Does pain make people short-sighted? The impacts of physical and psychological pains on intertemporal choice

We conducted three experiments to investigate the effects of physical and psychological pains on intertemporal choices. In Experiments 1 and 2, physical pain was induced by the self-created Shiatsu sheet treading method (SSTM) and the classical cold pressor task (CPT), respectively. In Experiment 3, psychological pain was induced by the video induction method. All types of pain increased preference for smaller immediate rewards. Theoretical implications and practical implications are discussed.


Introduction
Pain has two-dimensional structure, including two components -physical pain and psychological pain. Psychological pain includes social pain, which is caused by social factors and denotes the feelings caused by damaged or broken social relations (such as suffering rejection and exclusion, being ignored by others, or experiencing the death of a loved one). Social pain is "the distressing experience arising from actual or potential psychological distance from close others or from the social group" (Eisenberger & Lieberman, 2014). Neurophysiological studies have provided evidences for the two-dimensional structure of pain, establishing that physical pain and psychological pain have overlapping neural circuitry. For example, the existing literature has shown that human brain provides similar neural alarm systems for physical pain and psychological pain, leading some studies to postulate the Social Pain/Physical Pain Overlap Theory (Eisenberger & Lieberman, 2004;Eisenberger et al., 2007).
The effect of pain on intertemporal choice has received some recent attention. For example, those experiencing migraine have been reported to be more impatient than a healthy control group (Wu et al., 2017). Likewise, the induction of acute thermal pain through injections of a stimulant into the forearm also rendered participants less patient (Koppel et al., 2017). Those authors conjectured that pain increases preference for immediate monetary rewards by motivating its sufferers to seek immediate relief. Moreover, some recent studies demonstrated that physical pain degraded participants' ability to envision remote places or traverse psychological distance, including the ability to plan for the future (Agerström et al., 2019). Few, if any, studies have shown that psychological pain has an impact on intertemporal choice.
This study attempts to add to this small body of research. Our first two experiments induce physical pain using the Shiatsu sheet treading method (SSTM) and the cold pressor task (CPT), respectively. In Experiment 3, we induce psychological pain using a documentary video clip.

Experiment 1 (SSTM)
In this experiment, 60 undergraduates (67% female; M age = 20.4 years, SD = 2.4) recruited from two universities in western China were randomly assigned to a pain group or a control group. All participants were right-handed, and had never participated in similar studies, but formally consented to participate in this one. We used three domains: Money, Vacation to graduate and undergraduate lab assistants (Libo He, Chunmei Yang, Rong Zhang & Jin Pan) and all study participants.
Copyright: © 2021. The authors license this article under the terms of the Creative Commons Attribution 3.0 License.
However, Rassu et al. (2018) found no such effect in a study in which a burning sensation in the forearm was produced by via the application of topical capsaicin. and Health. As described in the Appendix, the Health items were probably misleading, so we report results for Health only in the Appendix, for all three experiments.

Materials
To induce pain, we used a plastic Shiatsu sheet containing 162 hard protrusions, which is shown in Figure 1. Participants were asked to stand barefoot on this sheet.

Experimental design and procedure
Respondents in all conditions made 27 choices between smaller immediate rewards and larger delayed rewards in each of three domains. Our monetary stimuli were similar but not identical to those used by Kirby et al. (1999). Table 1 shows the respective amounts (in Yuan), the delay (in days) and the imputed "k" value which is a metric of how impatient a respondent would have to be to choose the smaller more immediate reward. Analogous stimuli were created for the vacation and health domains (see Appendix for details). After entering the laboratory, each participant was asked to rest and enter a calm state before starting the formal experiment to ensure a relatively calm baseline emotional state. Participants in the pain group were instructed to step barefoot onto the Shiatsu sheet -and report their pain intensity with Numerical Rating Scale (NRS, shown in Appendix, Part 5; Ferreira-Valente et al., 2011) which ranging from 0 (not at all painful) to 10 (extremely painful). They were also asked to report their emotional state using Seventy healthy college students (59% female; M age = 21.2 years, SD = 1.6) were recruited to assess physical pain intensity by NRS, and we found that 2-3 minutes on the Shiatsu sheet induced moderate physical pain (M = 5.74, SD = 1.33). the emotion self-assessment questionnaire. After this, while still standing on the Shiatsu sheet, the participants completed the 27 intertemporal choices for all the money, vacation and health domains with a 3 min breaks in between (The presentation order of the three domains was counterbalanced among the participants). Participants in the non-pain group only reported their emotional state and made intertemporal choices. The post-interview results revealed that no participants guessed this study's objective. After the experiment, each participant received a fixed payment of RMB 5 Yuan, and one half of 1% of the option they chose for one of the 27 trials in the money domain. As a result, the final payment for each participant ranged from RMB 5.55 -9.25 Yuan. Table 2 displays the proportion of participants' impatient choices in each condition for each task type (In this experiment and later experiments, all decision results with health domain are presented in the Appendix). Those experiencing pain were more impatient in both domains (F(1, 57) = 27.30, p < .001, 2 = .324).

Results
Basic results from the emotional self-assessment questionnaire are reported in the Appendix for this and other experiments. Several attempts to show that the responses mediated the effects of pain on intertemporal choice failed to show any significant mediation, in any of the three experiments. It is not clear that the negative results can be interpreted, as mediation analysis can fail for many reasons.

Experimental design and procedure
The experimental design, procedure and compensation were identical to Experiment 1, though in this study we induced physical pain using a CPT in which subjects must hold their hand in cold water. Once again, the post-interview results revealed that no participant guessed the objective of this study. Table 3 displays the proportion of participants' impatient choices in each condition for each task type. Those in pain were more impatient in both domains (F(1, 57) = 51.43, p < .001, 2 = .474).

T 3:
The proportion of participants' impatient choices in each stimulus type (non-pain vs. pain) and task type (money vs. vacation) in Experiment 2.

Materials
The experimental design, procedure and payment were similar to those used in the prior two experiments, except that we attempted to trigger psychological rather than physical pain and participant should assess intensity of psychological pain by an 11-point (0 = not at all painful, 10 = extremely painful) Distress Thermometer (DT, shown in Appendix, Part 7;Roth et al., 1998;National Comprehensive Cancer Network, 2003). A Chinese documentary "human world" (the 9th episode) was shown to the participants of the pain group. In this video clip, the protagonist (Mrs. Zhang) is diagnosed with terminal pancreatic cancer when she is 5-month pregnant, so she is facing the prospect of saying goodbye to her loved ones, as well as feeling sorrow of missing the opportunity to see her child grow up. Once again, no participants guessed the objective of this study.
A total of 66 healthy college students (71% female; M age = 21.0 years, SD = 1.2) were recruited to assess the pain induction intensity of CPT by NRS. The results revealed that CPT could induce high-intensity pain (M = 7.08, SD = 1.49), and the duration was 2 -3 min.
No participant reported having experienced psychological pain within the last 6 months. Therefore, the interference of non-experimentally induced psychological pain on the experimental results can be eliminated.
A total of 65 healthy college students (66% female, M age = 21.0 years, SD =1.1) were recruited, and evaluated the psychological pain of the aforementioned video on DT. The results revealed that the video material could induced considerable distress (M = 7.17, SD = 1.58). Table 4 shows that the effect observed earlier extends to psychological pain: those participants forced to watch the sad video clip made more impatient choices in both domains (F(1, 66) = 203.58, p < .001, 2 = .755).

T 4:
The proportion of participants' impatient choices in each stimulus type (non-pain vs. pain) and task type (money vs. vacation) in Experiment 3.

General discussion
In this study, the effects of pain on intertemporal choice were investigated from both physical and psychological aspects. We demonstrated that pain increases individuals' preference for more immediate rewards, making them exhibit a "short-sighted" decision-making tendency of the intertemporal choice, regardless of whether the pain is physical or psychological. We found that physical pain could make individuals more immediate satisfaction on intertemporal choice. This result is mutually supportive of existing relevant findings. Becker et al. (2013) reported that physical pain increases individuals' motivation to obtain money benefits, and seeking instant gratification might be a compensatory response to pain because money benefits could reduce the subjective intensity of pain experience and unpleasant feeling. After that, Koppel et al. (2017) reported the effect of physical pain on intertemporal choice by inducing acute thermal pain, which confirmed the above conclusion. Besides the physical pain effect, the present study also found that the psychological pain can also make individuals more immediate satisfaction, established the consistent pattern of psychosomatic results under the pain state, and supported the Social Pain/Physical Pain Overlap Theory (Eisenberger & Lieberman, 2004;Eisenberger et al., 2007).
Pain may affect intertemporal choice mainly through two aspects. On the one hand, pain may result in negative emotional state, and participants will use immediate rewards as an analgesic to repair bad mood, so they are more likely to choose immediate options in the context of pain induction (Koppel et al., 2017). On the other hand, pain may cause individuals to increase their attention to pain-related stimuli, leading to a degree of attentional bias (Khatibi et al., 2009;Haggman et al., 2010). When pain occurs, as the dominant cue occupying the attention resource of individuals, decreasing it or eliminating it becomes the primary task of individuals, which would decrease the attention resource required by individuals to focus on other tasks (Eccleston & Crombez, 1999;Buffington et al., 2005;Wager et al., 2013). This may make individuals more inclined to adopt the way of intuitive processing for intertemporal decision-making, and thus tend to instant satisfaction.
This study is the first time to prove the impact of psychological pain on intertemporal choice; the first time to find the cross-domain consistency of the pain effect on intertemporal choice based on the multiple domains perspective of decision-making; and establishes a new physical pain induction paradigm (SSTM) which is more convenient and safer than the classical paradigm (CPT), which has certain application value. However, this study also has some limitations. As we did not elucidate the internal mechanism of pain effect in intertemporal choice, we call for future research to investigate the role of emotions and cognitive resources, such as the role of attention in the pain effect on intertemporal choice. In addition, our scope of sampling was limited, comprised only college students, although who had the maximum cross-cultural consistency, the same level of individual education, and came from different socioeconomic status. Thus, the scope of application of the study conclusions might be also limited, suggesting future studies to use multiple samples to verify the stability of the pain effect on intertemporal choice in large population.

Part 1. Experimental Material of the Money Intertemporal Choice Task
Welcome to participate in this study! We promise that all information contained in this questionnaire is only for research and we will not disclose your information in any way.
This experiment requires you to choose between the amount you can get immediately and the amount you can get after a period of time (Please tick " √ " in front of each item that

Part 3. Experimental Material of the Vacation Intertemporal Choice Task
Welcome to participate in this study! We promise that all information contained in this questionnaire is only for research and we will not disclose your information in any way. Please imagine that you are a company employee. You draw a lottery ticket at the company's annual meeting. You can redeem the lottery ticket in two ways. (A) Get a period of vacation now. (B) Work after a period of time to get more vacation time. Please select from the following 27 groups of options that you can accept (Please tick " √ " in front of each item that you think is appropriate. There are no right or wrong answers). in anything, often angry or annoyed, and feel hard to concentrate.
If there are two choices, to return to the state of health immediately or to continue the sub-health state for a period of time but can obtain a longer state of health, please choose from 27 options below that you can accept (please tick " √ " before each item you think is appropriate). There are no right or wrong answers.
The effect of pain on intertemporal choice in health domain is inconsistent with that in other two domains. In the opinion of reviewers, this inconsistency likely resulted from difficult that participants had with this scenario. Specifically, in the short-term condition, participants were asked to imagine that an unspecified bad health state got better for a relatively short time and then got bad again. Although the items were based on those of Chapman & Elstein (1995), those original items had a much longer introduction, explaining the illness and its symptoms and asking participants to imagine "that this state of health will continue unchanged for the rest of your life." Without this extended explanation, our participants may have found it difficult to assume that the baseline was a chronic health condition. They may have assumed that, once they got relief, they could continue to get relief, or would get permanent relief again at some future date.

Part 2. Results of Emotion
The emotion reported by the participants are shown in Tables A1, A2 and A3. The questionniare did not ask participants to rate all emotions but rather to select some for rating, using the scale shown above in Part 5. The non-selected emotions were assigned a rating of "1" (the lowest) for analysis of the datal. Although this was our best guess, it might have induced additional error if the selected emotions were chosen for some other reason aside from being the strongest. Yet we would lose too many observations if we simply treated the omitted emotions as missing data. Moreover, the selected emotions might have been the ones that participants in the pain condition thought were most relevant to the experimental manipulation. If this happened, it would show a stronger effect of the manipulation than would otherwise be found. Thus, on the recommendation of a reviewer, we include these results here.

T
A1. The reported emotion in both the pain and the non-pain group in Experiment 1. Note: The emotion of the non-pain group is the baseline emotion before the experimental manipulation, while the emotion of the pain group is the pain-derived emotion.