have an edge. On the otherhand, if all people trust each other, the cheater will have a huge edge!