This was a little harder to find than I expected:

http://www.ncbi.nlm.nih.gov/pubmed/16555779

"These findings demonstrate that female facial appearance holds detectable cues to reproductive health that are considered attractive by other people."

Found via http://asutriplehelix.org/node/99 which also cites some other related papers

EDIT: I really, really enjoy finding cases where something I assumed to be true is questioned on the grounds of scientific correctness. It is always a wonderful opportunity to bring my thinking closer to reality, no matter if my assumptions are true or false.

I'm a research scientist for a face recognition company and have done a lot of work with attractiveness estimators and sex classification. I've found that the sex classification score and female attractiveness has a pretty high correlation. I think Alice O'Toole has a paper that has a similar finding although I would have to search for the exact reference.

With her 'perceived' reproductive capacity would have been already a bit more palatable.

> Source please.

BOOBS.