To cite the \emph{ probit.gam } Zelig model:
 \begin{verse}
 Skyler J. Cranmer. 2007. "probit.gam: Generalized Additive Model for Dichotomous Dependent Variables" in Kosuke Imai, Gary King, and Olivia Lau, "Zelig: Everyone's Statistical Software,"\url{http://gking.harvard.edu/zelig} 
\end{verse}