The hypothalamic peptide oxytocin (OT) is a member of the ancient class of nonapeptides, conserved throughout evolution from nematodes to humans (Donaldson & Young Reference Donaldson and Young2008). OT controls lactation and parturition, and in the brain it influences social cognitive functions and a diverse repertoire of social behaviors, ranging from pair-bond formation to mating and parenting (Insel Reference Insel2010; Insel & Young Reference Insel and Young2001; Rilling & Young Reference Rilling and Young2014). Consistent with this profile, the OT receptors are enriched in a widely distributed network of brain regions engaged in social and cognitive plasticity (Mitre et al. Reference Mitre, Marlin, Schiavo, Morina, Norden, Hackett, Chao and Froemke2016).

The central role of OT in pair bonding has been revealed in various species, including zebra finches (Klatt & Goodson Reference Klatt and Goodson2013) and teleost fish (Oldfield & Hofmann Reference Oldfield and Hofmann2011), and is particularly evident in comparisons between the monogamous prairie vole and the polygamous montane vole (Carter Reference Carter1998; Young & Wang Reference Young and Wang2004). In contrast to the latter, prairie voles exhibit a high density of OT receptors in the prelimbic cortex and nucleus accumbens (NAcc), which receive dense inputs from other signaling pathways, including dopamine (DA) and arginine-vasopressin, to facilitate monogamous behavior (Johnson & Young Reference Johnson and Young2015).

Findings linking OT to infant-caregiver bonding (Hurlemann & Scheele Reference Hurlemann and Scheele2016) encompass increased peripheral OT levels during parent-child interactions, on the one hand, (Feldman et al. Reference Feldman, Gordon, Schneiderman, Weisman and Zagoory-Sharon2010) and variations in the OT receptor gene that relate to differences in parenting sensitivity, on the other (Feldman et al. Reference Feldman, Zagoory-Sharon, Weisman, Schneiderman, Gordon, Maoz, Shalev and Ebstein2012). Because it is still unclear whether peripheral levels of OT predict central levels (Carson et al. Reference Carson, Berquist, Trujillo, Garner, Hannah, Hyde, Sumiyoshi, Jackson, Moss, Strehlow, Cheshier, Partap, Hardan and Parker2015; Landgraf & Neumann Reference Landgraf and Neumann2004), stronger evidence for a key role of OT in infant-caregiver bonding comes from experimental studies relying on exogenous administration of OT, usually realized through nasal delivery of the peptide (Quintana et al. Reference Quintana, Guastella, Westlye and Andreassen2016; Striepens et al. Reference Striepens, Kendrick, Hanking, Landgraf, Wullner, Maier and Hurlemann2013). Exogenously administered OT augments affective parenting (Weisman et al. Reference Weisman, Zagoory-Sharon and Feldman2012) and modulates fathers' neural responses to pictures of their own children (Wittfoth-Schardt et al. Reference Wittfoth-Schardt, Grunding, Wittfoth, Lanfermann, Heinrichs, Domes, Buchheim, Gundel and Waller2012). Furthermore, OT promotes relationship stability by stimulating positive communication during couple conflict (Ditzen et al. Reference Ditzen, Schaer, Gabriel, Bodenmann, Ehlert and Heinrichs2009) and by increasing trust following betrayal (Baumgartner et al. Reference Baumgartner, Heinrichs, Vonlanthen, Fischbacher and Fehr2008). Although these subtle, modulatory effects of OT are influenced by a panoply of person-dependent factors including sex (Scheele et al. Reference Scheele, Striepens, Kendrick, Schwering, Noelle, Wille, Schlapfer, Maier and Hurlemann2014b), personality (Scheele et al. Reference Scheele, Kendrick, Khouri, Kretzer, Schlapfer, Stoffel-Wagner, Gunturkun, Maier and Hurlemann2014a), and early life adversity (Meinlschmidt & Heim Reference Meinlschmidt and Heim2007), they clearly support the highly adaptive role of the peptide in the formation and maintenance of social bonds.

In their superb article, Maestripieri et al. focus on attractiveness-related prosocial biases that occur when adults interact with one another. Endogenous OT is increasingly released during experiences of romantic love (Schneiderman et al. Reference Schneiderman, Zagoory-Sharon, Leckman and Feldman2012), social support (Grewen et al. Reference Grewen, Girdler, Amico and Light2005), and generosity (Zak et al. Reference Zak, Kurzban and Matzner2005). Studies relying on exogenous OT administration have shown that the peptide modulates a wide range of prosocial and courtship behaviors in humans, including interpersonal distance in people engaged in monogamous relationships (Preckel et al. Reference Preckel, Scheele, Kendrick, Maier and Hurlemann2014; Scheele et al. Reference Scheele, Striepens, Güntürkün, Deutschlander, Maier, Kendrick and Hurlemann2012), altruism (Hu et al. Reference Hu, Scheele, Becker, Voos, David, Hurlemann and Weber2016; Marsh et al. Reference Marsh, Scheele, Gerhardt, Strang, Enax, Weber, Maier and Hurlemann2015), trust (Kosfeld et al. Reference Kosfeld, Heinrichs, Zak, Fischbacher and Fehr2005), generosity (Zak et al. Reference Zak, Stanton and Ahmadi2007), and empathy (Domes et al. Reference Domes, Heinrichs, Michel, Berger and Herpertz2007; Hurlemann et al. Reference Hurlemann, Patin, Onur, Cohen, Baumgartner, Metzler, Dziobek, Gallinat, Wagner, Maier and Kendrick2010; Radke & de Bruijn Reference Radke and de Bruijn2015). As Maestripieri et al. point out, several of these behavioral changes affect the perceived attractiveness of a person as a potential mating partner. As a direct confirmation of this proposition, a recent study confirmed that perceived attractiveness of a person depends on that person's affective behavior (Anders et al. Reference Anders, de Jong, Beck, Haynes and Ethofer2016).

The behavioral effects of OT may result from perceptual changes, including increased attention to the socially informative eye region (Guastella et al. Reference Guastella, Mitchell and Dadds2008), improved recognition of cues related to sex and relationship (Unkelbach et al. Reference Unkelbach, Guastella and Forgas2008), and facilitated sensing of and responding to emotional stimuli (Shahrestani et al. Reference Shahrestani, Kemp and Guastella2013). Of particular relevance to the emergence of attractiveness-related biases are observations that OT increases ratings of trustworthiness and attractiveness of male and female targets in judgments of both sexes (Theodoridou et al. Reference Theodoridou, Rowe, Penton-Voak and Rogers2009, but see also Lambert et al. Reference Lambert, Declerck and Boone2014; Thienel et al. Reference Thienel, Heinrichs, Fischer, Ott, Born and Hallschmid2014). Noteworthy is that OT also improves men's ratings of the likeability of physically formidable male peers (Chen et al. Reference Chen, Mayer, Mussweiler and Heinrichs2015), clearly showing that OT-mediated attractiveness biases are not restricted to individuals of the opposite sex. In addition, OT enhances an attractiveness bias for the romantic partner (Scheele et al. Reference Scheele, Wille, Kendrick, Stoffel-Wagner, Becker, Güntürkün, Maier and Hurlemann2013; Reference Scheele, Plota, Stoffel-Wagner, Maier and Hurlemann2016), and this behavioral effect is accompanied by enhanced responses in reward-associated brain areas including the ventral tegmental area and the NAcc, with the latter being rich in OT-DA d2 receptor heteromers (Romero-Fernandez et al. Reference Romero-Fernandez, Borroto-Escuela, Agnati and Fuxe2013). However, a positron emission tomography study employing the DA d2 receptor radioligand [¹¹C]raclopride and a facial attractiveness rating task failed to detect altered striatal DA release as a correlate of an OT-mediated attractiveness bias (Striepens et al. Reference Striepens, Matusch, Kendrick, Mihov, Elmenhorst, Becker, Lang, Coenen, Maier, Hurlemann and Bauer2014). Instead, there is growing evidence from rodent studies for OT interactions with the serotonin system (5-HT) in the NAcc during encoding of social reward (Dölen et al. Reference Dölen, Darvishzadeh, Huang and Malenka2013; see also Mottolese et al. Reference Mottolese, Redouté, Costes, Le Bars and Sirigu2014). Hence, OT-mediated attractiveness biases may be anchored in interactions of the peptide with diverse signaling pathways, including 5-HT, but also gonadal steroids such as testosterone, as mentioned by Maestripieri et al. Interestingly, OT stimulates testosterone release (Frayne & Nicholson Reference Frayne and Nicholson1995; Gossen et al. Reference Gossen, Hahn, Westphal, Prinz, Schultz, Grunder and Spreckelmeyer2012; Weisman et al. Reference Weisman, Zagoory-Sharon and Feldman2014, but see also Wirth et al. Reference Wirth, Gaffey and Martinez2015), heightened OT levels co-occur with elevated testosterone levels (Jaeggi et al. Reference Jaeggi, Trumble, Kaplan and Gurven2015), and prenatal exposure to testosterone moderates later OT effects (Kret & De Dreu Reference Kret and De Dreu2013; Weisman et al. Reference Weisman, Pelphrey, Leckman, Feldman, Lu, Chong, Chen, Monakhov, Chew and Ebstein2015).

In quintessence, OT has a key role in modulating social cognition and behavior, with the ultimate aim to foster social bonding in general and the stability of monogamous pair bonds and offspring care in particular. Given this empirical background, we propose that the attractiveness-related prosocial biases described by Maestripieri et al. may very likely be orchestrated by, and depend on, OT signaling and represent a relict of our evolutionary past.