The principles of the 2013 Declaration of Helsinki were followed. The study was approved by University College London Research Ethics Committee (0847/006). All participants provided written informed consent prior to the experiment. No participant withdrew, and no participant reported distress at debriefing or at later follow-up.

Sixty right-handed student female participants were recruited in pairs and were paid £15–£25 for their participation. Only female participants were tested in order to control for potential effects of gender, both within the participant pairs, and also between participants and the (female) experimenters. Data from a number of standard questionnaires, including Big Five personality and trait empathy, were available from collection prior to participation. The protocol for matching participants in pairs stipulated that participants could not be relatives, friends, or from the same course or faculty. Thus, there was no particular relation between co-participants prior to the experiment. Data exclusion criteria were decided in advance of the experiment: failure to produce temporal intervals covarying monotonically with actual action-tone interval and any general failure to follow instructions. To identify participants for whom the action-tone intervals did not gradually increase with action-tone intervals, we performed linear trend analyses with contrast coefficients −1, 0, 1 for the three delays of the action-tone intervals (see the next section). Two participants were excluded due to non-significant linear trend. After this procedure, 39 participants remained in group 1 (mean age = 22.92, SD = 3.82) and 19 in group 2 (mean age = 22.79, SD = 3.63).

Materials and Procedure

25 Caspar E.A.

Cleeremans A.

Haggard P. The relationship between human agency and embodiment. On arrival at the experimental laboratory, participants read an information sheet about the experimental procedure and the aim of the experiment. The two co-participants signed their individual consent forms simultaneously, ensuring that they were both aware of the other’s consent. Roles were assigned randomly so that one of the participants was told they were the agent and the other was the “victim.” These roles were reversed for the second half of the experiment, making the procedure fully reciprocal. Participants sat at a table, face to face. An external, silent SODIAL Flexible Foldable USB keyboard was placed between them, oriented toward the agent but visible to both. The experimental task ran on a computer, which was located on the agent’s right side with the screen visible only to the agent (see Figure 2 ). The agent was instructed to press a key on the keyboard at a time she chose after the start of the trial, using the right index finger. This caused a tone to occur. The delay between key press and tone was set to vary randomly between 200, 500, and 800 ms. The participants’ task was to estimate the delay between the key press and the tone. They were informed that the delay would vary randomly on a trial-by-trial basis, between 0 and 1,000 ms (they were reminded that 1,000 ms makes 1 s). Participants were also told to make use of all possible numbers between 1 and 1,000, as appropriate, avoiding restricting their answer space (e.g., not only use numbers between 1 and 100), and avoiding rounding (cf. []). Each participant received a paper sheet with 60 empty boxes for their time estimates in each condition of the task. Participants’ answers were hidden from view of the other participant by a barrier, so as to avoid participants being biased by the other participant’s answers.

There were two experimental and two control conditions. In the active control condition, the agent pressed the space key whenever she wanted. In the passive control condition, the experimenter pressed the agent’s index finger down on the space bar, making sure to be unpredictable in her movements so as to minimize motor preparation in the agent. During these conditions, nothing was displayed on the computer screen. The predictions focused on two experimental conditions: free-choice and coercion. Participants were informed that they would both start with a specific amount of money (i.e., £20 for group 1 and £15 for group 2; this difference was due to the fact that participants in group 1 could lose money, but we needed to make sure participants would leave the experiment with the mandated minimum payment of £7.50/hr). In the free-choice condition, agents were instructed that they could freely choose to increase their remuneration for the experiment by delivering, or not delivering, a financial harm (group 1) or a physical pain (group 2) to the other participant, using the appropriate keys on the keyboard. They were told that they were totally free to choose how to act. The computer screen displayed which key press would be associated with which action (for instance, the “F” key for taking money/delivering a shock and the “H” key for refraining from taking money/delivering a shock). In group 1, the agents earned 5p each time they chose to inflict financial pain to the “victim,” who then lost 5p. In group 2, the agents earned 5p each time they decided to deliver a painful electric shock to the “victim.” They earned no money if they decided not to deliver a shock. During this condition, the experimenters did not look at the participants, but focused their attention on task irrelevant objects. In the coercive condition, the experimenters stood up next to the agent and ordered her, on each trial, to take money or not (group 1) or to administer a shock or not (group 2) to the “victim.” The tone played after the key press was the same for both keys. In group 1, the “victim” did not know on each trial whether the agent had chosen to inflict financial harm or not and did not know which of the agent’s response keys was mapped to financial harm—this information was available only on the agent’s feedback display screen. Two new response keys (“J” and “L”) were used when roles were reversed in group 1, so that former “victims” could not now simply repeat harm done to them by repeating the agent’s previous key presses. Thus, group 1 was prevented from imitative behavior. In group 2, participants inevitably experienced the action selected by the agent on each trial, in the form of physical pain. Therefore, imitative behavior would become unavoidable: we thus decided to use the same key-press mappings throughout.

The gains and losses were displayed on the screen visible to the agent. In group 1, the agent saw two moneybags, 1 with her own money and 1 with the “victim’s” money. Each time the agent inflicted financial harm, a coin was shown moving from the “victim’s” bag to her bag, and the total amount of money increase was displayed. When the agent did not take money, no animations were displayed. In group 2, the agent only saw her own moneybag on the screen since the “victim” did not lose money, but instead received a painful shock to their left hand. The shock caused a twitch of the “victim’s” hand that was readily visible to the agent.

Two experimenters participated in group 1, each testing half the sample. One experimenter instructed agents to take money 50/60 times. The other experimenter instructed agents to take money 30/60 times. This variation allowed some control over possible effects of experimenter’s nastiness on participants’ behavior and experiences under coercion. In group 2, both experimenters were simultaneously present, but one gave the coercive instructions. Both experimenters instructed agents to deliver shocks 30/60 times.

We used a partially randomized order of conditions. Participants performed the active or the passive control condition first, then the free-choice condition, then the coercive condition, and then the remaining control condition, either active or passive. We chose not to randomize free-choice and coercive conditions in order not to bias participants in the free-choice condition from previous experience in the coercive condition (e.g., attempting to match the coercive experimenter’s instructions in their “free” choices). Participants went through the same four conditions twice, once as agent and once as “victim,” that is, eight conditions in total. There were 60 trials per condition (20 trials at each action-tone delay, in randomized order), giving a total of 480 trials. Participants performed 240 actions as agents and observed 240 actions as “victims.” The order of the conditions was the same within each pair.

Details of the painful stimulation and other measures are given in the Supplemental Experimental Procedures

At the end of the experiment, participants were paid separately based on their earned financial gain during the experiment. For one dyad in group 1, the experimenter judged that the relation between the agent and the “victim” had become conflictual and hostile. The experimenter made an on-the-spot decision to pay both of these participants the same amount (£20), to reduce the possibility of subsequent distress or conflict.