:4D) moderates the impact of sexual cues on men's decisions in ultimatum games
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Copyright © 2006 The Royal Society Digit ratio (2D ![]() ![]() *Author for correspondence (Email: bram.vandenbergh/at/econ.kuleuven.be). Received February 27, 2006; Accepted March 16, 2006. | |||||
Abstract Three experimental studies demonstrate that ‘sex-related cues’ impact human decision-making in ultimatum games. In the ultimatum game, two individuals divide a sum of money. The proposer offers a portion of the money to the other player, the responder. If the responder accepts the offer, the money is distributed in agreement with the proposer's offer. If the responder rejects the offer, neither player receives anything. Our studies show that exposure to pictures of sexy women or lingerie increases the likelihood of accepting unfair offers. Digit ratios of responders are reliably associated with their behaviour: males with lower digit ratios are more likely to reject an unfair split in neutral contexts, but more likely to accept unfair offers in sex-related contexts. Keywords: ultimatum game, sex, digit ratio, cooperation | |||||
1. Introduction Many objectives are achieved more effectively if everyone cooperates (e.g. group hunting). However, collaboration requires complicated decisions on how resources (e.g. prey) should be divided among collaborators. The current research focuses on decisions about accepting or rejecting a division of resources. Fairness considerations reduce the likelihood of accepting unfair offers (Brosnan & de Waal 2003), whereas visceral factors, such as hunger or thirst, may increase the acceptance likelihood of unfair distributions of resources (Loewenstein 1996). Evidence points out that testosterone may be related to increased fairness considerations (high testosterone males are more likely to reject unfair distributions; Burnham 2003), while an increase in testosterone levels after exposure to potential mates (Roney et al. 2003) may be associated with a higher likelihood of accepting small offers (Wilson & Daly 2004). We propose that sex cues moderate the relationship between testosterone and decisions about accepting or rejecting a distribution of resources. The most widely used instrument in research on bargaining is the ultimatum game, developed by Güth et al. (1982). In the ultimatum game, two players are given the opportunity to split a sum of money. One player, the proposer, offers a fraction of the money to the other player, the responder. If the responder accepts the offer, the money is distributed in agreement with the proposer's offer. If the responder rejects the offer, neither player gets anything (Page et al. 2000). Proposers should realize that responders are willing to accept any positive take-it-or-leave-it offer since any amount of money is better than no money. Thus, standard economic theory predicts that proposers should offer very small amounts and responders should be willing to accept them (Rubinstein 1982). Nevertheless, systematic deviations from game theoretic equilibria are frequently observed. Proposers make generous offers and responders often reject small, albeit positive, offers (e.g. Thaler 1988; Nowak et al. 2000; Henrich et al. 2004). We hypothesize that exposure to sex cues will impact ultimatum game behaviour. Sex cues lead to changes in what is perceived ‘just’ (Loewenstein et al. 1997; Ariely & Loewenstein 2006). Fairness norms prescribe a fifty-fifty split in ultimatum games (Nowak et al. 2000; Henrich et al. 2004). We hypothesize that the preference for this normative fair split is reduced after exposure to sex-related stimuli. Also, exposure to highly attractive women increases male subjects' preference for smaller, earlier rewards (Wilson & Daly 2004). We propose that the presence (versus the absence) of sex cues increases the likelihood of accepting small and unfair offers made by proposers because sex cues reduce fairness considerations and lead to a greater preference for small, immediate rewards. We conjecture that this general effect will be more pronounced among men with lower digit ratios. Evidence suggests that ratio between the length of the index finger (2D) and the ring finger (4D) reflects the degree to which humans are exposed to prenatal androgen. 2D | |||||
(a) Study 1 (i) Participants Heterosexual male students (n=44) were recruited via the Internet. Their ages ranged from 18 to 28 years (m=21.5; s.d.=2.05). Participants were seated in partially enclosed cubicles, which prevented them from communicating with each other. Our sample comprised 41 students participating in return for a fee and three students participating for course credit.(ii) Procedure In the picture rating task, 15 photographs were randomly displayed on a computer screen and each participant had to rate the attractiveness of these pictures. Two sets of 15 pictures comprised the stimulus materials. In the landscape condition, 15 pictures of landscapes (beaches, fields, mountain ranges, riversides, etc.) were displayed, and in the sexy women condition, 15 pictures of non-nude female models were shown to the participants. All pictures were artificially created to enhance experimental control and contained a female model wearing a bra or swimsuit/bikini. The full torso was seen in all photographs.Subsequently, participants engaged in a seemingly unrelated task and were informed about the rules of the ultimatum game: the proposer had to suggest a split of €10 and the responder had to accept or reject the proposer's offer. Each participant made two relevant and consequential choices. They recorded both, a maximum offer as proposer and a minimum offer they were willing to accept as responder. To avoid anchoring effects, subjects recorded their choices on a visual analogue scale with endpoints €0 and €10. If the maximum offer equalled or exceeded the responder's minimum demand, the responders would accept the offer, and both the proposer and responder would be paid according to their choices. If the proposer's offer was less than the responder's demand, the responder rejected the offer and no deal was made, leaving both the proposer and the responder empty handed. Participants were informed that 20% of participants would be randomly chosen, that they would be designated either the role of a proposer or a responder and that they would be partnered with another individual participating in the same experiment. After the game, participants entered their email address that was used to inform the participants several weeks later about their eventual gains. After the ultimatum game, the right-hand was scanned by a male experimenter to evaluate digit ratios (we used the right-hand because relations with right-hand digit ratio are typically stronger; Manning 2002). Participants positioned their hand palms on the glass plate of a scanner. Lengths of the second and fourth digits were gauged from the ventral proximal crease of the digit to the finger tip by means of an Adobe Photoshop tool (‘measure tool’). We measured from the most proximal crease when there was a band of creases at the base of the digit. The lengths of index (2D) and ring (4D) fingers were measured twice by the same rater with a time-lapse of several weeks. Digit ratio measures had a high level of repeatability (intraclass correlation coefficient (ICC)=0.99). In the analysis, we used the average between the two measurements. (iii) Results and discussion We assessed the minimum acceptance level of male responders in the ultimatum game. As predicted, participants who had been visually exposed to pictures of sexy women recorded significantly lower minimum acceptance levels (m=3.19, s.e.=0.29) than participants who had been visually exposed to pictures of landscapes (m=3.41, s.e.=0.31), F1,40=4.75, p<0.05. This suggests that exposure to sexy women leads to a lower preference for the normative fair split in an ultimatum game. We did not find a main effect of 2D![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]()
(b) Study 2 (i) Participants Heterosexual male students (n=37) were recruited via the Internet and their ages ranged from 18 to 27 years (m=21; s.d.=1.86). Our sample comprised 25 students participating in return for a participation fee and 12 students participating for course credit. The same laboratory setting as in study 1 was used.(ii) Procedure In the rating task, one piece of clothing had to be rated on different dimensions. In the T-shirt condition, one T-shirt was evaluated on several aspects (such as quality, colour, appreciation, etc.) and in the bra condition, one bra was rated on the same dimensions. Subsequently, participants were given instructions about the seemingly unrelated ultimatum game: the proposer had to suggest a split of €10 and the responder had to accept or reject the proposer's offer. Study 1's procedure was used. After the ultimatum game, the right-hand was scanned by the same male experimenter as in study 1. The lengths of index (2D) and ring (4D) fingers were measured twice by the same rater with a time-lapse of several weeks. Digit ratio measures had a high level of repeatability (ICC=0.98). In the analysis, we used the average between the two measurements.(iii) Results and discussion The minimum acceptance level of male responders was assessed. After visual and tactile exposure to a bra, participants recorded significantly lower minimum acceptance levels (m=3.42, s.e.=0.39) than participants who had been exposed to a T-shirt (m=3.80, s.e.=0.38), F1,33=8.88, p<0.01. This suggests that exposure to lingerie leads to a lower preference for the normative fair split in an ultimatum game. We did not find a main effect of 2D![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() (c) Study 3 (i) Participants Heterosexual male students (n=95) were recruited via the Internet and their ages ranged from 19 to 30 years (m=22; s.d.=1.67). All students participated in return for a participation fee. The same laboratory setting as in studies 1 and 2 was used.(ii) Procedure As in study 1, 15 photographs were randomly displayed on a computer screen. Three sets of 15 pictures comprised the stimulus materials. In the elderly women condition, the photographs contained models who appeared to be in their 70s. In the young women condition, the photographs contained models who appeared to be in their 20s. All models in the elderly and young women condition were displayed with relatively modest, unrevealing clothing. In the sexy women condition, 15 pictures of non-nude female models (the same stimuli as used in study 1) were shown to the participants. Subsequently, participants were given instructions about the supposedly unrelated ultimatum game. Two participants were omitted from analyses because they did not comply with the instructions. After the ultimatum game, the right-hand was scanned by the same male experimenter as in studies 1 and 2. The lengths of index (2D) and ring (4D) fingers were measured twice by the same rater with a time-lapse of several weeks. Digit ratio measures had a high level of repeatability (ICC=0.99). In the analysis, we used the average between the two measurements.(iii) Results and discussion Consistent with studies 1 and 2, participants who had been visually exposed to sexy women recorded significantly lower minimum acceptance levels (m=2.75, s.e.=0.32) than participants who had been visually exposed to pictures of elderly women (m=3.2, s.e.=0.33), or young women (m=3.54, s.e.=0.27), F2,87=5.49, p<0.01. Planned contrasts revealed that the minimum acceptance level in the sexy women condition was significantly lower than the responder's demand in the young women condition, F1,87=9.99, p<0.005, and significantly lower than the demand in the elderly women condition, F1,87=7.39, p<0.01. This suggests that exposure to sexy women leads to a lower preference for the normative fair split in an ultimatum game. We did not find a significant main effect of 2D![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]()
(d) Meta-analysis In the ‘meta-analysis’ of our three experiments, two conditions were constructed. The control condition comprised the landscape (study 1), the T-shirt (study 2), the elderly women and the young women conditions (study 3). The sex-cue condition comprised the sexy women (studies 1 and 3) and the bra condition (study 2). Participants who had been exposed to a sex cue recorded significantly lower minimum acceptance levels (m=3.07, s.e.=0.19) than participants in the control condition (m=3.46, s.e.=0.15), F1,170=23.67, p<0.0001. We did not find a significant main effect of 2D![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() | |||||
3. General discussion The ultimatum game may be considered as a lab model of cooperative hunting or reflect dilemmas of food sharing (Page et al. 2000). Lower minimum acceptance levels in ultimatum games are thus not trivial. Three experimental studies demonstrate that a subtle cue related to sex is able to impact decision-making. Consistent with the predictions we derived from Wilson & Daly (2004) and Ariely & Loewenstein (2006), we found that heterosexual males are more likely to accept unfair offers in an ultimatum game after exposure to sex cues. In three experiments, 2D | |||||
Acknowledgements B.V.d.B. is supported by a Junior Researcher (Aspirant) grant from the Fund for Scientific Research—Flanders, Belgium (FWO). S.D. is supported by grant G.0391.03 from the Fund for Scientific Research—Flanders, Belgium (FWO) and a research grant from the Katholieke Universiteit Leuven (OT 03/07). Financial support by Censydiam-Synovate is gratefully acknowledged. | |||||
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