Pupil-mimicry conditions trust in partners: Moderation by oxytocin and group membership

Across species, oxytocin, an evolutionary ancient neuropeptide, facilitates social communication by attuning individuals to conspecifics’ social signals, fostering trust and bonding. The eyes have an important signaling function and humans use their salient and communicative eyes to intentionally and unintentionally send social signals to others, by contracting the muscles around their eyes and pupils. In our earlier research, we observed that interaction partners with dilating pupils are trusted more than partners with constricting pupils. But over and beyond this effect, we found that the pupil sizes of partners synchronize and that when pupils synchronously dilate, trust is further boosted. Critically, this linkage between mimicry and trust was bound to interactions between in-group members. The current study investigates whether these findings are modulated by oxytocin and sex of participant and partner. Using incentivized trust games with partners from in-group and out-group whose pupils dilated, remained static, or constricted, this study replicates our earlier findings. It further reveals that i) male participants withhold trust from partners with constricting pupils and extend trust to partners with dilating pupils, especially when given oxytocin rather than placebo; ii) female participants trust partners with dilating pupils most, but this effect is blunted under oxytocin; iii) under oxytocin rather than placebo, pupil dilation mimicry is weaker and pupil constriction mimicry stronger and; iv) the link between pupil constriction mimicry and distrust observed under placebo disappears under oxytocin. We suggest that pupil-contingent trust is parochial and evolved in social species in and because of group life.

跨物种视角下，催产素（oxytocin）作为一种进化上古老的神经肽（neuropeptide），通过使个体敏锐感知同类的社会信号，进而促进社会沟通、增进信任与社会联结。
眼部具有重要的信号传递功能，人类可通过收缩眼周肌肉与瞳孔，有意或无意地向他人传递兼具显著性与交际属性的社会信号。
在过往研究中，我们发现相较于瞳孔收缩的互动对象，瞳孔扩张的互动者更易获得他人的信任。而在此基础效应之外，我们还观察到互动双方的瞳孔大小会出现同步变化：当双方瞳孔同步扩张时，个体的信任感会进一步提升。尤为关键的是，这种瞳孔模仿与信任之间的关联仅局限于内群体（in-group）成员的互动场景中。
本研究旨在探究催产素与参与者、互动对象的性别是否会对上述研究结果产生调节作用。本研究采用针对内群体与外群体（out-group）互动对象的激励型信任博弈范式，设置瞳孔扩张、保持静止与收缩三种实验状态，成功复现了过往研究的核心结论。研究还进一步揭示：
① 男性参与者会对瞳孔收缩的互动对象表现出不信任，而倾向于信任瞳孔扩张的对象，且该效应在注射催产素而非安慰剂（placebo）时尤为显著；
② 女性参与者最信任瞳孔扩张的互动对象，但该效应在催产素作用下会被削弱；
③ 相较于安慰剂组，催产素给药组中个体对瞳孔扩张的模仿行为会减弱，而对瞳孔收缩的模仿行为则会增强；
④ 安慰剂组中观察到的瞳孔收缩模仿与不信任之间的关联，在催产素作用下会完全消失。
综上，我们认为以瞳孔变化为依据的信任具有群体局限性，该机制是社会物种在群体生活中演化形成并依托群体生活得以存续的。