Positive emotions produce no or weak and highly variable increases in autonomic nervous system activity

Autonomic Nervous System Activity During Positive Emotions: A Meta-Analytic Review. Maciej Behnke et al. Emotion Review, February 4, 2022. https://doi.org/10.1177/17540739211073084

Abstract: Autonomic nervous system (ANS) activity is a fundamental component of emotional responding. It is not clear, however, whether positive emotional states are associated with differential ANS reactivity. To address this issue, we conducted a meta-analytic review of 120 articles (686 effect sizes, total N = 6,546), measuring ANS activity during 11 elicited positive emotions, namely amusement, attachment love, awe, contentment, craving, excitement, gratitude, joy, nurturant love, pride, and sexual desire. We identified a widely dispersed collection of studies. Univariate results indicated that positive emotions produce no or weak and highly variable increases in ANS reactivity. However, the limitations of work to date – which we discuss – mean that our conclusions should be treated as empirically grounded hypotheses that future research should validate.

Keywords: positive emotions, autonomic nervous system, cardiovascular activity, electrodermal activity

Discussion

In this quantitative review, we aimed to inspect, evaluate, and synthesize (to the extent possible) findings from past research that measured a physiological component of positive emotions. We found the available data to be quantitatively imbalanced, with many studies focused on some positive emotions and physiological signals and few studies focused on other positive emotions and physiological signals. Furthermore, we found high variability in methods used for emotion elicitation and data collection. Recognizing that the empirical evidence might be insufficient to test some effects, we aimed to use all available empirical data and stringent criteria for multiple hypothesis testing to examine whether the currently available empirical findings allow us to conclude that positive emotions elicit ANS reactivity. We also explored whether the ANS reactivity is specific to discrete emotions (in terms of patterns and magnitudes) or—alternatively—whether similar ANS reactivity accompanies all positive emotions. Finally, we tested participant characteristics and methodological factors as moderators of the ANS reactivity to discrete positive emotions. One main and three secondary findings emerged.

Based on univariate analyses, most discrete positive emotions elicited no or weak ANS reactivity. Moreover, half of the effect sizes in ANS responses were highly inconsistent, suggesting that other significant physiological variability sources exist. We also found that similarities outweighed differences in ANS responses during positive emotions. This contrasts the literature suggesting a stronger physiological differentiation among discrete positive emotions (Kreibig, 2010; Shiota et al., 2017). Finally, we found few moderating effects of study or participant characteristics. Thus, the current empirical material supports the view that positive emotions produce no or only a weak and nonspecific ANS response relative to baseline and neutral conditions (Cacioppo et al., 2000; Lench et al., 2011; Siegel et al., 2018).

However, we emphasize that these conclusions must be considered tentative because they are based upon imbalanced and incomplete data and one type of analysis (univariate, not multivariate). This suggests the need for more systematic research on the physiology of positive emotions that will fill existing gaps and provide material for future robust evaluation of positive emotions and ANS activity. For instance, impedance cardiography that is often applied to the study of stress and negative emotions is relatively underrepresented in the study of positive emotions. Moreover, the psychophysiological study of amusement is greatly overrepresented relative to gratitude, pride, or love. Finally, we advocate more multivariate sampling and analysis of emotional responses in positive emotions.

Positive Emotions and ANS Reactivity: The State-Of-The-Art

We based this review on the most extensive collection of available studies, which produced over 686 effect sizes derived from 6,546 participants. However, in the coding process, we observed substantial variability across this large number of studies, which resulted in the collection of a widely dispersed dataset. The studies were conducted in different settings. For instance, the laboratories used various equipment, procedures, and data cleaning and analysis techniques. Most studies examined only reactivity related to two emotions using 2-3 ANS measures with a single elicitation method. Furthermore, when coding the possible moderators, in many cases, we were unable to determine whether participants were alone during the experimental task or the experimenter stayed in the room after placing the physiological sensors. We found that only 44% of studies explicitly stated that they video recorded participants during the experiments. In this way, our meta-analysis supports the message from the recent comment on the current state of the science of positive emotion (Shiota, 2017) - although the field made incredible progress in the last decades, affective scientists are still far away from the promise of this field being fully realized. This also includes the call for more detailed reporting of procedures (e.g., were participants explicitly observed) and data use (e.g., duplicate datasets).

The data collected so far on positive emotion is insufficient to strongly support the ANS specificity versus similarity for a wide range of positive emotions presented in our investigation. Our analyses were challenging due to a widely dispersed dataset with small numbers of studies per emotion and per ANS measure. Moreover, some comparisons and analyses were performed on a relatively small number of effect sizes. Although the field of affective science struggles to just-decide-already whether specificity or similarity of ANS emotion-related reactivity is the ground truth, with this review, we observed that the current state-of-the-art is not sufficient to address this expectation for definite conclusions. While reviewing hundreds of studies, we also observed that researchers moved quickly to asking complex questions related to functions of targeted emotion without addressing more basic questions, e.g., which ANS parameters are adequate for studying a specific positive emotion. We suggest that a return to more basic questions might advance the field of psychophysiology of emotions. It would be beneficial for the field of positive emotions to further examine ANS reactivity, in particular, to positive emotions that have not been explored yet, such as hope or schadenfreude, using multiple ANS measures. We advocate that the field of positive emotions would benefit from greater integration and uniform standards/rigor for emotion elicitation and data curation, analysis, and reports.

Do Positive Emotions Produce Robust Changes in ANS Reactivity?

We found that the set of inspected positive emotions produce no or weak increases in ANS reactivity in both SNS and PNS. Our findings are consistent with models of positive emotions that assumed that positive emotions do not generate independent sympathetic responses (Fredrickson & Levenson, 1998; Fredrickson et al., 2000; Fredrickson, 2013; Folkman, 2008; Levenson, 1988, 1999). Moreover, we did not observe increased PNS reactivity in positive emotions, as suggested by the polyvagal theory (Porges, 2011). One explanation for these null results is that large differences across experiments might be responsible for the responding range. Figure 2 presents that even the mean effects interpreted as medium sizes had wide confidence intervals that prevented them from being significant. These findings are consistent regardless of focusing only on separate ANS measures, measures that had more than ten studies, or measures aggregated into broader categories.

Our findings match previous meta-analyses focused on happiness that concluded that happiness produces weak ANS reactivity and that this reactivity is not different from neutral conditions (Lench et al., 2011; Siegel et al., 2018). Our findings do not support conclusions from qualitative reviews in which some positive emotions such as contentment and love decreased cardiovascular or electrodermal activity (Kreibig, 2010). However, more than half of the physiological responses’ directions in the qualitative review were based on fewer than three studies, suggesting that these findings were preliminary (Kreibig, 2010). With additional studies that were published over the last decade, we found support for the previously found directions of the electrodermal reactivity (increases) to positive emotions. We also found support for increased ANS activity to joy and amusement (Kreibig, 2010).

Are ANS Reactivity Patterns Specific to Particular Positive Emotions?

The main goal was to provide a quantitative review of the body of research related to ANS reactivity and positive emotions. However, we also evaluated the specificity or generality of ANS reactivity to discrete emotions. The basic expectation in this meta-analysis was that discrete positive emotions produce specific adaptive changes in physiology (Ekman & Cordaro, 2011; Levenson, 2011; Panksepp & Watt, 2011; Kreibig, 2010).

We found that similarities outweighed differences in ANS responses during positive emotions. This finding is consistent with the models that view ANS reactivity to emotion as context-sensitive and not discrete-emotion-sensitive (Barrett, 2013, 2017; Quigley & Barrett, 2014). Thus, the ANS reactivity is not random but is specific and supports actions in the specific context, which could vary for the same discrete emotion (Barrett, 2006; Barrett & Russell, 2015; Quigley, & Barrett, 2014). Theorists suggest that multiple distinct, context-sensitive physiological responses to discrete emotion are possible, as long as both serve the same adaptive function, e.g., freezing versus fleeing from a threat in fear (Ekman, 1992).

However, the ANS reactivity is only one component of emotional responding. Thus, major judgments about the structure of emotions should be interpreted along with affective and behavioral responses and should not be based solely on any one component.

Shared ANS reactivity to positive emotions might be related to common neural origin from a highly conserved circuit of neural structures, namely the mesolimbic pathway, often called the “reward system” (see Shiota et al., 2017 for discussion). The activation along the mesolimbic pathway has been linked to a wide range of stimuli associated with the family of positive emotions, including delicious foods (Berridge, 1996), monetary incentives (Knutson et al., 2001), babies (Glocker et al., 2009), loved ones (Bartels & Zeki, 2004), humor (Mobbs et al., 2003), and favorite music (Blood & Zatorre, 2001). It may explain the mechanism by which the discrete positive emotions share some overlapping properties that might be further differentiated depending on the conditions in which positive emotions are activated. Overlapping properties of positive emotions and continuous gradients between discrete emotion categories have been found in recent large-scale investigations (Cowen & Keltner, 2017). That study has shown that emotions were more precisely conceptualized in terms of continuous categories, rather than discrete emotions, showing smooth gradients between emotions, such as from calmness to aesthetic appreciation to awe (Cowen & Keltner, 2017).

Supporting the dimensionality of emotions, we found differences along the dimension of approach motivation (Gable & Harmon-Jones, 2010; Harmon-Jones et al., 2013). Positive emotions characterized by strong approach tendencies, such as joy and excitement, were accompanied by a higher sympathetic reactivity (e.g., DBP, MAP) than low-approach positive emotions like amusement. Our investigation may serve future studies to conceptualize positive emotions in terms of physiological arousal starting from the least arousing and ending with the most arousing positive emotions, namely awe, attachment love, gratitude, nurturant love, contentment, excitement, amusement, pride, craving, sexual desire, and joy.

Are There Moderators of ANS Reactivity to Positive Emotions?

We investigated several moderators that we thought might influence physiological responsiveness to emotions, but most did not moderate the observed effects. Only in craving did we observe a significant moderating effect of the elicitation method on the physiological response. We observed that behavioral methods, namely, exposure to food, produced stronger ANS reactivity than pictures, films, and imagery. This observation indicates the advantage of using active rather than passive emotion elicitation methods. Furthermore, in line with Lench and colleagues (2011), we found that reactions to craving were stronger when the proportion of women in samples increased.

Contrary to our expectations, we found no influence of several continuous variables on physiological reactivity to positive emotions, including age, sex proportion, participant number, and study quality (Kret & De Gelder, 2012; Stevens and Hamann, 2012; Mill et al., 2009; Sullivan et al., 2007). Like the previous meta-analysis of Lench and colleagues (2011), we found no evidence that the participants’ age influenced the degree of emotional reactivity. However, this may be due to imbalanced age distribution (skewed young), meaning we were underpowered to detect age differences. A thorough examination of how age influences the emotional experience's intensity requires additional research with older participants. In line with previous meta-analyses, we found no sex differences in response to positive emotions (Joseph et al., 2020; Siegel et al., 2018). Finally, although we found indications of publication bias for some pairs of ANS reactivity and positive emotion, we conclude that most mean effect sizes seem to be robust and unlikely to be an artifact of systematic error.

Limitations and Future Directions

First, as we emphasize throughout the paper, the conclusions we present are provisional and contingent upon current data availability. More definitive conclusions will await additional research, particularly on under-researched positive emotions and measures.

Second, in this project, we used a univariate approach to analyze the mean ANS reactivity to discrete positive emotions in a series of meta-analyses. Although available multivariate meta-analytic approaches (Riley et al., 2017) would provide a better fit to the characteristic of emotions (Kragel & LaBar, 2013; Stephens et al., 2010), several factors militated against using a multivariate approach. For instance, a multivariate meta-analysis requires a correlation matrix between the ANS measures. This was not possible to obtain because only 7 out of 128 articles included in our investigation reported correlations between some ANS measures. Along similar lines, for many analyses (e.g., amusement), two or more ANS measures were never observed jointly in the same study. Moreover, a previous meta-analysis found that multivariate pattern classifiers did not provide strong evidence of a consistent multivariate pattern for any emotion category (Siegel et al., 2018). Of note, the multivariate and univariate models produce similar point estimates, but the multivariate approach usually provides more precise estimates. Thus, the benefits of a multivariate meta-analysis are small (Riley et al., 2017). The advantages of using multivariate meta-analysis of multiple outcomes are greatest when the magnitude of correlation among outcomes is large, which was not the case for most of our analyses. In conclusion, our approach can produce statistically valid results for each pair of positive emotion and ANS reactivity measures (Pustejovsky & Tipton, 2021). Future studies might collect multiple physiological measures when studying ANS reactivity to emotions (Cacioppo et al., 2000) to provide data that allows for robust multivariate analyses.

Third, we used univariate statistics, which disrupt the physiological response's continuity and treat the entire emotion manipulation as a separate piece. Although univariate methods have historically dominated the literature (see Cacioppo et al., 2000; Kreibig, 2010, for the reviews), future reviews may use multivariate approach data to replicate our findings.

Fourth, during the coding, we relabeled examined emotions in many studies. However, we found no differences in effect sizes extracted from studies with the original emotion label and effect sizes extracted from studies for which we renamed the emotion label. Moreover, the conclusions that come from our literature research stress the importance of using precise terminology in emotion-related literature. The overview of existing empirical and theoretical models indicates not only a variety of discrete positive emotions but also a variety of terms used to describe them. For instance, researchers used different labels for emotions elicited by funny situations, such as amusement (Kreibig et al., 2013), happiness (Kring & Gordon, 1998), or mirth (Foster et al., 2003). The heterogeneity of labels suggests problems with discrete emotions’ construct validity and measurement invariance. Future research would benefit from a more uniform nomenclature and definitions accepted by researchers within affective science.

Fifth, most of the theoretical and empirical models conceptualize individual positive emotions without clearly addressing how different positive emotions might be interrelated (e.g., Cowen and Keltner, 2017; Ekman and Cordaro, 2011; Tong, 2015; Weidman & Tracy, 2020, with the exception of Kreibig, 2014; and Shiota et al., 2017). It might be useful to group discrete emotions into families or clusters based on their similarities. For instance, joy and excitement are similar emotions associated with progress in achieving one's goals, but excitement has an anticipatory response compared to joy that brings well-being and good fortune after an event (Lazarus, 1991; Shiota et al., 2017; Smith & Kirby, 2010). Future studies may focus on examining similarities rather than differences between positive emotions. Researchers should also balance between generalization and differentiation in studying emotions.

A sixth limitation resulted from including studies that examined physiological reactivity from baseline or neutral conditions. The results produced by these studies differ due to differences in the design of these studies. Some research used neutral movies as a baseline (e.g., De Wied et al., 2009), whereas other studies used neutral videos in the experiment (e.g., Codispoti et al., 2008). We followed the theoretical premise that both baselines and neutral conditions should be emotionally impartial. A moderation analysis showed that the type of comparison used in primary studies had no effect on the size of the physiological reactivity. These results allowed us to examine further hypotheses, but both decisions of including different types of comparison and relabeling the emotion categories may have produced bias in the results of our meta-analysis. Given the considerable increase in psychophysiological research on emotions in recent years, future meta-analytic work would provide empirical support for the emotional impartiality of baseline and neutral conditions.

Seventh, we found substantial research methodology variability in primary studies. We tested whether the study quality moderated the effect sizes by assessing the presence of exclusion criteria, manipulation check procedures, and protocols for reporting or handling artifacts and missing data. We found those three measures to be objective indicators of study quality. However, the study quality had no effect on the size of physiological reactivity. More studies that include multiple ANS measures and multiple discrete positive emotions with diverse samples are required to strengthen broad inferences about ANS responses to positive emotions. Future studies might also examine how emotions differ in ANS reactivity rather than asking whether emotions generally differ physiologically (e.g., Berntson et al., 1991; Stemmler, 1992). ANS reactivity produces the optimal bodily milieu to provide physiological support for behaviors associated with discrete emotion (Levenson, 2014). This requires unique configurations of multiple physiological responses rather than a single unique physiological change (Levenson, 2014). Single ANS measures might not be sufficient, given that most physiological measures used in the emotion-related literature constitute the physiological outcome of emotion-related states, showing a one-to-many relation between the physiological measure and emotions (see Cacioppo et al., 2000; Richter & Slade, 2017 for the discussion). Groups of emotions may lead to similar general activation that occurs in response to an upcoming action (Brehm, 1999; Fredrickson & Levenson, 1998; Frijda, 1987). For instance, excitement, craving, or sexual desire prepare the organism to “be ready for action,” and they produce similar sympathetic activation. However, more specific activity might be observed in targeted organs. For instance, craving might be observed in the gastrointestinal tract, sexual arousal in the genital system, and excitement in the locomotor system (Levenson, 2011, 2014).

Eighth, most of the ANS variables included in our meta-analysis are blends of SNS and PNS activation (e.g., HR). The measures more specifically related to PNS or SNS measures (e.g., RSA or PEP) were not broadly assessed across the positive emotions. Thus, we could not fully address whether positive emotions produce pure SNS or PNS activity but rather the co-product of one of the two systems.

Nineth, we believe that our meta-analysis opened the discussion for methodological issues in the psychophysiology of emotions that would be worth testing empirically. For instance, for moderators, we focused on the length of the time interval used to calculate the physiological levels for baselines and emotion manipulations. Although we did not find effects of the time interval on mean ANS reactivity to positive emotions, we believe that comparing different time intervals of the same physiological measure is problematic. Similarly, researchers usually used the same time intervals to present the reactivity of all ANS measures despite differences across the family of ANS variables. Thus, scientists tended to sacrifice the specificity of particular ANS measures for the sake of a uniform data analysis strategy.

Women perceived greater competitive tendencies in their fellow females when resources were plenty, rather than scarce

Bradshaw, H. K., Krems, J. A., & Hill, S. E. (2022). Resource availability differentially influences women’s perceptions of same- (versus cross-) sex others’ competitiveness. Evolutionary Behavioral Sciences, Feb 2022. https://doi.org/10.1037/ebs0000289

Abstract: Across four studies, the current research tested the prediction that women would perceive greater competitive tendencies in same- (vs. cross-) sex others when resources were scarce. Contrary to predictions, results found evidence that women perceived more competitive tendencies in same- (vs. cross-) sex targets when resources were abundant. Study 1 demonstrated that women (but not men) perceived greater competition within groups of female same-sex targets (vs. groups of male same-sex targets and groups of cross-sex targets) residing in ecologies where resources were widely available; no such pattern emerged when judging competition within groups residing in ecologies where resources were scarce. In Study 2, women (but not men) who held relatively low levels of resource scarcity beliefs (i.e., those who believed resources were relatively abundant) attributed greater competitive tendencies to same-sex targets than cross-sex targets. Study 3 showed that enacting a resource abundance (but not a scarcity) mindset led women to expect same-sex targets to behave more competitively toward them than cross-sex targets; this effect, however, did not replicate in Study 4. With the exception of Study 4, these data suggest that, contrary to intuition—and our predictions—women perceive same-sex others to be more competitive than cross-sex others when resources are abundant.

Do restaurant customers who receive an unreasonably low bill bring it to the server’s attention? A field experiment on dishonesty

Do restaurant customers who receive an unreasonably low bill bring it to the server’s attention? A field experiment on dishonesty. Yossef Tobol, Erez Siniver, Gideon Yaniv. Journal of Economic Psychology, February 4 2022, 102491. https://doi.org/10.1016/j.joep.2022.102491

Abstract: Inspired by Azar et al., 2013, Azar et al., 2015 study of customers’ tendency to return excessive change in a restaurant, the present paper reports the results of a field experiment destined to examine restaurant customers’ honesty in a different real-life situation where customers receive an unreasonably low bill. To ensure that customers noticed the error in the bill, the experiment was restricted to single customers who ordered two items only (e.g., coffee and a toasted sandwich), presenting them with a bill from which one item, either the more or the less expensive one, was omitted. A majority of customers (169 out of 278) failed to bring the error to the server’s attention. Several factors appeared to have a large effect on the decision of whether to report the bill error. Female customers reported the error in the bill much more often than male customers, customers whose more expensive item was omitted from the bill reported the error twice as often than customers whose less expensive item was omitted, repeated customers (presumably) reported the error much more often than one-time customers and customers who paid with a credit card reported the error much more often than customers who paid in cash. We also consider the possibility that the means of payment is a choice variable determined by the decision of whether or not to bring the error in the bill to the server’s attention.

Keywords: Field ExperimentRestaurant CustomersBill ErrorDishonest Behavior

Check also A field experiment on dishonesty: A registered replication of Azar et al. (2013). Jakub Prochلzka, Yulia Fedoseev, Petr Houdek. Journal of Behavioral and Experimental Economics, September 2020, 101617. https://www.bipartisanalliance.com/2020/09/replication-of-azar-et-al-2013-70-of.html

It appears some demonstrated experimental effects depend more heavily on personality than previously thought: Serious Problems With Interpreting Rubber Hand “Illusion” Experiments

Serious Problems With Interpreting Rubber Hand “Illusion” Experiments. Warrick Roseboom; Peter Lush. Collabra: Psychology (2022) 8 (1): 32274. https://doi.org/10.1525/collabra.32274

The rubber hand “illusion” (RHI), in which participants report experiences of ownership over a fake hand, appears to demonstrate that subjective ownership over one’s body can be easily disrupted. It was recently shown that existing methods of controlling for suggestion effects in RHI responding are invalid. It was also shown that propensity to agree with RHI ownership statements is correlated with trait phenomenological control (response to imaginative suggestion). There is currently disagreement regarding the extent to which this relationship may cofound interpretation of RHI measures. Here we present the results of simulated experiments to demonstrate that a relationship between trait phenomenological control and RHI responding of the size reported would fundamentally change the way existing RHI results must be interpreted. Using real participant data, each simulated experiment used a sample biased in selection for trait phenomenological control. We find that using experiment samples comprised only of participants higher in trait phenomenological control almost guarantees that an experiment provides evidence consistent with RHI. By contrast, samples comprised of only participants lower in trait phenomenological control find evidence for RHI only around half the time – and of greater concern, evidence specifically for “ownership” experience just 4% of the time. These findings clearly contradict claims that the magnitude of relationship between phenomenological control and RHI responding is a minor concern, demonstrating that the presence of participants higher in trait phenomenological control in a given RHI experiment sample is critical for finding evidence consistent with RHI. Further study and theorising regarding RHI (and related effects) must take into account the role that trait phenomenological control plays in participant experience and responses during RHI experiments.

Keywords: embodiment, phenomenological control, hypnosis, imaginative suggestion, rubber hand illusion

Discussion

Quantitative study of subjective body ownership in the RHI is accomplished using participant ratings of (dis)agreement with a series of subjective statements about referred touch (S1 and S2) and (putatively) ownership (S3). Propensity to agree with these statements has previously been shown to be related to trait phenomenological control – the domain general ability to meet expectancies arising from direct or implicit imaginative suggestion (including demand characteristics; Dienes, Lush, et al., 2020; Lush et al., 2020). It has been claimed that this reported relationship is too small to be of concern (Ehrsson et al., 2021; Fan et al., 2021). To concretely demonstrate the potential influence of this relationship, here we simulated a series of experiments with different degrees of sampling bias, selecting a disproportionate number of participants with higher trait phenomenological control. Given the relationship between subjective agreement and phenomenological control, this induced sampling bias may be analogous to the deliberate exclusion of participants (e.g. Chancel & Ehrsson, 2020; Ehrsson et al., 2005) who don’t report a strong experience of the RHI (up to ~30% of people; Riemer et al., 2019). We show that the frequency with which a RHI experiment will provide evidence consistent with subjective agreement for a RHI is directly related to the proportion of participants in the experiment sample that are higher in phenomenological control. Put more concretely - only experiments run with a large proportion of participants higher in phenomenological control will provide evidence for the RHI. This finding directly contradicts the statement that the reported relationship between RHI responding and trait phenomenological control is too small to be of concern.

Why does selecting participants based on trait phenomenological control affect whether a RHI study will conclude in favour of evidence for RHI or not? It can only be because they are substantially related – reiterating the result reported by Lush et al. (2020). Why is it a problem if the same people who are likely to report agreement with RHI questions also happen to be higher in phenomenological control? To interpret the clearly substantial relationship, we must first consider the nature of the constructs these measures are supposed to represent – body ownership for RHI responding; and propensity to respond to suggestion, implicit or explicit, with imagination for phenomenological control. Phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. This is evident both in the measure of phenomenological control used in Lush et al. (2020; SWASH), which interrogates a variety of possible experiences, not just those related to bodily experience or ownership (e.g. abilities related to amnesia or musical and visual hallucination, in response to suggestion) and in several recent findings: Relationships between trait phenomenological control and anomalous experiences when demand characteristics (and therefore participant hypothesis awareness) have not been adequately controlled have been reported (Lush et al., 2020) for other body-related effects including vicarious pain (reports of pain in response to seeing people in apparently pain-inducing situations) and mirror-touch synaesthesia (reports of felt touch in response to seeing people touched), and also for non-body related experiences like the visually-evoked auditory response (Lush, Dienes, Seth, et al., 2021; reports of sound experienced when watching silent videos). The ability to control experience in response to suggestion is evident across perceptual and decision domains and there is much evidence that imaginative suggestion effects can be experienced as subjectively ‘real’ (see Dienes, Palfi, et al., 2020; Lynn et al., 2020; McConkey, 2008), and that hypnotisability is distinct from social compliance (e.g. see Moore, 1964; Tasso et al., 2020; but see Polczyk & Pasek, 2006). In an explanatory framework, a domain general process subsumes domain specific demonstrations of its operation. Therefore, we are left with limited options for interpretation of the result from Lush et al. (2020):

1. participants who can control their experience across sensory and decision domains through phenomenological control are also controlling their experience in RHI experiments (as suggested by Lush et al., 2020)

2. participants who can control their experience in other domains through phenomenological control happen to be the same participants who can experience the RHI, but for distinct reasons (e.g. pure multisensory integration account)

3. the same participants who can control their experience in other domains don’t control their experience in RHI but respond as though they do for other unspecified reasons – perhaps a response bias induced by social context or pure confabulation just for RHI case

The first interpretation is perhaps the most parsimonious in that it requires only that we reconsider the RHI as an act of phenomenological control in response to suggestion and that it allows for group level report of RHI ownership experience to reflect genuine experience. It is important to note that this position does not rule out a multisensory contribution to the RHI, but rather that another cognitive process is in play. This is an important distinction. If we were to say that there is no problem treating an act of phenomenological control (which is necessarily creative and interpretative) as evidence for a sense of ownership, we might also study visual perception through phenomenological control in response to imaginative suggestion. If people report the experience of “seeing” a unicorn in response to a direct or indirect imaginative suggestion, we would be obliged to take this as evidence for the existence of unicorns on the same basis that RHI researchers claim that reports of the RHI are evidence for a sense of body ownership. Certainly, visual imagery is constrained by the properties of visual processing and previous perceptual experience, as is the RHI likely constrained by the properties of (multi)sensory processing and previous experience. The existence of such constraints is not in contention. But the interpretation of imaginatively suggested experience is not the same as studying the properties of multisensory perception, as is the common interpretation of RHI studies (e.g. Ehrsson et al., 2005). Saying that you are “studying the rubber hand instantiation of the influence of phenomenological control on perception and decision making” is perfectly reasonable. But this statement is not consistent with the broader claims made in most papers regarding RHI where it is stated that the RHI represents a fundamental case study in understanding how subjective body ownership is determined in humans.

The second interpretation, while less parsimonious than the first may still be possible and is not explicitly ruled out by the results presented here or previously. However, given that hypothesis awareness is not controlled for in the existing RHI literature (Lush, 2020; Lush, Seth, et al., 2021; Reader, 2021), evidence for this interpretation cannot depend on that existing literature until the possible influences of phenomenological control are ruled out. There are methods by which phenomenological control effects may be controlled; control conditions which are not confounded by demand characteristics could be developed by a two-step procedure which takes into account both expectancies and differences in the relative difficulty of suggestion effects (see Lush, 2020 for details). This requires a difficult process of matching expectancies and direct imaginative suggestion response across candidate illusion and control measures. Note that while this makes the study of embodiment illusions more challenging than has been the tradition, the combined evidence from many small sample studies in which demand characteristics were uncontrolled has only generated illusory evidence which is of no value to theories of embodiment (except perhaps as a cautionary tale). Similar issues have been dealt with successfully in other fields. For example, the suggested approach has much in common with the use of placebo controls in clinical trials, which, although it adds considerable cost to experiments, has been standard practice for decades (Beecher, 1955).

The third interpretation should be considered in light of evidence that phenomenological control may confound interpretation of other effects. The RHI is one of four distinct effects for which such relationships have now been shown (Lush, Dienes, Seth, et al., 2021; Lush et al., 2020) which involve a range of modalities. Correlation is not causation, of course, and relationships between the RHI and phenomenological control may be attributable to some unknown cause (perhaps some difference in multi-sensory integration mechanisms common to RHI response and phenomenological control - even for cases of phenomenological control that require no multisensory percept). However, given the simplicity of the proposal that demand characteristics can act as implicit imaginative suggestions in measures of experience, and the growing evidence that this occurs in a range of effects, it is a more parsimonious explanation that all of these effects are confounded by phenomenological control than that some other explanation is in play for each individual case.

Note that, whenever expectancies are consistent with experimental aims (e.g. participants are hypothesis aware), any given result may be attributable to hypothesis awareness effects other than phenomenological control (e.g., faking or imagination; see Corneille & Lush, 2021). However, for both the RHI and response to imaginative suggestion, there is extensive evidence that at least some reports reflect genuine experience (see Dienes, Palfi, et al., 2020 for a discussion of this evidence as it relates to phenomenological control). Of course, if phenomenological control scales were to reflect trait differences in faking (for example), this would be an important consideration for interpretation of the RHI (and other reported experiences which correlate with trait phenomenological control).

Researchers familiar with the RHI will be likely to note that these results (and those of Lush et al., 2020) only relate to the subjective statements, while there is extensive evidence from ‘implicit’ measures such as proprioceptive drift, and convergent evidence from neuroimaging and animal studies. As mentioned previously, there are only two aspects linking RHI to phenomenological change – extended report and subjective ratings. All other findings, such as proprioceptive drift or changes in measured blood flow (fMRI) or scalp potential (EEG), are linked to changes in subjective ownership only through broad correspondence or co-occurrence with these subjective reports. If human participants’ ratings on the subjective statements are predominantly related to their trait phenomenological control, then proprioceptive drift (or neuroimaging) results are correlating with phenomenological control, not specifically rubber hand subjective ownership. Again, this is because phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. Note that the asynchronous control measure is also employed for “implicit” RHI measures (e.g., proprioceptive drift), and participants show hypothesis awareness for these effects (Lush, Seth, et al., 2021).

Regarding animal studies (e.g. Fang et al., 2019; Wada et al., 2016), while it may be completely reasonable to posit that, for instance, a rat may have a sense of body ownership, the reverse inference on which such claims rely when referencing RHI is problematic. Because it is not possible to simply ask an animal about their subjective experience, to conclude that an animal has a change in subjective experience of body ownership during, for example, a “rubber tail illusion” paradigm (Wada et al., 2016) we start with a potential relationship in human participants between a behavioural observation and subjective reports (e.g. correlate proprioceptive drift measure in hand with magnitude or time course of change in subjective ownership reports). We then attempt to obtain observations of a similar behaviour in rodents (e.g. something like proprioceptive drift of tail). We must then make the conceptual leap that the observed behavioural phenomena (proprioceptive drift in humans and something like it in rodents) are equivalent. Through this combination of directly observing one relationship related to subjective experience of ownership (subjective ownership responding and proprioceptive drift are related in humans), then observing this single phenomenon in rodents (behaviour with similar properties to proprioceptive drift), that we have assumed to be equivalent to a similar observation in humans (hand proprioceptive drift), we can end up with the reverse inference that the rodent also has change in subjective experience of ownership. Importantly, there is, and can be, no direct evidence to support this claim. It is important to also note that the same issue of reverse inference is present in any study using implicit measures that doesn’t provide evidence for concurrent changes in subjective experience (e.g. Tsakiris & Haggard, 2005). If the foundation statement about human subjective body ownership on which this set of inferences is based is instead reflecting some other ability (e.g. to control experience in response to suggestion – phenomenological control), then it is clear that all subsequent inferences are questionable. This is not to say that nothing can be discovered about multisensory perception of the body using neuroimaging or animal models. Our claim in no way invalidates behaviourist approaches that seek to map body perception to neural function through observed behaviour. However, any instances wherein these findings are related to subjective ownership through their connection with RHI-like paradigms are clearly problematic if the original basis for claims of changes in subjective body ownership during rubber hand exposure are not being correctly interpreted.

It is sometimes claimed (e.g. Ehrsson et al., 2021; Tsakiris, 2010) that RHI effects reflect a combination of top-down and bottom-up processes. However, these top-down effects are believed to be distinct from demand characteristic effects and RHI response is considered to be “resistant to suggestions, thoughts, and high level conceptual knowledge” (Ehrsson et al., 2021). Only a few researchers have stated theoretical positions consistent with the theory that demand characteristics play an important role in these effects (Alsmith, 2015; Dieguez, 2018). An influential theory proposes that top-down effects arise when internal representations of body image are matched against incoming sensory signals (Tsakiris, 2010; though see Chancel & Ehrsson, 2020 for a counter-argument that apparently top-down effects are attributable to multisensory congruency). However, given that demand characteristics have not been controlled in existing RHI studies, it is premature to consider theoretical proposals of top-down effects which are not related to demand characteristics.

In sum, agreement with three different subjective statements is the standard basis for linking all aspects of RHI phenomena with body ownership. Propensity to agree with these reports is strongly related to the domain general ability for phenomenological control. Experiments wishing to link RHI experience specifically with subjective body ownership must take this relationship into account or their results cannot be taken to indicate general properties of human body ownership.