We found that the effects of meditation interventions on prosociality were limited. The methodological quality of 61% of the studies was graded as weak. Although there was a moderate increase in prosociality when considering all studies, further analyses indicated that this effect was qualified by two factors: category of prosociality and methodological quality. Meditation interventions had an effect on the categories of compassion and empathy, but not on aggression, connectedness, or prejudice. The low number of empathy studies prevented a more detailed analysis of moderators. For the category of compassion, we found that methodological quality impacted the outcomes so that an increase from baseline to post-intervention was moderated by the use of a meditation teacher. Specifically, the moderation results showed that a significant increase in compassion only occurred if the intervention teacher was a co-author in the published study, but not when the intervention was delivered by other means (written/audio) or by a teacher that was not a co-author in the publication. The results for compassion were also moderated by the type of control group; specifically, the effect became non-significant when an active control group was used. Although the effect size remained similar for active and passive controls, the confidence intervals showed a much wider variation in the results for studies employing an active control group (−0.04, 0.78) indicating that some studies produced results where there were no changes in compassion, or there was a reduction from baseline to post-intervention. Overall, the weak methodological quality of the studies and the results for the moderator analysis indicates low confidence in the validity and replicability of the examined studies. The studies used a wide range of prosocial outcome measures, including self-report instruments, implicit association tasks, and behavioural measures. Not all of these had been previously validated, but the crux of the methodological shortcomings lies elsewhere, as the quality of studies analysis and moderator analysis reveals. Below we expand on these shortcomings and how they might be avoided in future studies.

On the whole, there was some evidence favouring the prosocial effects of meditation, but this was limited by various factors, including: (1) the finding that the initial results were moderated by the meditation effects on empathy and compassion alone; (2) that the effects on compassion were moderated by the type of control group used and the role of the teacher; (3) the weak methodological quality of the studies. Concerning point (1), it is intriguing that we found that the effects of meditation on empathy and compassion were significantly stronger than for the other types of prosociality. One explanation could be that the meditation interventions focused on the development of empathic and compassionate qualities. This is clearly the case for the majority of the compassion studies analysed, where the meditation training (LKM and CM) used statements that explicitly asked participants to focus on feelings such as ‘love’ and ‘kindness’ towards oneself and others. Also, these studies used as the major outcome measure a scale of Self-Compassion that assessed the same emotions that were elicited during the intervention35,37,38,47. Only one study attempted to extricate the affective element in the intervention by including two different types of meditation11. On the other hand, the studies on aggression, prejudice and connectedness tended to use a type of mindfulness-based meditation which did not directly mention qualities of reduced aggression, prejudice or increased connectedness.

Regarding point (2), that the effects of meditation on compassion were only significant when compared to passive control groups suggests that other forms of active interventions (like watching a nature video) might produce similar outcomes to meditation. Another meta-analysis has shown a similar pattern of non-significant or weak results concerning the effects of meditation on psychological stress and well-being when compared to active controls20. The second moderator we found is more controversial, and this seems to be a novel finding. In what way does the joint status of study co-author and meditation teacher affect the results of the compassion studies? At best this shows that a motivated meditation teacher will impact to a greater extent one’s students; at worse, it suggests that experimenter biases are introduced which affect the outcomes of the studies. These are just one kind of bias that are likely to be affecting studies in this area and which we review below (point 3), alongside offering potential solutions to overcoming them.

Our assessment of the quality of the studies identified several methodological weaknesses, which increase the likelihood that biases were introduced. First, despite Rosenthal’s48 well-known work on experimenter biases and the importance of using double blind designs in experimental psychology, meditation studies seldom try to avoid this particular bias. Recent work indicates that experimenter biases are not a thing of the past. When Doyen and colleagues49 attempted to replicate a previous experiment suggesting that priming participants with age-related stereotypes had an effect on walking speed50, they failed to find any significant results when using a double-blind procedure (prior studies were not blinded). They further showed that when making some experimenters believe that priming participants with age-related stereotypes would slow them down, this indeed had a significant effect, but only in those experimenters that were made to believe in the stereotypes. This example illustrates how experimenter beliefs can directly influence the outcome of a study.

In the context of the studies included in our review, authors provided the training in 48% of the studies. Only one study stated that the meditation instructor, although being part of the research team, had no part in the data analysis (Neff and Germer, personal communication) and another study engaged an external instructor who was not connected to the study in any other way41. Information about the intervention teacher could not be obtained for 10% of the studies and the remaining 42% used audio recordings or online instructions. In sum, for about half of the studies we reviewed, unintentional experimenter biases could have been introduced by researchers/teachers with a personal interest in the intervention (e.g., by giving preferential treatment or being particularly enthusiastic to participants in the experimental group).

But the prevalence of experimenter effects is only one side of the coin. The media portrayal of meditation as a cure for a range of mental health problems or to improve well-being51 is very likely to feedback into participants who will have a high expectation of the benefits of a meditation intervention. Despite potential to introduce unintentional expectation bias in participants, only one of the studies we examined controlled for expectation effects and this methodological concern is generally absent in the meditation literature. The exception was a study by Creswell and colleagues22, which included a four item scale assessing beliefs about the efficacy and relevance of the intervention (meditation versus analytical training) and found that the meditation group had substantially higher expectations of a positive effect for the intervention, even though participants were not explicitly told that they were engaging in meditation.

What is the solution to unintentional experimenter and expectation effects? Acknowledging them is a good starting point and supplementing this with short scales that try to measure participants’ expectations can only improve the validity of studies — for an example from the anxiety treatment literature, see the Credibility and Expectancy Questionnaire52. But more can be done. Importantly, larger gains can be made by introducing at least some aspect of blinding procedures in randomized controlled studies. Then, the challenge is to find suitable interventions that can function as active control conditions. An interesting solution was developed by Smith53, who developed a 71-page manual describing the rationale and benefits of a made-up meditation technique. He gave the manual to a research assistant, who was unaware that it was a placebo, and who then proceeded to give a lecture to participants in the control group about the merits of the technique (very much like in the experimental group that used Transcendental Meditation). When it came to the actual placebo technique, participants were instructed to sit quietly for 20 minutes twice per day in a dark room, and to think of anything they wanted. Although this was an innovative approach, we acknowledge that it may create other problems such as the elimination of intentional positive expectancy.

In sum, the negative impact of experimenter and expectation biases should not be overlooked in modern meditation studies, especially given the lack of double-blind designs in this field. Planning double-blind studies that use a placebo is possible and desirable in meditation research, particularly when dealing with the elicitation of positive emotions, such as compassion or empathy. Having a meditation teacher who knows nothing about the hypotheses of the study and has no part in designing, analysing and writing the results, would also reduce the likelihood of methodological biases.

Methodological flaws allow for many other biases, such as those concerning data analysis and reporting. Interpreting statistical results and choosing what to highlight is challenging. Kaptchuk54 has summarised a number of potential interpretative biases that have become widespread in science reporting, including a confirmation bias, where one tends to evaluate evidence that supports one’s beliefs more favourably than evidence that challenges it.

A confirmation bias was particularly prevalent in the studies we reviewed in the form of an over-reporting of ‘marginally significant’ results. In addition to statistically significant results (p < 0.05), 48% of studies reported marginally significant results (p > 0.05), which varied considerably — p-values ranged from 0.06 to 0.14. Further, the majority of studies failed to urge caution in the interpretation of these marginally significant results and, in some cases, discussed them a par with other statistically significant effects. This over-reliance on marginally significant results to generate theoretical interpretations naturally increases the chances of a Type I error55. Just to illustrate this bias, let us take an example from one of the studies we reviewed and meta-analysed. On p. 461, the authors reported a marginally significant difference (p = 0.069) in favour of the meditation intervention relative to the control group. However, on the following page, when the authors reported a different set of results that did not favour the meditation intervention they claimed the exact same p-level as non-significant: “The results confirmed our hypotheses for intergroup anxiety. Contrast 1 was not significant, t(75) = 1.85, p = 0.069” (p. 462,44).

Another potential instance of a confirmation bias we identified was the inconsistency of reported results in the way that meditation intervention effects were assessed. Some studies reported within-subjects effects from pre- to post-test, whereas others reported changes from post-testing to follow up, yet others only compared between-subject effects at post-test. It is unclear what exactly underpins this inconsistency, but it is likely to be the result of a bias to report significant results and neglect non-significant ones.

Potential suggestions to ameliorate confirmation biases include: a full disclosure of results, including all non-significant ones; a clear treatment of p-values as either significant or non-significant; and to run two different families of statistical tests, such as traditional null-hypothesis testing and non-parametric tests (e.g. bootstrapping), or Bayesian tests, and see if the findings converge. Meditation studies would benefit from being pre-registered to prevent ad-hoc analysis and reduce the experimental degrees of freedom during analysis. Finally, the presence of a confirmation bias has an impact on the interpretation of the results by biasing the generation of theoretical assumptions about their meaning.

The majority of studies we reviewed presented very tenuous and unclear justifications for why a meditation intervention ought to improve prosocial outcomes. The research literature tends to swiftly reference the health benefits of meditation and/or mention the alleged prosocial effects of meditation in the Buddhist tradition36,38,41. Further, this literature generally conveys the impression that Buddhism is particularly concerned with the promotion of prosociality and that meditation is the means to achieve it. This is a rather inaccurate understanding of a rich and plural religious tradition. Leading academics of South Asian religions have highlighted the Western misreading and reconstruction of Buddhism as a rational form of inquiry focused on meditation, which has been uncritically accepted by psychology researchers56. For example, such authors highlight that for most forms of Buddhism, it is not meditation but the study of sacred scriptures that is the most valued means to achieve deep personal transformation. Other scholars have also cast a critical light upon the definition of mindfulness as a process of paying attention, in the present moment and non-judgmentally57, regarding it as something different from what the Buddha scriptures describe – less than a form of attention or awareness to one’s thoughts, feelings and sensations, but rather a reflection upon the impermanence of all things, starting with one’s body58.

This is not the place to dwell upon the lack of agreement between psychology and the Eastern spiritual traditions on what meditation is and its precise role in effecting personal change. We simply wish to point out the conceptual mist which comes across in the reports we examined, either in the lack of an overall coherent theoretical framework, or even the lack of an attempt to theorise about how meditation works. Most of the reports focus on meditation as a tool that can be used for various purposes, such as the cure of social isolation13. Only rarely do studies try to look at underpinning mechanisms, such as the role of meditation in increasing empathic accuracy43 or in decreasing psychological stress41. However, the results either failed to show that the mechanism in question played any significant role or it only worked partially (reducing psychological distress when dealing with prejudice regarding homeless but not Black people)41.

The lack of a clear attempt to address underpinning mechanisms of meditation makes the literature more vulnerable to implicit magical beliefs about the power of Eastern contemplative techniques, even when adapted into medical and mental health settings. Previous systematic reviews and meta-analyses have voiced parallel concerns. For example, a recent review of LKM and CM found these techniques ill-defined and lacking standardised protocols19. Also, the majority of meta-analyses on the benefits of meditation acknowledge the pervasive methodological shortcomings of the studies analysed, but still suggest that such results are ‘encouraging’ or ‘promising’20,21,26,59,60,61,62. Unfortunately, such note of optimism is premature in what concerns the literature on the prosocial effects of meditation. We need new studies that take seriously the potential biases and methodological limitations we highlight above, as well as providing a clear theoretical grounding, including the role of potential psychological processes underpinning the prosocial effects of meditation.

Conclusion: Can Meditation Make the World a Better Place?

All world religions promise that the world would change for the better if only people were to follow its rules and practices. The popularisation of meditation techniques in a secular format is offering the hope of a better self and a better world to many. In the early 1970s, Transcendental Meditation conveyed this message openly, announcing that the rising number of individuals practising this technique would lead to world peace in the short term63. Psychologists using mindfulness or other Buddhism-derived meditation techniques are now advancing similar ideas about the prosocial effects of meditation. In the foreword to the Mindfulness Initiative UK (2015) report launched at the British Parliament, Kabat-Zinn wrote of the profound potential of meditation to bring about societal changes. Despite these high hopes, our analysis suggests that meditating is likely to have a positive, but still relatively limited effect in making individuals feel or act in a substantially more socially connected, or less aggressive and prejudiced way. Compared to doing no new emotionally engaging activity, it might make one feel moderately more compassionate or empathic, but our findings suggest that these effects may be, at least in part, the result of methodological frailties, such as biases introduced by the meditation teacher, the type of control group used and the beliefs and expectations of participants about the power of meditation.

This, of course, does not invalidate Buddhist or other religions’ claims about the moral value and eventually life changing potential of its beliefs and practices. However, the adaptation of spiritual practices into the lab suffers from methodological weaknesses and is partly immersed in theoretical mist. Before good research can be conducted on the prosocial effects of meditation, these problems need to be addressed.