Volume 433 (2011), pp. 65–74

The Author Contribution section neglected to credit Brendan O'Leary for data analysis and for writing this article. This has been corrected here.

Brendan O'Leary, Srinath Rao and William Plaxton designed the experiments; Brendan O'Leary and Srinath Rao performed the experiments; and Brendan O'Leary and William Plaxton analysed the data and wrote the paper.