How does the second hypothesis explains both of those tendencies at the same time? It would rather seem to predict that people of mixed race would be found less attractive then those of the same race as the subject and more attractive then those of the other race. I.e. an averaging out of attractiveness rather then an increase of it.
Oh, it depends where you're brought up. What you really find attractive is a mix of all the races in the community where you were brought up. If everyone you meet in childhood is of the same race then you'll probably find that race attractive, but if you (say) spend half your time hanging out with whites and half your time hanging out with Asians...
