Participants

Forty participants (34 females, 6 males) between 18 and 31 years (M = 20.23, SD = 2.97 years) were recruited from the Plymouth University (School of Psychology) participant pool and participated for course credit. All participants had normal or corrected-to-normal vision and were right-handed. This research received ethical approval from Plymouth University Ethics Committee and informed consent was obtained from all participants. All experiments were performed in accordance with the guidelines and regulations set out by the ethics committee.

Measures

Participants were asked to fill out an electronic questionnaire comprising four self-report questionnaires:

The Levenson Psychopathy Scale (LSRP) 24 is a self-report measure of trait Psychopathy designed for non-institutionalised populations. It comprises a two-factor structure measuring both primary (i.e., callousness) (16 items; α = 0.90) and secondary psychopathic traits (i.e., impulsivity) (10 items; α = 0.66). The scale contains 26 items in total that are rated on a 4-point Likert scale (from 1 = strongly disagree to 4 = strongly agree). The scale includes items such as “Success is based on survival of the fittest; I am not concerned about losers”.

The Hexaco-IP-PR 25 is a personality inventory assessing six dimensions of personality. The inventory measures the following characteristics: Honesty-Humility (Items 10; α = 0.81), Emotionality (Items 10; α = 0.83), Extraversion (Items 10; α = 0.74), Agreeableness (Items 10; α = 0.87), Conscientiousness (Items 10; α = 0.80) and Openness to experience (Items 10; α = 0.78). The inventory contains 60 items, rated on a 5-point Likert scale (from 1 = strongly disagree to 5 = strongly agree). The inventory contains items such as “I want people to know that I am an important person of high status”.

Moral Judgment and Simulated Action Measures

The between-groups experiment comprised two conditions to which participants were randomly allocated; a judgment condition (N = 20) and a simulated action condition (N = 20). In both conditions, participants were presented with vignettes describing the footbridge dilemma and 14 further personal force dilemmas. These moral dilemmas were selected from those originally used in Greene, et al.2 and also included five personal force dilemmas adapted from this database (see Supplementary Material: Appendix A online). Of the dilemmas originally used in Greene, et al.2, personal dilemmas were modified to ensure that each involved personal force. For example, consider the “euthanasia” dilemma:

You are a leader of a small group of soldiers. You are on your way back from a completed mission deep in enemy territory when one of your men has stepped in a trap that has been set by the enemy and is badly injured. The trap is connected to a radio device that by now has alerted the enemy to your presence. They will soon be on their way.

If the enemy finds your injured man they will torture him and kill him. He begs you not to leave him behind, but if you try to take him with you your entire group will be captured. The only way to prevent this injured soldier from being tortured is to shoot him yourself.

This dilemma, while categorised as personal, does not involve personal force according to its original definition6. As such, in the present experiment it was adapted as follows:

If the enemy finds your injured man they will torture him and kill him. He begs you not to leave him behind, but if you try to take him with you your entire group will be captured. The only way to prevent this injured soldier from being tortured is to kill him yourself by stabbing him.

Replacing a gun with a knife ensures that “the force that directly impacts the other [person] is generated by the agent’s muscles”6.

In both conditions, utilitarian and non-utilitarian responses were represented as a binary variable and the mean utilitarian proportion across all dilemmas was subsequently calculated for each individual by dividing the number of their utilitarian responses by the total number of dilemmas.

Simulated Action Variables

In the simulated action condition, as well as generating an overall response to the dilemma, the vBOT system provided additional measures including force, speed and subsequent power.

Force

Baseline force measurements were calculated for each participant in order to control for varying strengths among participants. The vBOT arm allowed participants to push forward when simulating a utilitarian action or to pull away when refusing to endorse an action (non-utilitarian). A normalised force measure was subsequently calculated for the simulated utilitarian actions. Baselines measurements were first created by averaging the force of endorsements of actions in non-moral dilemmas (baseline force for endorsements). The normalized force applied by each participant when producing a simulated utilitarian response was then calculated as a proportion of their baseline force.

Speed

Speed was defined as the maximum speed (cm/s) that a participant moved the vBOT arm across the movement trajectory. Using the same procedure for force measurements, a normalised maximum speed was conditionally calculated for the simulated utilitarian actions. Baselines measurements were created by averaging the speed of endorsements of actions in non-moral dilemmas. The normalized speed was then calculated for the simulated utilitarian actions as a proportion of this baseline speed.

Power

The relative force and speed with which individuals simulated utilitarian actions were strongly correlated, (r(18) = 0.51, p = 0.021). Given that the product of speed and force equates to a measure of power, we used normalised force and speed scores to create a relative measure of power for each participant. This represented the power exerted by an individual when simulating a utilitarian action with the vBOT arm.

Procedure

In both conditions, participants first completed the electronic questionnaire comprising the trait assessments. All personal force dilemmas were presented in a randomised order. In the judgment condition, dilemmas were presented to participants on a computer running E-Prime software and each dilemma was presented in three blocks of text that could be read at a speed determined by the participant. After each dilemma, participants were asked a morality question (“Is it morally acceptable to [specific to the scenario]?”) followed by a behavioural question (“Would you do it?”). Responses were given by selecting “Yes” (Y-key) or “No” (N-key). Participants were given 8 seconds to respond. This time frame was selected as it is long enough to allow responses that are not time-pressured but short enough to prevent a long elaborate decision-making process that would be unrealistic in an action framework. Similar time frame windows have been adopted in previous VR moral action paradigms10,16.

In the simulated action condition, participants initially completed an electronic pre-questionnaire assessing their gaming experience (hours per week of video game play and the number of games played annually). Participants were then presented with three non-moral dilemmas (see Supplementary Material: Appendix B online) selected from an existing database2 to provide a baseline measure of force. The personal-force moral dilemmas were then presented. All dilemmas were presented using the vBOT system (see Fig. 1).

Figure 1 The vBOT System. The diagram shows the side-view of the set-up with a participant holding the handle of the vBOT arm with their right hand whilst viewing the monitor via a semi-silvered mirror. Text-based vignettes of dilemmas are displayed on the monitor for participants to read. Full size image

At the start of each trial the vBOT handle first pulled the participant’s right hand to the starting position, which was at a central location (in the mid-sagittal plane 30 cm below the eyes and 30 cm in front of the chest). Participants were prevented from viewing their hand directly, and the VR system was used to overlay images of the hand cursor (0.5 cm radius red disk).

Participants were able to read the dilemmas in the semi-silvered mirror and as in the judgment condition, these were presented in blocks of text that could be read at a speed determined by the participant. Participants used a button press with their left hand to scroll forward through these blocks of text. After the end of each dilemma had been reached, upon a final left-hand button click the participant was asked (“Are you going to [specific to scenario]?”) followed by the phrase (“If so, move the arm forward to [specific to scenario]. If not, then pull away [specific to scenario]”). A final button press was then used to cue the response action and this generated the message (“Act now”). Matched to the judgment condition, after reading each dilemma in full, participants were given 8 seconds to respond.

A utilitarian endorsement was achieved when the handle was pushed forward into a soft object (which required the application of force) by more than 2 cm. The simulated soft object was located immediately forward of the start position and implemented using the combined effect of a weak spring (k = −4Ncm−1) and a resistive viscous field (k = −0.5Ncm−2). As such, when simulating a utilitarian response, the vBOT arm produced the physical resistance force that would be experienced when making contact with an object, thereby generating haptic feedback to the participant. A refused (non-utilitarian) response was achieved when the handle was pulled backwards more than 1.25 cm from the start position. No resistance was experience when pulling back. In this system, the handle position of the manipulandum is measured using optical encoders sampled at 1000 Hz and it uses motors operating under torque control to allow the application of end-point forces. A force transducer (Nano 25; ATI) is mounted under the handle to measure the applied forces.

In both conditions, if a response was omitted (no response was given after 8 seconds), the program would then move on to the next dilemma and the omitted scenario would be presented again at a later stage during the experiment. In the judgment condition, the proportion of utilitarian endorsements for morality and behavioural questions was recorded. In the simulated action condition, the proportion of utilitarian actions, force, and speed were all recorded for further analyses. Response times in the judgment and simulated action conditions were recorded by different systems and as such, could not be directly compared.

Data availability

The datasets generated during and/or analysed during the current investigation are available from the corresponding author on reasonable request.