Oxytocin Effect on Collective Decision Making: A Randomized Placebo Controlled Study

Abstract

Collective decision making often benefits both the individuals and the group in a variety of contexts. However, for the group to be successful, individuals should be able to strike a balance between their level of competence and their influence on the collective decisions. The hormone oxytocin has been shown to promote trust, conformism and attention to social cues. We wondered if this hormone may increase participants' (unwarranted) reliance on their partners' opinion, resulting in a reduction in collective benefit by disturbing the balance between influence and competence. To test this hypothesis we employed a randomized double-blind placebo-controlled design in which male dyads self-administered intranasal oxytocin or placebo and then performed a visual search task together. Compared to placebo, collective benefit did not decrease under oxytocin. Using an exploratory time dependent analysis, we observed increase in collective benefit over time under oxytocin. Moreover, trial-by-trial analysis showed that under oxytocin the more competent member of each dyad was less likely to change his mind during disagreements, while the less competent member showed a greater willingness to change his mind and conform to the opinion of his more reliable partner. This role-dependent effect may be mediated by enhanced monitoring of own and other's performance level under oxytocin. Such enhanced social learning could improve the balance between influence and competence and lead to efficient and beneficial collaboration.

Fig 1. CONSORT flow diagram.

Flow diagram of the progress through the phases of a parallel randomised trial of two groups [66].

Fig 2. Experimental and task design.

(A) Experimental timeline. The experiment started with intranasal self-administration of oxytocin or placebo (double blind). Participants waited quietly for 20 minutes and the performed a 5-minute practice block. The experiment proper started 25 minutes after administration. The experiment duration varied but lasted at least 35 minutes in all dyads, by which time the average of 224 were carried. (B) Pairs of male participants (dyads) performed together a 2-alternative forced choice oddball detection task. Visual stimuli consisted of six vertically oriented Gabor patches displayed equidistantly around an imaginary circle. One randomly selected interval contained the target of higher contrast. Participants indicated their individual decisions privately. If they disagreed, a joint decision was negotiated and announced. Feedback about accuracy was provided. (C) Participants sat in the same testing room, each viewing his own display. Display screens were placed on separate tables at a right angle to each other. (D) We measured the proportion of trials on which the observer reported that the target was presented in the second interval in different target contrast levels, and fitted a psychometric curve to these data. For each dyad three curves were fitted: two for the individual decisions made by each dyad member, and one for the dyadic decisions. Here we depict the psychometric curves fitted to one exemplar dyad. We present the better member data (dark square) and fitted curve (dark line), worse member data and fitted curve (light line and light circles), and dyadic data and fitted curve (dark dots and dashed line). In this case dyadic slope is steeper than the best member’s slope, indicating positive collective benefit.

Fig 3. Oxytocin effect on egocentric bias collective benefit.

(A) When announcing the joint decision participants could keep their original choice or go with the other member’s decision. We used ANCOVA model to estimate the relation between total number of joint decisions made by a participant and the number of egocentric (agree with self) joint decisions. This relationship is captured by the slope and intercept estimated by the ANCOVA model: steeper slope indicates an egocentric inclination, i.e. tendency to not change one’s mind during disagreements. We did not find a significant treatment effect, and no difference in slopes between worse and better members. (B) We fitted psychometric curves to the data from the entire experiment duration, estimating the dyadic sensitivity and the individual sensitivities of dyad members (s_Dyad, s_Max, s_Min). Collective benefit is the difference between the dyadic sensitivity and the better dyad member (Sdyad-Smax). Under oxytocin dyads’ collective benefit was significantly higher than 0 (p = 0.026), but not under placebo (p = 0.2). However, collective benefit was not significantly different between oxytocin and placebo.

Fig 4. Time dependent analysis: overcoming equality bias under oxytocin.

(A) We used an exploratory time dependent analysis of collective benefit, to examine whether oxytocin effect is dependent on time from administration. We estimated collective benefit in three 12 minutes time windows, with the last window in the expected time window for oxytocin effect (49–60 minutes after administration). A significant interaction effect was found (Treatment X Time F(2,82) = 3.14 p = 0.048). This interaction effect depends on the marked increase in collective benefit under oxytocin in the [–60] minutes, the only time collective benefit was significantly higher than 0 (p = 0.045). (B) The relation between number of egocentric joint decisions and number of overall joint decisions made by the better dyad members (left panel), and the worse dyad members (right panel), during the peak in collective benefit under oxytocin. In this time window, 49–60 minutes after administration, a significant interaction is seen (p = 0.008) as better members become more egocentric (p = 0.048) and worse members become less egocentric (p = 0.1), both overcoming equality bias.