Index of articles, click here.
Oxytocin Increases Generosity in Humans
Seville, 26. July 2010
Human beings routinely help strangers at costs to themselves. Sometimes the help offered is generous?offering more than the other expects. The proximate mechanisms supporting generosity are not well-understood, but several lines of research suggest a role for empathy. In this study, participants were infused with 40 IU oxytocin (OT) or placebo and engaged in a blinded, one-shot decision on how to split a sum of money with a stranger that could be rejected. Those on OT were 80% more generous than those given a placebo. OT had no effect on a unilateral monetary transfer task dissociating generosity from altruism. OT and altruism together predicted almost half the interpersonal variation in generosity. Notably, OT had twofold larger impact on generosity compared to altruism. This indicates that generosity is associated with both altruism as well as an emotional identification with another person.
Human beings show considerable generosity toward strangers. In 2005, over $260 billion was given to U.S. charities, with $199 billion (77%) of this given by individuals . The absolute amount of charitable giving is not only high, but the proportion of income donated has grown. In 1954, the average individual in the U.S. gave 1.9% of after-tax income to charity ($222), while in 2005 giving averaged 2.2% of after-tax income ($656, inflation adjusted) . In 2005, approximately one-third of this giving was directed to religious organizations, followed by 19% to health and human services, and 15% to education. People give of their time as well as money. In 2005, over 65 million Americans volunteered to help charities . Ninety-six percent of volunteers said that one of their motivations was ?feeling compassion toward other people. In the midst of all this giving, the physiologic mechanisms that support altruism and generosity are little understood.
Several evolutionary mechanisms have been proposed to explain altruistic giving. These include kin selection, direct and indirect reciprocity, group or multi-level selection, and strong reciprocity. Kin selection [3]?[4] does not explain all altruistic giving because a proportion of giving is to nonrelatives. A recent study found that an average of 1.3% of household income was given to nonrelatives (roughly the same amount given to religious organizations), and an average of 20.3 person-days was spent helping nonrelatives during a year [5]. Reciprocal altruism is giving with an expectation of equal or larger future return from the same person. Yet much charitable giving and direct helping of others does not appear to provide direct reciprocation, for example, volunteering or donating blood [2]. Indirect reciprocity is giving to one person in the expectation of return from another [6]?[8]. This relies on reputation and does not explain anonymous giving [9] that is the focus of this paper. Group selection can support altruistic giving to nonkin as an evolutionarily stable strategy if individuals can be excluded from the group [10]?[12]. Exclusion is difficult when giving to large organizations like the Red Cross, even though much giving is in-group directed [2]. Another multi-level selection theory, strong reciprocity, was recently proposed to explain altruistic acts [13]?[14]. Strong reciprocity, defined as altruistically rewarding cooperators and punishing defectors, does not explain generosity when resources are scarce. Indeed, none of these evolutionary models explicitly predicts generosity in anonymous one-shot interactions.
In this paper we investigate a mechanism that may produce generosity while dissociating generosity from altruism. Altruism is defined as helping another at a cost to oneself [Sober, p 17, 15]. Generosity is defined as ?liberality in giving? [16] or offering more to another than he or she expects or needs. Generosity is therefore a subset of altruism. For example, one may give a homeless person 25 cents (altruism) or ten dollars (altruism and generosity).
The role of empathy in prompting altruistic acts has been proposed by behavioral scientists [17]?[23], though the roots of this idea come from Thomas Aquinas (1225-1274)[24], David Hume (1711-1776) [25], and Adam Smith (1723-1790) [26]. Neuroimaging experiments in humans measuring empathic responses have revealed activity in a network of brain regions, including areas that process emotional and social information, premotor regions, as well as pain pathways. Nonhuman primates have also been shown to exhibit empathy [27], indicating that human empathy has evolutionary roots. Studies measuring human brain activity during charitable giving have shown that giving appears to activate reward regions of the brain, as well as areas associated with emotions and social behaviors [28]?[30]. Coincident brain regions associated with empathy and charitable giving are primarily found in subcoritical areas that process emotional stimuli.
We investigated the role of empathy in producing generosity by manipulating a physiologic mechanism hypothesized to instantiate empathy, the neuromodulator oxytocin (OT). A substantial animal literature has established that OT facilitates attachment to offspring, and in monogamous mammals, cohabiting sexual partners and same-sex conspecifics [31]?[33]. Recent human studies have shown that OT facilitates a temporary attachment between strangers, increasing trust and reciprocity [34]?[38]. In the present paper, we test whether OT is a proximate mechanism prompting generosity between anonymous human strangers. Two tasks were used to dissociate the physiologic role of empathy in producing generosity and altruism using monetary transfers. Monetary transfers were used to obtain objective and active measures of generosity and altruism.
A simple mathematical model will clarify our experimental approach (related models have been proposed [39]?[42], and others). Consider a dyadic interaction between two individuals, i and j. Let bi be the benefit that i receives, and bj the benefit to j. Individuals obtain utility from receiving their own benefit, and possibly from the other person receiving a benefit. We include a parameter ??[0,1] that captures the empathy one has for the other person. We use the term empathy in its standard meaning of ?an identification with and understanding of another's situation or feelings? [43]. We expect ? to be higher when one is induced to explicitly consider one's dyadic partner's feelings regarding the benefits being offered. This has been called ?perspective taking? by social psychologists [44].
For simplicity, let utility be given by a standard form b?, where ??(0,1). Let total benefits be limited, bi+bj = M
When ? = 0, individual i is completely selfish, when ? = 1 s/he is egalitarian. It is straightforward to show that i's choice of the benefit offered to j increases when ? is higher.
When ? = 0, individual i is completely selfish, when ? = 1 s/he is egalitarian. It is straightforward to show that i's choice of the benefit offered to j increases when ? is higher.
Our experimental strategy was to induce participants to consider another's reaction to a split of benefits by giving j a chance to punish i for a stingy offer. In a separate task, i was prompted to make a unilateral monetary transfer to j absent punishment. Because a unilateral transfer does not require considering another's perspective, we expected this task to produce a lower ? and an associated smaller monetary transfer. In order to demonstrate the causal effect of OT on generosity, we infused one-half of the participants with OT intranasally while the other half received the same amount of normal saline.
Two decision tasks from experimental economics, the ultimatum game (UG) and the dictator game (DG) were used. In both tasks, participants in randomly-formed dyads were assigned the role of decision-maker 1 (DM1), or decision-maker 2 (DM2). In the UG, DM1 was endowed with $10 and was asked to offer a split of this money to DM2. DM2 has no endowment. If DM2 accepted the split, the money was paid. But, if DM2 rejected the split, both DMs earned nothing. Participants were asked to make decisions as both DM1 and DM2, with subsequent random assignment of roles. As DM2s, they were asked to state the minimum amount they would accept from a DM1. The rejection threshold was not reported to the other DMs. By asking subjects for the minimum acceptable offer, the UG task was designed to have participants consider how the DM2 in the dyad would react to an offer (perspective taking). A rejection of DM1's offer in the UG allowed DM2 to punish DM1 for stingy offers, but at a cost.
We define a generous transfer in the UG as a DM1 offer that exceeds the average minimum acceptable offer. That is, generous offers are those which are greater than are expected for acceptance.
The DG is similar to the UG in that DM1 has a $10 endowment and DM2 has nothing. The difference in the DG is that DM2 has no choice?he or she must accept whatever DM1 offers. As a result, the DG does not compel DM1 to consider how DM2 will feel about the split of benefits (reduced perspective taking). The consensus view in experimental economics is that the transfer in the DG is a measure of altruism [39], [45]. The inclusion of both the UG and DG allows us to dissociate generosity from altruism within subjects.
In both the UG and DG, subjects whose identities were masked to each other and the experimenters, were randomly assigned to dyads without pre- or post-decision communication. All participants received the same instructions, and there was no deception. Participants were infused with 40 IU oxytocin (OT) or placebo (normal saline) intranasally (see Materials and Methods). The infusion was double-blind. Participants were privately paid their earnings in cash at the conclusion of the experiment. Our approach followed a related study using OT to examine interpersonal trust by means of monetary transfers [37].