The present study shows that the reward system may be involved in this attentional bias towards babies. Little is known about the precise interaction of OT and T in the brain. In the female brain the majority of T may be converted to estradiol via aromatase47, and estradiol can increase the number of OT receptors in the brain29. Yet, the observation of higher systemic T concentrations in women may indicate an impaired ability to convert it to estradiol, thus either leaving a higher amount of T in the circuitry that may counteract maternal behavior through an unknown mechanism or by leaving the number of OT receptors unaffected by reduced influence of estradiol. In a similar vein one may speculate that high OT concentrations may serve as a compensatory mechanism for high T concentrations, we presume that women with low T concentrations were less sensitive to OT administration, because low T concentrations should support nurturing behavior per se6. This would indeed explain why we could not find any activation in the brain reward system in women with low endogenous T after OT administration. But since babies nevertheless remain a strong social cue, this does not explain why neither in the low T nor in the placebo group the baby pictures were rewarding by themselves. We suppose that the young adult stimuli may also be socially salient for young participants, for instance as potential mates, and therefore could be equally rewarding and evoke similar activation in the brain reward system48. Our experimental design does not allow the isolation of the response to adult or infant faces, since pictures were always presented in combination, but only enables the identification of activational differences during processing of these stimuli. So we cannot answer this question. However, for reasons discussed before we presume that the brain of women with a high endogenous T concentration may be more sensitive to OT administration than women with a low endogenous T. One could therefore speculate that same age adult stimuli as well as infant pictures both constitute a strong social stimulus, but OT could function as an enhancer of reproductive behavior and direct attention towards infant stimuli in women with high T level, who may be more sensitive to OT. In this study, we focused on nulliparous women that reacted to unfamiliar infant or adult stimuli to investigate the neuroendocrine basics of the key stimulus in general. As oxytocin is especially known to influence attachment behavior15 it could be of great interest to use the same paradigm with familiar infants in further research. The results of this study are therefore limited to the general mechanism of the key stimulus BS and are not necessarily transferable to attachment behavior in mothers or other caretakers. Gordon et al.26 found a positive interaction between increased T levels and OT on maternal touch in mothers, while the effect in fathers was in the opposite direction. Their results may indicate that the interaction between T and OT may be sexually dimorphic. Therefore, our results on nulliparous women may resemble those in mothers but it is possible that they deviate to the results in fathers and men who have never sired a child. Although the findings of Gordon and colleagues26 were focused on parents, we assume that the results in mothers support our previous findings that OT seemed to promote adult-infant discrimination and increased attention to baby pictures, but only in women with high endogenous T6. OT is a peptide hormone that is critically involved in mother-infant attachment49. Especially lactating mothers show increased OT levels50. The artificial increase through intranasal administration could be closely related to the concentration found in new mothers (see Weisman et al.51 for the active drug effects reflected in saliva). Although there already exists excellent research about the active drug effects and reflection in saliva49–51, a direct comparison of mothers and non-mothers salivary OT concentration with and without OT administration would be of great interest. But yet, aside from oxytocin, postpartum hormonal changes have large transitions which makes the results of nulliparous women not necessarily comparable with mothers52,53.