Table 1. Summary of the current state of data on pair bonding from prairie voles, titi monkeys, and humans.

	Prairie voles	Titi monkeys	Humans
Oxytocin	OTRa in the NAcb are crucial to pair bonding based on multiple methods: pharmacology [30], viral vectors [38], RNAic technology [39]	OTR present in some dopaminergic areas, like the LSd [113]; these areas are implicated in imaging studies of pair bonding [105,109]; however, no OTR present in the NAc as in voles	Peripheral OTe responsive to relationship cues [44-46]; intranasal OT enhances partner attractiveness, distance from strangers [52,53]; genotype associations [43]; OTR distribution known only for brainstem [117]
Vasopressin	AVPR1af in VPg are crucial to pair bonding based on multiple methods: pharmacology [30], viral vectors [41], RNAi technology	Intranasal AVPh causes changes in partner contact [126]; AVPR1a present in NAc rather than VP [113]; these areas implicated in imaging studies of pair bonding [105,109]	Peripheral AVP associated with relationship quality [50,51]; genotype associations [49]; AVPR1a distribution known only for brainstem [117]
Dopamine	Dopamine type D2 receptors involved in formation [64]; type D1 receptors involved in maintenance via up-regulation of aggression [33]	D1 receptors in LS up-regulated in males after pairing [130]; AVPR1a present in NAc [113]	Dopaminergic areas implicated in imaging studies [62]
Opioids	µ receptors regulate formation and ĸ receptors regulate maintenance [33,77]; pharmacology, gene expression in striatum	µ and ĸ receptors involved in maintenance, grooming [78] and responses to separation [128]	µ agonism promotes visual attention to faces and eyes [148]; touch from partner alters µ availability [149]; kappa system not studied
Visual system	Not studied, expected NOT to be heavily involved	Contains OTR and AVPR1a in titis [113]	Not studied directly, involvement expected (but see above)

^aOTR = oxytocin receptor, ^bNAc = nucleus accumbens, ^cRNAi = RNA interference, ^dLS = lateral septum, ^eOT = oxytocin, ^fAVPR1a = arginine vasopressin receptor type 1a, ^gVP = ventral pallidum, ^hAVP = arginine vasopressin