\usepackage{url}
@article{ balas:plos_2007,
    author={Balas, B. and Cox, D. D. and Conwell, E.},
    title={{The effect of real-world personal familiarity on the speed of face information processing}},
    journal={PloS One},    
    volume={2},
    number={11},
    pages={1223},
    issn={1932-6203},
    url={http://www.rowland.harvard.edu/cox/pdfs/pone.0001223.pdf},
    year={2007}
}