APA   Click to copy
Panchanathan, K., & Boyd, R. (2005). Second-order free-riding problem solved? Nature, 437, E8–E9.

Chicago/Turabian   Click to copy
Panchanathan, Karthik, and Robert Boyd. “Second-Order Free-Riding Problem Solved?” Nature 437 (2005): E8–E9.

MLA   Click to copy
Panchanathan, Karthik, and Robert Boyd. “Second-Order Free-Riding Problem Solved?” Nature, vol. 437, 2005, pp. E8–E9.

@article{karthik2005a,
  title = {Second-order free-riding problem solved?},
  year = {2005},
  journal = {Nature},
  pages = {E8-E9},
  volume = {437},
  author = {Panchanathan, Karthik and Boyd, Robert}
}