Statistics
To examine the relationship between plasma OT, Trust and Trustworthiness, we used regression models with robust standard error in STATA 11. In the first model, only plasma OT is included in the analysis to test a linear effect of OT on trust behaviors. Since U-shaped dose response curves are often observed for the action of peptide neuromodulators and steroid hormone actions in the brain [32], [33], we added a quadratic in a second model to test for nonlinear relationships between plasma OT and trust related behaviors. In addition, as pointed out by a recent paper [34], the data in Zak et al [22] might indeed suggest a revered U-shaped relationship between plasma oxytocin and trust. Here, a significantly positive quadratic term would indicate a U-shaped relationship while a significant negative quadratic term would indicate an inverse U-shaped relationship. To examine gender effects, we carried out a separate analysis for each gender. All the statistics reported here are two-tailed.