Quadratic, p,0.029). We also consider whether there might be a relationship buy Calcitonin (salmon) between the decision to send and return. In our 
data, 11.5 of the subjects send nothing, and 10.2 of the subjects return nothing. However, in the probit regression, we do not observe a significant relationship between plasma oxytocin and the decision to send (OT, p.0.839; OT quadratic, p.0.832), nor the decision to return (OT, p.0.892; OT quadratic, p.0.895). This might be due to lack of power as the data is skewed for the binary decisions. Alternatively, it might suggest that the plasma OT is more reflecting the degree of trust and trustworthiness, rather than per se trust or not, and 12926553 trustworthy or not.Trust by the first mover is risky since the trustee may not reciprocate the amount transferred. As anticipated, risk attitude and trust are significantly correlated (corr = 0.141, p,0.001) and the direction is positive viz., subjects with a propensity to financial risk are more trusting in the TG. Towards testing the specificity of plasma OT levels as a measure of the level of trust rather than degree of risk tolerance, we conduct a regression analysis with risk attitude as an additional independent variable. The inverted Ushaped association between trust and plasma OT remains robust (the whole sample: OT, p,0.001; OT quadratic, p,0.001; male only: OT, p,0.002; OT quadratic, p,0.002; female: OT, p,0.121; OT quadratic, p,0.133). We further test the relationship between risk and OT; no significant relationship is found (the whole sample: OT, p,0.693; OT quadratic, p,0.594; male only: OT, p,0.695; OT quadratic, p,0.812; female: OT, p,0.247; OT quadratic, p,0.264). This further supports the view that the effect of OT is specifically on trust but not risk attitude.DiscussionWe have examined whether baseline plasma OT is a biomarker which partially predicts the levels of trust and trustworthiness observed in the TG in a large Han Chinese undergraduate sample. Notably, we observed a U-shape relation between the levels of trust and marginally trustworthiness and subjects’ baseline plasma OT level. Specifically, subjects with more extreme levels of plasma OT were more likely to be trusting as well as trustworthy than those with moderate levels of plasma OT. U-shape relationship in hormone and neurotransmitter action is a frequent observation and documented in [32,40]. In the brain the U-shaped doseresponse relationship is particularly characteristic of Pleuromutilin web peptide hormones often presented as an inverted U [41,42] and sometimes as a simple U shape [32,33] as observed in the current investigation. We suggest the notion that the current findings, viz., the extremes 
of plasma OT predict trust and trustworthiness, reflect the complexities of the mechanisms underlying the role of OT in social cognition and especially, but not exclusively, the trust phenotype. As Bartz et al underscore in a cogent and recentPlasma Oxytocin and TrustFigure 1. Plasma oxytocin and trust. (A) Scatter Plot on the relationship between plasma oxytocin and trust. (B) Histogram on the relationship between plasma oxytocin and trust. doi:10.1371/journal.pone.0051095.greview [43], the effects of OT are constrained by features of situations ?simply put `context matters’. Indeed, the role of OT on social cognition and prosociality like trust/trustworthiness is nuanced and subtle. The current findings are consistent with this view and, moreover, help refine predictions about plasma OT levels and how this measu.Quadratic, p,0.029). We also consider whether there might be a relationship between the decision to send and return. In our data, 11.5 of the subjects send nothing, and 10.2 of the subjects return nothing. However, in the probit regression, we do not observe a significant relationship between plasma oxytocin and the decision to send (OT, p.0.839; OT quadratic, p.0.832), nor the decision to return (OT, p.0.892; OT quadratic, p.0.895). This might be due to lack of power as the data is skewed for the binary decisions. Alternatively, it might suggest that the plasma OT is more reflecting the degree of trust and trustworthiness, rather than per se trust or not, and 12926553 trustworthy or not.Trust by the first mover is risky since the trustee may not reciprocate the amount transferred. As anticipated, risk attitude and trust are significantly correlated (corr = 0.141, p,0.001) and the direction is positive viz., subjects with a propensity to financial risk are more trusting in the TG. Towards testing the specificity of plasma OT levels as a measure of the level of trust rather than degree of risk tolerance, we conduct a regression analysis with risk attitude as an additional independent variable. The inverted Ushaped association between trust and plasma OT remains robust (the whole sample: OT, p,0.001; OT quadratic, p,0.001; male only: OT, p,0.002; OT quadratic, p,0.002; female: OT, p,0.121; OT quadratic, p,0.133). We further test the relationship between risk and OT; no significant relationship is found (the whole sample: OT, p,0.693; OT quadratic, p,0.594; male only: OT, p,0.695; OT quadratic, p,0.812; female: OT, p,0.247; OT quadratic, p,0.264). This further supports the view that the effect of OT is specifically on trust but not risk attitude.DiscussionWe have examined whether baseline plasma OT is a biomarker which partially predicts the levels of trust and trustworthiness observed in the TG in a large Han Chinese undergraduate sample. Notably, we observed a U-shape relation between the levels of trust and marginally trustworthiness and subjects’ baseline plasma OT level. Specifically, subjects with more extreme levels of plasma OT were more likely to be trusting as well as trustworthy than those with moderate levels of plasma OT. U-shape relationship in hormone and neurotransmitter action is a frequent observation and documented in [32,40]. In the brain the U-shaped doseresponse relationship is particularly characteristic of peptide hormones often presented as an inverted U [41,42] and sometimes as a simple U shape [32,33] as observed in the current investigation. We suggest the notion that the current findings, viz., the extremes of plasma OT predict trust and trustworthiness, reflect the complexities of the mechanisms underlying the role of OT in social cognition and especially, but not exclusively, the trust phenotype. As Bartz et al underscore in a cogent and recentPlasma Oxytocin and TrustFigure 1. Plasma oxytocin and trust. (A) Scatter Plot on the relationship between plasma oxytocin and trust. (B) Histogram on the relationship between plasma oxytocin and trust. doi:10.1371/journal.pone.0051095.greview [43], the effects of OT are constrained by features of situations ?simply put `context matters’. Indeed, the role of OT on social cognition and prosociality like trust/trustworthiness is nuanced and subtle. The current findings are consistent with this view and, moreover, help refine predictions about plasma OT levels and how this measu.
