Touch plays a vital role for social and psychological well-being and is said to have a “Midas effect” on judgments and decisions, promoting prosocial behavior (Crusco and Wetzel, 1984; Schirmer et al., 2016). Pleasant touch is also thought to increase the release of oxytocin (Walker et al., 2017). Oxytocin, in turn, has been extensively studied with regards to its effects on trust and prosocial behavior, but results remain inconsistent. In this study, we indirectly investigated the presumed effect of endogenously released oxytocin by gently stroking participants on their forearm while they performed a series of decision tasks assessing betrayal aversion, altruism, and risk taking. Our findings contribute to the understanding of touch—and, by extension, oxytocin—in social behavior and decision making.

The first evidence for a causal link between oxytocin and trust was provided by Kosfeld et al. (2005), who found that intranasally administered oxytocin increased investments in a trust game. However, this finding has been difficult to replicate. Some researchers have found that intranasal oxytocin has no effect on initial investments in the trust game, but that it influences investments following trust betrayal (Baumgartner et al., 2008); others have found that the effect of oxytocin on trust and responses to trust betrayal is moderated by gender (Yao et al., 2014) or that it only applies to individuals high in attachment avoidance (De Dreu, 2012). A recent review and meta-analysis found no consistent effect of intranasal oxytocin on trust (Nave et al., 2015). One potential caveat of these studies is the controversial assumption that intranasal oxytocin passes the blood–brain barrier and reaches target brain areas (Leng and Ludwig, 2016). However, studies correlating plasma levels of endogenously released oxytocin with trust have also yielded mixed results. Some researchers have found that oxytocin has no effect on investments in the trust game, but that it influences the amount returned by trustees (Morhenn et al., 2008) or that the level of oxytocin is higher following the receipt of an intentional monetary transfer compared to an equivalent transfer that is determined by a random lottery (Zak et al., 2005). Others have found a U-shaped pattern such that individuals who are either high or low in plasma oxytocin are both more trusting and more trustworthy than participants with moderate levels of oxytocin (Zhong et al., 2012). A drawback of several of these studies that could help explain the inconsistent findings is that they have used unextracted samples of plasma oxytocin that have been shown to be unreliable (McCullough et al., 2013; Christensen et al., 2014). In addition, the oxytocin literature suffers from issues such as publication bias (Lane et al., 2016) and low statistical power (Walum et al., 2016). In sum, the evidence that oxytocin directly influences behavior remains sparse. If there is an effect, it is likely moderated by a variety of factors.

In the present study, we aimed to experimentally manipulate the levels of endogenously released oxytocin by gently stroking participants' forearm with a soft brush. Slow, gentle touch is perceived as pleasant and activates areas of the brain that are associated with interoception and reward, such as the insula, caudate, and dorsolateral prefrontal cortex (Perini et al., 2015). Gentle stroking of the skin at a speed of 1–10 cm/s also activates a specific type of nerve fibers, C-tactile (CT) afferents, that respond optimally to the type of touch that is perceived as most pleasant (Löken et al., 2009). The pleasant and relaxing effects of CT-optimal touch mirror those of exogenously administered or endogenously released oxytocin, suggesting that activation of CT fibers increases the release of oxytocin (Walker et al., 2017, see also Uvnäs-Moberg et al., 2015), although this link has yet to be established empirically. Furthermore, it has been shown that people spontaneously stroke other humans, but not objects, at CT-optimal speeds (Croy et al., 2016), which supports the idea that touch, in particular the kind of touch that activates CT fibers, plays a vital role in the formation and maintenance of social bonds (Olausson et al., 2010). Thus, it seems reasonable to hypothesize that affective touch influences economic behavior; however, to the best of our knowledge, no such studies exist.

We investigate the effect of touch on betrayal aversion, altruism, and risk taking. Betrayal aversion refers to the reluctance to take risk when the outcome depends on a human counterpart rather than when it is determined by nature (i.e., chance; Bohnet and Zeckhauser, 2004). The first evidence for this tendency was provided by Bohnet and Zeckhauser (2004), who elicited participants' minimum acceptable probability (MAP) of getting an even split (the good outcome) for which they were willing to take a risk in a standard trust game compared to an equivalent risk-only trust game. They found that participants' MAPs were greater in the trust game than in the risk-only trust game, indicating that people infer a cost from the possibility of being betrayed by another person, above and beyond the monetary cost. This finding has been replicated across several cultures (Bohnet et al., 2008). More recent neuroimaging studies have shown that playing a trust game with a human counterpart rather than a computer activates areas of the brain that are associated with emotion regulation and negative affect, including the right anterior insula, medial frontal cortex, and right dorsolateral prefrontal cortex (Aimone et al., 2014). Furthermore, betrayal averse participants show less amygdala activity before choosing a risky compared to certain option in a non-social risk task but not in an equivalent social risk task and show greater activity in the striatum, which is involved in reward, after receiving a social than a non-social outcome (Lauharatanahirun et al., 2012).

Betrayal aversion has been suggested as one of the mechanisms by which oxytocin increases trust (Engelmann and Fehr, 2017). The prediction that follows is that touch, because it presumably increases oxytocin levels, reduces betrayal aversion. For instance, Baumgartner et al. (2008) gave male participants intranasal oxytocin or placebo and compared decisions made by investors in a trust game both before and after they received feedback that the trustees did not reciprocate in 50% of cases. Oxytocin had no significant effect on investments before feedback, but after feedback participants who had received oxytocin invested more than those who had received placebo. This suggests that oxytocin reduces the sensitivity to betrayal of trust. However, note that this study relied on intranasal oxytocin despite controversial underlying assumptions. More recent research has failed to replicate the findings (Klackl et al., 2013). Another study that is of particular relevance to the present study was conducted by Morhenn et al. (2008), who compared participants' behavior in a one-shot trust game following either a 15-min massage or a 15-min rest. They found no difference in investors' behavior, but trustees who had received a massage returned more money than trustees who had rested. Most importantly, for participants who had rested, both oxytocin levels and the amount received from the investor predicted the amount returned by the trustees, but for participants who had received a massage, only oxytocin predicted the amount returned. These findings suggest that touch—and oxytocin—promotes prosocial behaviors, although note that these researchers used unextracted samples of plasma oxytocin that may be unreliable (see McCullough et al., 2013; Christensen et al., 2014).

The effect of touch on altruism is more difficult to predict. Previous research suggests that touch increases positive valuations and makes people more prosocial overall, an effect known as the Midas effect (Crusco and Wetzel, 1984; Schirmer et al., 2016). For instance, restaurant guests give larger tips after having been touched on the shoulder by the waitress (Crusco and Wetzel, 1984) and people who have been touched are more likely to help a stranger (Kleinke, 1977; Guéguen and Fischer-lokou, 2003). The prediction that follows from this line of research is that touch increases altruism. However, the oxytocin literature gives a more complex picture. Some researchers have found that oxytocin increases donations to charitable organizations (Barraza et al., 2011; Marsh et al., 2015) and that it increases monetary contributions in a social dilemma (Israel et al., 2012). Others have found that the effect of oxytocin on altruism depends on contextual factors, such as whether the target is a stranger or a close other (Pornpattananangkul et al., 2017) and whether they belong to the in-group or to the outgroup (De Dreu et al., 2010). An alternative explanation of these inconsistencies is that oxytocin promotes mentalizing, i.e., the ability to take someone else's perspective (Domes et al., 2007; but see also Radke and de Bruijn, 2015, and Leppanen et al., 2017, who found no support of this suggestion). Zak et al. (2007) found that intranasal oxytocin increased monetary offers in an ultimatum game but not in an equivalent dictator game. The difference between these two tasks is that in the ultimatum game, the investor has to take the recipient's reaction into account because the recipient can reject the investor's offer, resulting in zero earnings for both players. In contrast, in the dictator game, the recipient simply obtains whatever amount the investor offers, which does not require perspective taking to the same extent. Furthermore, a recent fMRI study showed that oxytocin had no effect on the frequency of altruistic decisions, but that it increased activity in the left temporo-parietal junction, a region that has been implicated in theory of mind, when participants observed others being helped (Hu et al., 2016). Following this line of research, touch should have no direct effect on altruism. However, note again that these studies used intranasal oxytocin.

Oxytocin has mostly been studied in terms of its role in social relationships and behavior, so its effect on risk taking in the non-social domain is unclear. Physical contact has been shown to increase financial risk taking, especially if the toucher is female and if the touch involves a tap on the shoulder rather than a handshake (Levav and Argo, 2010). Somatosensory stimulation in the form of thermal pain also increases risk seeking (Koppel et al., 2017). On the other hand, individuals who have received oxytocin are not more risk seeking than participants who have received a placebo, as shown in studies comparing the effect of intranasal oxytocin on decisions in a trust game to decisions in an equivalent risk game (e.g., Kosfeld et al., 2005). To our knowledge, only one published study has investigated the effect of intranasal oxytocin using a risk-taking task that does not involve another person, and it found no main effect of oxytocin on risk taking (Patel et al., 2015). However, a three-way interaction appeared such that men (but not women) who had received oxytocin were less risk taking if they were told that others were watching them perform the task (which resulted in social stress). Thus, if touch influences risk taking, it may do so via some mechanism other than increased oxytocin, such as increased positive affect.

To the best of our knowledge, our study is the first to investigate the effect of CT-optimal touch on economic decision making. We implemented a crossover design in which all participants completed the decision tasks both with and without touch (in counterbalanced order), which allowed us to explore the effects both within and between subjects. Furthermore, we investigated betrayal aversion, altruism, and risk taking using three standard economic decision-making tasks: the Betrayal Aversion Elicitation Task (BAET), a dictator game, and the Balloon Analog Risk Task (BART).

We investigated the effect of pleasant touch on betrayal aversion, altruism, and risk taking. Pleasant touch activates CT fibers in the skin, which are thought to mediate the oxytocin-enhancing effects of touch (Walker et al., 2017). Our results indicate no effect of touch on any of the outcome variables, neither within subjects nor between subjects. Furthermore, there were no significant interactions between touch and gender or attachment styles.

Given the lack of consistency in previous studies investigating the effect of oxytocin on trust (Nave et al., 2015), it is perhaps unsurprising that we find no effect of touch on betrayal aversion. Several issues have been pointed out in the oxytocin literature, including publication bias (Lane et al., 2016), low statistical power (Walum et al., 2016), lack of evidence that intranasal oxytocin reaches target brain areas (Leng and Ludwig, 2016), and unreliable measures of plasma oxytocin (McCullough et al., 2013; Christensen et al., 2014). This suggests that what we think we know about oxytocin in humans may not be true. Furthermore, as suggested by Bartz et al. (2011; see also Shamay-Tsoory and Abu-Akel, 2016), the effect of oxytocin on trust and prosocial behavior—if there is one—is likely constrained by both individual and contextual factors. For example, previous studies have suggested that oxytocin reduces investments in a trust game following betrayal in women but not in men (Yao et al., 2014) and that oxytocin increases trust and reduces betrayal aversion in individuals that are high, compared to low, in attachment avoidance (De Dreu, 2012). However, in our study, we found no significant interactions between touch and gender or attachment. Regarding contextual factors, previous studies have shown that oxytocin increases trust when trustees are described as trustworthy but not when they are described as untrustworthy (Mikolajczak et al., 2010) and that oxytocin increases trust and altruism toward the in-group but results in defensive behaviors toward the outgroup (De Dreu et al., 2010). Oxytocin also increased cooperation (which requires some degree of trust) in a coordination game when there had been prior contact between participants but reduced cooperation when there had been no prior contact (Declerck et al., 2010). This finding is particularly noteworthy because in the study by Kosfeld et al. (2005), which provided the initial evidence for a causal link between oxytocin and trust, participants introduced themselves to each other before they played the trust game. In contrast, participants in our study played with anonymous counterparts. Therefore, given that oxytocin may enhance the salience of social cues (Shamay-Tsoory and Abu-Akel, 2016), the absence of an effect of touch on betrayal aversion in our study could, at least in part, be due to the lack of social information.

An alternative explanation for our null effect of touch on betrayal aversion is that the size of betrayal aversion was small to begin with, indicating that participants made little difference between the trust game and the risk-only trust game. Early studies reported betrayal aversion sizes ranging from 0.08 to 0.22 (Bohnet et al., 2008). Studies using the Betrayal Aversion Elicitation Task, which assesses betrayal aversion within subjects, have reported betrayal aversion sizes of 0.04 (Aimone et al., 2015) and 0.07 (Quercia, 2016). Betrayal aversion in our study was −0.005 (indicating slightly betrayal seeking or betrayal neutral) in the touch condition and 0.017 (slightly betrayal averse) in the control condition. Furthermore, the proportion of participants that could be categorized as betrayal averse was lower than the proportion of participants that could be categorized as betrayal neutral, which contradicts previous findings that people generally are betrayal averse (Bohnet and Zeckhauser, 2004; Bohnet et al., 2008; Aimone et al., 2015). One possible explanation for these discrepancies is that participants in previous studies (e.g., Bohnet and Zeckhauser, 2004; Bohnet et al., 2008; Aimone et al., 2015; Quercia, 2016) were tested in groups, meaning that any betrayal occurred there and then as a result of the decision of another participant that was present in the same room. In contrast, participants in our study were tested individually and played with anonymous counterparts who had already made their decisions prior to the study. Therefore, the potential betrayal may have felt less personal, which, in turn, may have reduced the negative affective experience associated with the possibility of being betrayed (Lauharatanahirun et al., 2012; Aimone et al., 2014).

The reason we found no effect of touch on altruism could, again, be that there is no direct, causal effect and/or that it depends on individual and contextual factors. Some researchers have found that oxytocin increases donations to charitable organizations (Barraza et al., 2011; Marsh et al., 2015) and that it increases monetary contributions both to the in-group and to the outgroup in a social dilemma (Israel et al., 2012). The prediction that follows from this line of research is that touch increases altruism, which is not what we found in the present study. Instead, our findings are in line with studies showing no effect of oxytocin on altruism (Zak et al., 2007; Hu et al., 2016). Nevertheless, other researchers have found that the effect depends on the closeness of the relationship to the target (Pornpattananangkul et al., 2017) and whether the target belongs to the in-group or the outgroup (De Dreu et al., 2010). We did not take such contextual factors into account. We did find a trend such that touch increased altruism in women more than in men, but this interaction was significant only at the 10% significance level and should be interpreted with caution.

The lack of an effect of touch on risk taking makes sense given that previous studies have found no effect of oxytocin on non-social risk taking (e.g., Kosfeld et al., 2005; Patel et al., 2015). However, it is at odds with studies showing that brief physical contact increases risk taking (Levav and Argo, 2010). A limitation of our study is that we did not measure actual hormone levels, so we cannot rule out the possibility that our lack of effects is due to a failure to increase oxytocin. An alternative possibility is that touch increases positive affect, which, in turn, reduces betrayal aversion and increases altruism and risk taking. Indeed, CT-optimal touch is perceived as pleasant and rewarding (Perini et al., 2015) and positive affect has been suggested as one of the mechanisms underlying the Midas effect (Schirmer et al., 2016). However, studies finding an effect of touch on altruism and risk taking have investigated touch in the form of brief physical contact, such as a tap on the shoulder (Kleinke, 1977; Crusco and Wetzel, 1984; Guéguen and Fischer-lokou, 2003; Levav and Argo, 2010). Here, we investigated the effect of continuous, gentle stroking that lasted throughout the decision phase. This distinction is important for several reasons. First, it is possible that any effect in our study was reduced because the manipulation was obvious to participants. Second, it is possible that participants attributed any affective changes to the touch and that the influence on behavior diminished as a result. Third, incidental affect from the touch may not have been strong enough to override integral affect from the BART, which is rich in affective cues (for a discussion of the integration of incidental and integral affect in decision making, see Västfjäll et al., 2016). Moreover, in studies reporting an effect of brief touch on prosocial behavior, the prosocial behavior was directed toward the toucher, such as the waitress receiving a tip (Crusco and Wetzel, 1984). In contrast, participants in our study donated money to a charitable organization. It is possible that increases in positive affect are attributed to the person delivering the touch, and that touch therefore promotes prosocial behavior only toward the toucher.

In conclusion, we found no effect of touch on betrayal aversion, altruism, or risk taking. These results add to a growing body of research suggesting that oxytocin has no direct, causal effect on trust and prosocial behaviors. Nonetheless, we remain optimistic that touch plays a vital role for social and psychological well-being. It is possible that its effects on economic decision making and behavior are dependent on the social context in a way that may be difficult to study in a laboratory setting. Future research should continue to investigate the circumstances under which affective touch—and its hormonal correlates—influences social behaviors and economic decision making.

This study was carried out in accordance with the recommendations of human research guidelines, Regional Ethics Board for East Gothland with written informed consent from all subjects.

LK, IM, DV, and GT developed the study concept and design. LK collected the data and LK and DA analyzed the results. All authors contributed to the interpretation of the data. LK wrote the first draft of the paper and DA, IM, DV, and GT provided revisions. All authors approved the final version.