Abstract Research was conducted to determine what the characteristics of good management and leadership are and how we can evaluate whether an intervention has successfully developed these characteristics in its participants. Initially, content on the Effective Altruism Forum and by 80,000 Hours was reviewed. Additional searches of Google Scholar were conducted. Researchers have identified a number of leadership styles, measured through various scales, that are correlated with organisational performance outcomes. Our search findings suggest that the use of contingent reward behaviours is similarly effective to “transformational leadership.” Transformational leadership also has similar results to several newer theories of leadership such as “servant leadership” and “ethical leadership.” However, given that these leadership behaviour types have different correlations with personality factors, they should probably still be seen as different approaches to leadership. We also conclude that if measuring whether an intervention to improve management and leadership expertise has had effects on an organisation’s performance is not feasible, it may be sufficient to measure whether the intervention has improved ratings of participants’ leadership skills on scales that measure skillsets such as transformational leadership.
Introduction One of the talent bottlenecks in the effective animal advocacy (EAA) community that Animal Advocacy Careers (AAC) is aiming to address is the undersupply of management and leadership expertise. This bottleneck was initially identified through AAC’s survey and interviews with 10 CEO’s and hiring professionals from some of the “top” or “standout” charities currently or formerly recommended by Animal Charity Evaluators and a short survey of 9 attendees of Effective Altruism Global suggest that a lack of leadership and management expertise is a common concern in the EAA community. This issue has also been identified in the wider effective altruism community. We anticipate this issue could be addressed through a number of interventions, such as training programmes for current employees of EAA organisations or the publication of resources that aggregate evidence on how to enter and excel in management and leadership roles. We intend to trial some interventions throughout the coming year to help us identify which might be the best ways to help make progress in this skill gap. The primary goal of this short literature review was to inform AAC’s upcoming evaluation of management training programmes; what skills and knowledge do we expect that the training programme will need to develop in its participants, and how can we evaluate whether it has done so successfully? A secondary goal was to begin the process of aggregating evidence on how to enter and excel in management and leadership roles. Although focused on AAC’s own goals and training requirements, this report may be useful for other organisations organising training programmes or individuals seeking to understand how to develop their own expertise. Methodology This was a time-capped report. A flexible limit was set of 20 hours on initial research and note-taking (3 hours on searches of the Effective Altruism Forum and 80,000 Hours and 17 hours on Google Scholar searches) and 5 additional hours clarifying the write-up of the findings. This research was only intended to secure low-hanging fruit of learnings from relevant research and to identify whether further, more rigorous research would be worthwhile. Research questions The two main research questions for this report were:
What are the characteristics of good management and leadership?
How can we evaluate whether an intervention has successfully developed these characteristics in its participants?
Since search results often had implications for both of these research questions, the results are not separated in the “Summary of findings” spreadsheet. Search strategy Initially, content on the Effective Altruism Forum and by 80,000 Hours was reviewed. Content on “operations management” and on “management consultancy” was not reviewed. Subsequently, searches of Google Scholar were conducted. All searches were limited to results from 1990 onwards, though for some search terms, an additional search was limited to 2014 onwards to more easily identify more recent research. The first 5 pages of results were skimmed or reviewed for each search term. Research items were identified non-systematically. That is, there were not strict inclusion and exclusion criteria, and the likely relevance of research was assessed predominantly by the phrasing of the title, rather than reviews of the abstracts or the content of all returned results. Nevertheless, the following criteria were used to decide which items to include:
Are the findings directly applicable or at least fairly comparable to the training contexts that AAC are likely to use, e.g. training for adult professionals?
Does the research item contain (or summarize) substantial empirical findings?
Is the research item unlikely to have been made predominantly redundant by subsequent research? Relevant factors affecting this criterion include the date of publication and any impressions that I have of the thoroughness of the literature on the subtopic that it covers.
Is the item published in a peer-reviewed academic journal?
Once it became clear that most research on this topic from the past few decades was clustered around a small number of leadership theories, it became possible to restrict searches primarily to literature reviews and meta-analyses relating to these theories. Hence, after this point, individual studies were only included if they seemed especially important and no meta-analysis had been identified on that topic.
The conclusions of the research items were not grounds for exclusion. That is, research items were not (intentionally) omitted if their conclusions were surprising or contrasted with the findings of other included research. The scoring system Each included item of research was assigned an “effects and importance” score for the main characteristic of good management or leadership that it provided evidence for. The scores were given on a possible range from -5 to +5. Each item of research was also assigned a “strength of evidence” score. Given the short timeframe for research and small number of included studies, I opted not to use any statistical or quantitative procedures to aggregate the results from this scoring system. Results and discussion Impressions of the existing literature Searches of the existing effective altruism content did not identify substantial evidence on what makes good management and leadership. The following characteristics and behaviours were identified as possibly being important:
Ability to avoid over- or under-managing
Approachability and openness
Coaching and developing your team
Complex problem solving
Estimation of capabilities
Innovativeness, creativity, and entrepreneurialism
Judgement and decision-making
Personal time management
Setting team directions
Team time direction and prioritisation
Google Scholar searches identified a handful of theories of leadership and approaches to leadership research that utilise some of the characteristics referred to in the effective altruism content. Survey scales have been used to evaluate leadership and outcomes in relation to these existing theories; the identified research tended to focus on the association of these scales with desired outcomes, rather than focus on individual characteristics or behaviours. It seems that in the 20th century, researchers were somewhat divided between theories and research agendas focused on either the innate, untrainable characteristics associated with effective leadership, or trainable behaviours associated with effective leadership. More recently, however, research has moved towards a synthesis of these two approaches and has incorporated the key insights from various other research paradigms from the 20th century. In examining correlations between particular competencies of leaders and the antecedents or outcomes of these competencies, a researcher does not necessarily need to make assumptions about how innate or trainable these competencies are. Hence, for example, competencies can be evaluated by practitioners or colleagues and the personality or psychological correlates of these competencies can be separately evaluated. Almost all of the primary studies that were aggregated in included reviews and meta-analyses used observational methods. For example, in Judge and Piccolo’s (2004) review, about two-thirds of the included studies used cross-sectional designs and the other third used longitudinal designs. This should reduce our confidence that any of the identified correlations represent causal relationships. It was also common for the studies to look for correlations between answers to questions provided by the same respondents. In this sense, lurking variables such as social desirability bias could affect individuals’ answers to multiple different question types and explain some of the observed correlations. Theories of leadership, leadership behaviours, and their usefulness Some scales and theories seem likely to be more worthwhile seeking to explicitly train and measure than others. In the rest of this section, several theories of leadership and leadership behaviour types are presented in order of their apparent importance for inclusion in management and leadership training programmes; i.e. how important it is to tailor training programmes towards the relevant characteristics and behaviours and how important it is to measure relevant constructs as part of the monitoring and impact evaluation of the training programme. They are ordered in relation to the following two criteria:
The extent to which scales that represent the theory’s evaluation of leadership quality are correlated with desirable outcome measures, such as objectively measured success outcomes, perceived team effectiveness, and various positive team attitudes.
The extent to which this scale has been thoroughly evaluated in the academic literature. For example, given the substantial overlap between transformational leadership and authentic leadership, the former seems preferable, since research has led to a greater understanding of transformational leadership and its correlates.
The “Summary of findings” spreadsheet contains findings relating to a number of other leadership concepts not discussed here (since they seemed lower priority to include and independently evaluate), such as leader-member exchange, authentic leadership, management by exception, and laissez-faire leadership. Transformational leadership Transformational leadership is comprised of the following characteristics and behaviours, as summarised by Piccolo and Colquitt (2006):
“Idealized influence” — “the degree to which leaders behave in charismatic ways that cause followers to identify with them.”
“Inspirational motivation” — “the degree to which leaders articulate visions that are appealing to followers.”
“Intellectual stimulation” — “the degree to which leaders challenge assumptions, take risks, and solicit followers’ ideas.”
“Individualized consideration” — “the degree to which leaders attend to followers’ needs, act as mentors or coaches, and listen to followers’ concerns.”
Transformational leadership has been measured through a number of scales, though Dumdum et al. (2013) note that “an initial review of the literature back to 1995 showed that the [Multifactor Leadership Questionnaire] was by far, the most frequently used measure for studying transformational leadership.” Meta-analyses find correlations with positive outcomes of varying strengths. See, for example, the summary below of 4 separate meta-analyses by Hoch et al. (2018):
Transformational leadership clearly seems to be the most widely-studied and well-known of the leadership theories discussed in this report. It has the highest number of search results and the original research items that developed the theory have a large number of citations. Additionally, reviews and meta-analyses of the other leadership theories often evaluate those theories in comparison to transformational leadership. Contingent reward (from transactional leadership) “Transactional leadership” is a theory which seems to have been developed as a contrast to transformational leadership, summarised by Wang et al. (2011) as being where “leaders clarify expectations and reward followers for fulfilling them.” There are three dimensions of transactional leadership; contingent reward, management-by-exception: active, and management-by-exception: passive. Contingent reward is the process of giving positive reinforcement to behaviours and performance that meets or exceeds expectations. Many of the meta-analyses of studies on transformational leadership also evaluate transactional leadership or contingent reward more specifically. Contingent reward or transactional leadership have correlations of similar strength with desirable outcomes as transformational leadership does; depending on the particular outcome variable and the specific meta-analysis, the point estimates may be slightly higher or lower for either transformational leadership or contingent reward, but the 95% confidence intervals usually overlap. Therefore, though transformational leadership has been contrasted to transactional leadership (with the former being suggested to be superior), the use of contingent reward behaviours seems similarly effective to transformational leadership. This skill therefore seems appropriate to encourage among leaders and to measure as part of leadership training programmes. Interestingly, there seems to be some overlap between the personality correlates of transformational leadership and contingent reward behaviours. Servant leadership Parris and Peachey (2013) conclude from their synthesis of empirical studies that “there is no consensus on the definition of servant leadership.” However, Van Dierendonck (2011) summarises that “[s]ervant leadership is demonstrated by empowering and developing people; by expressing humility, authenticity, interpersonal acceptance, and stewardship; and by providing direction.” Hoch et al. (2018) conclude that servant leadership “showed more promise” than ethical leadership or authentic leadership “as a stand-alone leadership approach that is capable of helping leadership researchers and practitioners better explain a wide range of outcomes.” For example, though doing little to help explain job performance outcomes beyond what was explained by transformational leadership, servant leadership helped to explain “organizational citizenship behavior” outcomes and did so better than ethical or authentic leadership. The point estimate for the correlation with job performance was similar for servant leadership (ρ = .23, from 8 studies) and transformational leadership (ρ = .27, from 74 studies) and the 95% confidence intervals overlapped. Ethical leadership Brown and Treviño (2006) include a definition of ethical leadership as “the demonstration of normatively appropriate conduct through personal actions and interpersonal relationships, and the promotion of such conduct to followers through two-way communication reinforcement, and decision-making.” It differs from transformational leadership in that “[e]thical leaders emphasize ethical standards, and moral management,” which is described as a “more transactional” approach, while “[t]ransformational leaders emphasize vision, values, and intellectual stimulation.” Bedi et al. (2016) found strong correlations between ethical leadership and both transformational and transactional leadership. Bedi et al. (2016) note that The “Ethical Leadership Scale” “remains one of the most widely used measures of ethical leadership.” Though Hoch et al. (2018) are less optimistic about the usefulness ethical leadership than authentic leadership, this is based primarily on an assessment of whether the concept is useful for explaining different outcomes to transformational leadership. Indeed, the point estimate for the correlation with job performance was similar for ethical leadership (ρ = .25, from 22 studies) and transformational leadership (ρ = .27, from 74 studies) and the 95% confidence intervals overlapped. Bedi et al. (2016) also found correlations with various outcome measures that were similar between both ethical leadership and transformational leadership. Nevertheless, they “also performed separate meta-analyses for each of the Big Five personality traits and compared [their] results with transformational leadership (Bono and Judge 2004)... the only personality trait that showed similar results for ethical leadership and transformational leadership was extraversion.” Agreeableness and conscientiousness were more strongly positively correlated (and neuroticism was more strongly negatively correlated) with ethical leadership than with transformational leadership. This suggests that those who perform highly on measures of ethical leadership may have similar outcomes to those who perform highly on measures of transformational leadership, but that the measures represent slightly different leadership styles. Shared leadership Wang et al. (2014) summarise that “[a] growing number of studies have examined the ‘sharedness’ of leadership processes in teams (i.e., shared leadership, collective leadership, and distributed leadership).” Sharedness is evaluated through surveys of team members using modified versions of the leadership assessment scales used for individuals. Their meta-analysis found correlations between the “sharedness” of leadership processes and team effectiveness across several outcome types, including a small correlation of ρ = .18 with objective measures of team performance. D’Innocenzo et al. (2016) and Nicolaides et al. (2014) also found correlations between the sharedness of leadership and various indicators of team effectiveness. Each of these three papers explores different aspects of this association. Strengths-based leadership Strengths-based leadership theory emphasises the importance of identifying one’s strengths and then developing them. This seems to imply that leadership involves encouraging specialisation of skillsets and supporting this team of specialists; the team is well-rounded, even if the individuals are not. This leadership theory differs from the others summarised in this report in several ways. Firstly, it was identified through a different mechanism to the others, following up a reference from an initial Google search on the topic, rather than through the Google Scholar searches. Secondly, it was evaluated by the company Gallup, rather than through a meta-analysis published in a peer-reviewed journal. Thirdly, the evaluations were from experimental or quasi-experimental trials of training interventions, rather than observational analyses. These differences make direct comparison with the findings for other leadership theories difficult. Gallup’s meta-analysis suggests small positive effects of completing the “StrengthsFinder” assessment and being made aware of one’s “top natural talents” on profit, sales, customer engagement, employee engagement, and turnover. There seems to be very little difference between using the $50 assessment tool and the shorter $20 assessment tool. Use of the shorter tool could be recommended to participants in a training programme as an optional follow-up activity, even if the training programme does not otherwise focus on strengths-based leadership. Other implications for the monitoring and evaluation of interventions to improve management and leadership Given the correlations between measures of leadership effectiveness and organisational performance, interventions that lead to improvements in leadership according to these measures — which previous research has shown to correlate with objective measures of organisational performance and which may be more easy to monitor before and after a particular intervention — may also lead to improvements in organisational performance. This rests on several assumptions:
That high performance on measures of leadership effectiveness causes organisational success, rather than organisational success inspiring high performance on (or at least more positive evaluations of) measures of leadership effectiveness. Given that the research is almost exclusively correlational, we cannot be confident that this assumption is correct. However, this seems to me to be intuitively likely.
That the relationship between leadership effectiveness and organisational success, identified in a number of meta-analyses, will hold true in the context in which the intervention is being conducted. In the absence of any evidence or reason to expect that animal advocacy organisations would be substantially more or less dependent upon quality leadership than any other organisation, it seems reasonable to assume that the relationship in this context will be similar to the relationship in other contexts.
Many of the details provided in the “Summary of findings” spreadsheet are relevant to the monitoring and evaluation of interventions to improve management and leadership. For example, the results of one paper suggest that shared leadership is correlated with team performance, but that how shared leadership is measured might have a substantial effect on the strength of the correlation. Suggestions for further research
The exploration of each of the topics considered here was very brief. A number of strategies could be used to more comprehensively check the evidence base for each claim:
Additional search terms could be added that vary from those used here.
Citations of the most rigorous and recent reviews or studies could be checked.
Some of the individual studies that were evaluated in reviews and meta-analyses could be examined, to better understand the features and methods of the underlying research.
The searches returned a number of results that were related to organisational systems, which could arguably form an important component of “management” (as opposed to leadership). This literature was not reviewed here, since it is large and mostly distinct from the literature on leadership theories. It also seems likely that changes to organisational systems would be implemented in a fairly different manner to the advice that one might learn during a leadership and management training programme.
Searching for the different leadership theories without additional terms such as “meta-analysis” returns books that are dedicated to the topic. For example, searching for “transformational leadership” returns books such as Bernard M. Bass and Ronald E. Riggio, Transformational Leadership (Mahwah, NJ: Lawrence Erlbaum, 2006). I did not read through such books for this report, but it would be useful to explore leadership theories in this way if they are going to be used as the basis of a training programme or its evaluation.
A more thorough literature review on this topic should also search for terms such as “predictors,” “correlates,” “antecedents,” “consequences,” and “effects” alongside the search terms for the leadership theories of most interest. It may be necessary to look into these topics at the level of individual studies, rather than continuing to rely on meta-analyses and literature reviews.
This research was not intended to summarise tips and recommendations from experienced leaders or from popular books like Simon Sinek, Leaders Eat Last: Why Some Teams Pull Together and Others Don't (London, UK: Penguin, 2014). Though AAC expects that research into this topic will be a less effective use of time, it may attempt to summarise and aggregate these recommendations in the future.
Unfortunately, the identified research did not tend to evaluate the features of management and leadership trainings that are associated with the highest effect sizes. It may be many years or decades before the relevant research field is sufficiently well-developed to attempt a systematic analysis of this sort. Though it could not be done very systematically, looking at some of the primary studies summarised in the reviews and meta-analyses included here, or separate Google Scholar searches for terms like “Leadership training” may provide insight into the formats of trainings that have been attempted and evaluated so far.
A number of related research areas might also provide actionable advice for training programmes to encourage good management and leadership. For example, if one concludes that communication skills are important, then research evaluating the importance of various components of communication could be useful.
Footnotes  In the end, 19.5 hours were spent on research (2 on on searches of the Effective Altruism Forum and 80,000 Hours and 17.5 on Google Scholar searches). 8 hours were spent on the write-up; the findings seemed more difficult to quickly understand than I had initially assumed that they would be, so a fuller write-up seemed more appropriate. Some additional time was spent on discussion and editing.  Relevant research was identified through the following two searches:
Management site:effectivealtruism.org OR site:80000hours.org
Leadership site:effectivealtruism.org OR site:80000hours.org
 The following search terms were used:
("management" OR "leadership") AND ("skills" OR "expertise" OR "characteristics")
("management" OR "leadership") AND ("skills" OR "expertise" OR "characteristics") AND ("meta-analysis" OR "systematic review" OR "literature review")
“transformational leadership” AND ("meta-analysis" OR "systematic review" OR "literature review")
(“Transactional leadership” OR “contingent reward”) AND ("meta-analysis" OR "systematic review" OR "literature review") [this search returned no additional useful results, since “transactional leadership” seems to have always been examined in relation to “transformational leadership.”]
“servant leadership” AND ("meta-analysis" OR "systematic review" OR "literature review")
“ethical leadership” AND ("meta-analysis" OR "systematic review" OR "literature review")
“authentic leadership” AND ("meta-analysis" OR "systematic review" OR "literature review")
“charismatic leadership” AND ("meta-analysis" OR "systematic review" OR "literature review")
“adaptive leadership” AND ("meta-analysis" OR "systematic review" OR "literature review") [returned no additional relevant reviews or meta-analyses]
(“strengths based leadership” OR “strengths-based leadership”) AND ("meta-analysis" OR "systematic review" OR "literature review") [returned no additional relevant reviews or meta-analyses]
An additional search was conducted to identify the “Gallup StrengthsFinder 2.0” that was mentioned by an initial, non-systematic search for ““theories of leadership” “transformational leadership.””  I did not exclude all items that failed to meet some of the inclusion criteria, if they seemed to perform especially well on others. These inclusion criteria were pre-planned, except for the last criterion.  -5 means that if this was the only relevant evidence on this issue, I would expect this characteristic of a manager or leader to have strong negative impacts on their teams and objectives, 0 means that I would expect it to have no impact on their teams and objectives (i.e. useless but not harmful), 1 means very low positive impacts, 2 means quite low positive impacts, 3 means moderate positive impacts, 4 means quite high positive impacts, 5 means very high positive impacts.  0 = no relevant evidence. 1 = observational evidence from a single study, observational evidence from multiple studies that has weak relevance to the importance of the characteristic of good management or leadership being discussed, or observational or indirect evidence from multiple sources where the findings are somewhat contradictory. 2 = Observational or indirect evidence from multiple sources or a single randomised controlled trial of low quality. 3 = a single randomised controlled trial of reasonable methodological quality, with little or no supporting observational or indirect evidence, or evidence from multiple randomised controlled trials where the findings are somewhat contradictory. 4 = multiple (2 to 5) randomised controlled trials of reasonable methodological quality, a single randomised controlled trial with substantial supporting observational or indirect evidence, a large number (6 or more) of randomised controlled trials of low quality, or convincing and consistent observational or indirect evidence. 5 = 6 or more randomised controlled trials of reasonable methodological quality, or 2 to 5 randomised controlled trials with substantial supporting observational or indirect evidence.  It is possible that this was due to the search terms used, rather than due to a lack of such research. It is also possible that research focusing on broad theories of leadership tended to be more highly cited than research focusing on individual characteristics or behaviours; this would have meant that these results were more easily identified via Google Scholar searches.  See, for example, the summary in J. Rodney Turner and Ralf Müller, “The Project Manager’s Leadership Style as a Success Factor on Projects: A Literature Review,” Project Management Journal 36, no. 2 (2005), 49-61.  See, for example, Joyce E. Bono and Timothy A. Judge, “Personality and Transformational and Transactional Leadership: A Meta-Analysis,” Journal of Applied Psychology 89, no. 5 (2004), 901-10.  Timothy A. Judge and Ronald F. Piccolo, "Transformational and Transactional Leadership: A Meta-Analytic Test of their Relative Validity," Journal of Applied Psychology 89, no. 5 (2004), 755-68. See also the scores in the column for “Strength of evidence (0 to 5)” on the “Summary of findings” spreadsheet, where scores of 2 or below represent exclusively observational research. However, Lauren D’Innocenzo, John E. Mathieu, and Michael R. Kukenberger, “A Meta-Analysis of Different Forms of Shared Leadership–Team Performance Relations,” Journal of Management 42, no. 7 (2016), 1964-91 note that “[c]lassroom and lab samples are often criticized in organizational research because they are believed to be nonrepresentative of organizational settings. The implicit assumption is that results of student samples, particularly ones working on laboratory tasks, are not likely to generalize to real working populations.” In this sense, lab experiments should not necessarily be seen as superior evidence for the topics being examined here, just as another form of evidence.  For example, Vias C. Nicolaides, Kate A. LaPort, Tiffani R. Chen, Alan J. Tomassetti, Eric J. Weis, Stephen J. Zaccaro, and Jose M. Cortina, “The Shared Leadership of Teams: A Meta-Analysis of Proximal, Distal, and Moderating Relationships,” The Leadership Quarterly 25, no. 5 (2014), 923-42 note that, “[a]s expected, criterion source moderated the relationship between shared leadership and performance... when performance was rated by the same source a very strong positive uncorrected true correlation emerged (μr = .80, p < .01) whereas when performance was rated by another source a moderate positive correlation emerged (ρ = .30, p < .01). The two validities were significantly different from one another (z = 4.17, p < .01).”  See the tab “Number of results for various search terms” in the “Summary of findings” spreadsheet. My understanding of the extent of overlap between various scales is mostly based on the findings of Julia E. Hoch, William H. Bommer, James H. Dulebohn, and Dongyuan Wu, “Do Ethical, Authentic, and Servant Leadership Explain Variance Above and Beyond Transformational Leadership? A Meta-Analysis,” Journal of Management 44, no. 2 (2018), 501-29, although some other reviews and meta-analyses also reported on overlap with other scales.  Ronald F. Piccolo and Jason A. Colquitt, “Transformational Leadership and Job Behaviors: The Mediating Role of Core Job Characteristics,” Academy of Management Journal 49, no. 2 (2006), 327-40.  Dumdum, Uldarico Rex, Kevin B. Lowe, and Bruce J. Avolio, "A Meta-Analysis of Transformational and Transactional Leadership Correlates of Effectiveness and Satisfaction: An Update and Extension," in Bruce J. Avolio and Francis J. Yammarino (eds.) Transformational and Charismatic Leadership: The Road Ahead (Oxford, UK: Elsevier Science, 2013), 35-66. They also note that “[a]lternative measures such as [P.M. Podsakoff, S. B. MacKenzie, R.H. Moorman, and R. Fetter, “Transformational leader behaviors and their effects on followers' trust in leader, satisfaction, and organizational citizenship behaviors,” The Leadership Quarterly 1 (1990), 107-42] provide a narrower assessment of the dimensions comprising the full range model.”  Hoch, Julia E., William H. Bommer, James H. Dulebohn, and Dongyuan Wu, “Do Ethical, Authentic, and Servant Leadership Explain Variance Above and Beyond Transformational Leadership? A Meta-Analysis,” Journal of Management 44, no. 2 (2018), 501-29. This table does not include all relevant meta-analyses. See also, for example, Timothy A. Judge and Ronald F. Piccolo, “Transformational and Transactional Leadership: A Meta-Analytic Test of their Relative Validity,” Journal of Applied Psychology 89, no. 5 (2004), 755-68.  See the tab “Number of results for various search terms” in the “Summary of findings” spreadsheet.  Gang Wang, In-Sue Oh, Stephen H. Courtright, and Amy E. Colbert, “Transformational Leadership and Performance Across Criteria and Levels: A Meta-Analytic Review of 25 Years of Research,” Group and Organization Management 36, no. 2 (2011), 223-70.  See the key findings for each of the following in the “Summary of findings” spreadsheet:
Gang Wang, In-Sue Oh, Stephen H. Courtright, and Amy E. Colbert, "Transformational Leadership and Performance Across Criteria and Levels: A Meta-Analytic Review of 25 Years of Research," Group and Organization Management 36, no. 2 (2011), 223-70.
Timothy A. Judge and Ronald F. Piccolo, “Transformational and Transactional Leadership: A Meta-Analytic Test of their Relative Validity,” Journal of Applied Psychology 89, no. 5 (2004), 755-68.
Dumdum, Uldarico Rex, Kevin B. Lowe, and Bruce J. Avolio, "A Meta-Analysis of Transformational and Transactional Leadership Correlates of Effectiveness and Satisfaction: An Update and Extension," in Bruce J. Avolio and Francis J. Yammarino (eds.) Transformational and Charismatic Leadership: The Road Ahead (Oxford, UK: Elsevier Science, 2013), 35-66.
 Joyce E. Bono and Timothy A. Judge, “Personality and Transformational and Transactional Leadership: A Meta-Analysis,” Journal of Applied Psychology 89, no. 5 (2004), 901-10.  Parris, Denise Linda, and Jon Welty Peachey, “A Systematic Literature Review of Servant Leadership Theory in Organizational Contexts,” Journal of Business Ethics 113, no. 3 (2013), 377-93.  Dirk Van Dierendonck, “Servant Leadership: A Review and Synthesis,” Journal of Management 37, no. 4 (2011), 1228-61.  Julia E. Hoch, William H. Bommer, James H. Dulebohn, and Dongyuan Wu, “Do Ethical, Authentic, and Servant Leadership Explain Variance Above and Beyond Transformational Leadership? A Meta-Analysis,” Journal of Management 44, no. 2 (2018), 501-29.  Michael E. Brown and Linda K. Treviño, “Ethical Leadership: A Review and Future Directions,” The Leadership Quarterly 17, no. 6 (2006), 595-616. They add that “ethical leaders “set clear and high ethical standards for others and follow these standards themselves. They also use rewards and punishments to influence followers' ethical behavior.” However, “empirical research tends to support the view that transformational leadership, at least as conceptualized and measured by Bass & Avolio (2000) via the Multi-Factor Leadership Questionnaire (Bass & Avolio, 2000), does describe a leader with an ethical orientation.”  Akanksha Bedi, Can M. Alpaslan, and Sandy Green, “A Meta-Analytic Review of Ethical Leadership Outcomes and Moderators,” Journal of Business Ethics 139, no. 3 (2016), 517-36.  Akanksha Bedi, Can M. Alpaslan, and Sandy Green, “A Meta-Analytic Review of Ethical Leadership Outcomes and Moderators,” Journal of Business Ethics 139, no. 3 (2016), 517-36, citing the scale by M. E. Brown, L. K. Treviño, and D. Harrison, “Ethical Leadership: A Social Learning Perspective for Construct Development and Testing,” Organizational Behavior and Human Decision Processes 97 (2005), 117–34.  Julia E. Hoch, William H. Bommer, James H. Dulebohn, and Dongyuan Wu, “Do Ethical, Authentic, and Servant Leadership Explain Variance Above and Beyond Transformational Leadership? A Meta-Analysis,” Journal of Management 44, no. 2 (2018), 501-29. Thomas W.H. Ng and Daniel C. Feldman, “Ethical Leadership: Meta-Analytic Evidence of Criterion-Related and Incremental Validity,” Journal of Applied Psychology 100, no. 3 (2015), 948-65 had previously conducted a meta-analysis on a similar topic and came to a more positive conclusion.  Akanksha Bedi, Can M. Alpaslan, and Sandy Green, “A Meta-Analytic Review of Ethical Leadership Outcomes and Moderators,” Journal of Business Ethics 139, no. 3 (2016), 517-36, citing Joyce E. Bono and Timothy A. Judge, “Personality and Transformational and Transactional Leadership: A Meta-Analysis,” Journal of Applied Psychology 89, no. 5 (2004), 901-10.  Danni Wang, David A. Waldman, and Zhen Zhang, "A Meta-Analysis of Shared Leadership and Team Effectiveness," Journal of Applied Psychology 99, no. 2 (2014), 181-98.  Danni Wang, David A. Waldman, and Zhen Zhang, "A Meta-Analysis of Shared Leadership and Team Effectiveness," Journal of Applied Psychology 99, no. 2 (2014), 181-98.  Vias C. Nicolaides, Kate A. LaPort, Tiffani R. Chen, Alan J. Tomassetti, Eric J. Weis, Stephen J. Zaccaro, and Jose M. Cortina, “The Shared Leadership of Teams: A Meta-Analysis of Proximal, Distal, and Moderating Relationships,” The Leadership Quarterly 25, no. 5 (2014), 923-42 and Lauren D’Innocenzo, John E. Mathieu, and Michael R. Kukenberger, "A Meta-Analysis of Different Forms of Shared Leadership–Team Performance Relations," Journal of Management 42, no. 7 (2016), 1964-91.  See the “key findings” for each paper in the first tab of the “Summary of findings” spreadsheet.  “Learn About the Science of CliftonStrengths,” Gallup, accessed 31st December, 2019, https://www.gallup.com/cliftonstrengths/en/253790/science-of-cliftonstrengths.aspx.  David Burkus, “Strengths-Based Leadership Theory” (2010), https://davidburkus.com/2010/04/strengths-based-leadership-theory/.  “The Relationship Between Strengths-Based Employee Development and Organizational Outcomes: 2015 Strengths Meta-Analysis,” Gallup (2015), https://www.gallup.com/services/193394/relationship-strengths-based-employee-development-organizational-outcomes.aspx.  “The Effect of CliftonStrengths 34 Feedback on Employee Engagement and Sales: 2018 CliftonStrengths Meta-Analysis,” Gallup (2018), https://www.gallup.com/cliftonstrengths/en/269090/2018-cliftonstrengths-meta-analysis-report.aspx.aspx.  See the paragraph above beginning “Almost all of…”  Lauren D’Innocenzo, John E. Mathieu, and Michael R. Kukenberger, “A Meta-Analysis of Different Forms of Shared Leadership–Team Performance Relations,” Journal of Management 42, no. 7 (2016), 1964-91.  As one example, see Ed Gerrish, “The Impact of Performance Management on Performance in Public Organizations: A Meta‐Analysis,” Public Administration Review 76, no. 1 (2016), 48-66.  By comparison, in the literature on health behaviour interventions, analyses have been undertaken by identifying the correlations between the use of particular “behaviour change techniques” (BCTs) and effect sizes, based on Charles Abraham and Susan Michie, “A Taxonomy of Behavior Change Techniques Used in Interventions,” Health Psychology 27, no. 3 (2008), 379-87. However, this approach requires numerous randomised controlled trials (RCTs) of interventions using various BCTs. As noted above, the literature on most leadership theories used few, if any, RCTs.  See, for example, “Communication” in John M. Ivancevich, Michael T. Matteson, and Robert Konopaske, Organizational Behavior and Management (New York: McGraw-Hill, 1990), 371-406.