Strategic Niche Management for Sustainability: A Systematic Literature Review

: With the aim of achieving a sustainable future, a new research area dealing with “sustain-ability transitions” has emerged. The focus on socio-technical niches and the related management activities are paramount to understand their aptitude to replace the dominant socio-technical regime. Strategic niche management (SNM) has become a renowned analytical framework for understanding the introduction and diffusion of very new sustainable innovations through societal experiments. With an emphasis on SNM, a systematic literature review (SLR) mainly looking at the environmental dimension of sustainability was carried out adopting the PRISMA statement. The increase in publications over the years, the enlargement of the geographical borders interested, the presence of points of reference in the literature, as well as the adaptability of the theoretical framework to both a generic and speciﬁc issue related to sustainability, demonstrate the centrality of SNM for studying the transition towards sustainability in socio-technical systems. vehicles


Introduction
Over the past 25 years, the way to reach the transition towards sustainability of the daily activities has increasingly interested innovation studies [1,2]. Sustainability transition (STR) refers to the transformation in response to the persistent problems challenging the current modes of production and consumption [3]. According to the multi-level perspective (MLP) there are three analytical levels for appreciating socio-technical systems [4]: (i) the landscape, that represents the macro-level of the analysis, the external context that allows and binds the opportunities for regime change, the whole of structures, cultures and practices with partial-external and partial-independent functioning; (ii) the regime, that constitutes the meso-level of the system, the set of rules and institutions that permits and limits the behavior of actors; and (iii) the niche, that represents the micro-level of the system, the protected space that allows to develop and apply an innovation, the new set of rules and institutions, the whole of alternative structures, cultures, and practices aimed at the accomplishment of social needs.
The particular relevance of transformations at niche level spurred the introduction of the concept of SNM [5]. It represents the creation, development, and controlled phase-out of protected niches for the improvement and use of innovations through experimentation, aiming at the understanding of their desirability and increasing the rate of their application [5]. The theoretical background of SNM represents an effort to bring insights social sciences and innovation studies into evolutionary economics, inverting the perception of the technological change from casual to controlled, and underlining the role of the niche as absolute and relative space [6] in which technological innovation are examined, developed, and sheltered from the ordinary competition [7]. Wrongly considered in its early stage as a traditional policy tool, such a framework does not suggest a top-down creation of niches by governments, but a direction of actions by societal groups oriented to a particular mission (see [8] for hydrogen and SDGs). Indeed, SNM can be perceived as a form of reflexive governance, emerged by means of collective enactments, which its basic idea is the substitution of dominant (polluting) technologies with new (sustainable) ones [9].
During the last few years-especially due to the growing interest given by different fields of research on the ways to reach a transition towards sustainability in the sociotechnical systems-SNM has become a renowned analytical framework. In this respect, the goal of this review is the understanding of the perspectives from which literature has been developed so far by means of a SLR. Differently from Jenkins and Sovacool, (2018) [10] who analyzed peer-reviewed articles published between 2002 and 2016, pairing the term "Strategic Niche Management" with "energy", "electricity", "buildings", "transport", and "vehicles"-this study examines the first 100 results obtained on Google Scholar by querying "Strategic Niche Management" without applying temporal frames or other keywords to the research.
Investigating the focus, the development and the implications of the SNM literature, the paper introduces in Section 1 the background of the research, in Section 2 presents the materials and methods adopted, in Section 3 shows the results obtained, and in Section 4 gives final conclusions.

Materials and Methods
SLR is a method for examining a corpus of scholarly literature, to develop insights, critical reflections, future research paths, and research questions [11][12][13]. Being a rigorous, robust, and transparent method to research for trends and possible future study paths [14]-by adopting the PRISMA statement [15]-the SLR has been selected as a research method to assess and evaluate the existing knowledge on SNM.

Search Strategy
This study was realized by means of Google Scholar and Discovery Sapienza. While the first provides a way to widely search for scholarly literature searching across articles, theses, books, abstracts, and court opinions from academic publishers, professional societies, online repositories, universities and other web sites, the second allows the access to scholarly eResources, utilizing the Sapienza University catalogue, subscribed eJournals, eBook collections, databases, and the Sapienza PhD Theses repository. The choice to use Google Scholar and to examine the first 100 results (out of the about 350,000) obtained by querying "Strategic Niche Management" is motivated by the possibility to implicitly classify the publications on their relevance.
First of all, is important to recognize that Google Scholar uses different classification algorithms based on whether the search of the term is applied on the full text of the document (adopted for this study), on the title, using the "related articles" or the "cited by" function. Secondly, is of paramount relevance the understanding the elements classification affecting the outcome of the query, namely: the quotation count of the publication, the presence of the search term in the title, the presence of the search term in the publication full text and its frequency, the age of the document, as well as the author and journal names.
The most influential factor is the quotation count, generally ranking documents with more citations in higher positions, and implying an easier finding of the literature owning a point of view close to the mainstream. The second relevant factor is the presence of the search term in the title (without the consideration of synonyms), and then its occurrence in the full text of the source, assigning to the frequency of the search term a scarce impact on the outcome. In addition, the database ranks recent publications in a higher position than the older ones, considers the relevance of the author as well as of the journal name, and does not index the text in the images [16]. On the other hand, the documents have been analyzed with the usage of the catalogue of Discovery Sapienza for filtering the sample identified by Google Scholar, permitting to focus the attention of the study exclusively on peer-reviewed articles.

Exclusion Criteria
After careful processing, the following were excluded from the results: book chapters; PhD, Master and Bachelor theses; reports; conference, discussion and working papers; duplicates; articles not peer-reviewed, not recognized by the database, not available, and not written in English. In doing so, the number of the publications observed passed from 100 to 57, subsequently classified for timing of publication, geographical distribution, number of quotations and content.
The process introduced is represented by the PRISMA 2020 flow diagram in Figure 1 below.
been analyzed with the usage of the catalogue of Discovery Sapienza for filtering the sample identified by Google Scholar, permitting to focus the attention of the study exclusively on peer-reviewed articles.

Exclusion Criteria
After careful processing, the following were excluded from the results: book chapters; PhD, Master and Bachelor theses; reports; conference, discussion and working papers; duplicates; articles not peer-reviewed, not recognized by the database, not available, and not written in English. In doing so, the number of the publications observed passed from 100 to 57, subsequently classified for timing of publication, geographical distribution, number of quotations and content.
The process introduced is represented by the PRISMA 2020 flow diagram in Figure  1 below.

Results
Ordering the titles for year of publication within a range that goes from 1998 to 2020, the results show an increase of the number of publications starting from 2007, with a peak in 2013 and a stabilization from 2018 ( Figure 2).
Even though the development of SNM involved scholars from several European countries, its seminal ideas come from the Netherlands [18,19]. In particular, it arose from a research program started in the middle 1990s by the University of Twente and the University of Maastricht, as an effort to shed light on the relationship between technical and socio-economical changes [20]. Classifying the titles for origins, the results show the involvement of 24 different countries/states, which combined with the number of publications, returned the following outcomes (Table 1).   The counties demonstrating the greatest number of publications are the Netherlands, respectively followed by the United Kingdom, Finland, Sweden, and Germany, evidencing a leading involvement of the Northern European countries. These results are principally due to the fact that during the years SNM has been studied as research model and policy tool by Dutch scientists through several research projects carried out over the years and funded by the EU. The analysis continues with the observation of the number of quotations obtained by the publications. For a more intuitive reading of the results, Table 2 divides the quotations into five ranges that score from 0 to 100, from 100 to 200, from 200 to 300, from 300 to 500, and greater than 500. The most relevant range, containing 31 publications, is the first one, followed by the second with 12 titles, the third with 8, and then by the fourth and the fifth, each one with 3 publications. The Dutch dominance over SNM literature is once again demonstrated by the articles of Kemp et al., (1998) [18] and Schot and Geels, (2008) [21] receiving respectively 3048 and 1869 citations. When moving to the content analysis and after carefully processing the results, the publications have been divided in two macro-categories, General and Specific, containing in turn sub-categories. The first macro-category gathers all the articles that apply SNM generically, without focusing on a single subject. The sub-categories identified are: sustainability transition, networks, renewable energy, and other (which deals with topics such as governance, radical innovations, SNM competence kit, etc.). On the other hand, the second macro-category collects articles that utilize SNM to study specific subjects. The sub-categories identified are: electric vehicles, biofuel, transport, solar power, community energy, biomass, and other (concerning topics such as organic food, sanitation, hydrogen economy, etc.).

Sustainability Transition
In the following part, first examining the general and then the specific articles, we organize the results for author(s)/year, title, synthesis, and sub-category. The tables are followed by deeper descriptions of the articles. Table 3 reports general topics of sustainability transition. Kemp et al. (1998) [18], referring to the non-sustainability of the directions of technical change in transport and agriculture industries, illustrated how technical change was locked in ruling technological regimes, presenting the niche management as a method to stimulate a conversion into a new regime. Caniëls & Romijn, (2008a) [22] reported about SNM as a theoretical framework that leads a socio-technical transition towards a more sustainable development. Referring to SNM, the authors focused on its facilitation in the introduction of new sustainable technologies by means of sheltered societal experimentations in fields such wind energy, biogas, public transport systems, electric vehicle transport, and eco-friendly food production, and on its contribution to a wider shift towards sustainable development. Schot & Geels, (2008) [21] examined experimental conclusions and theoretical contributions of the 10 years before in SNM, underlining that sustainable innovation paths can be eased by creating technological niches. The study concentrated on the function of different nicheinternal mechanisms (visioning, networking, and learning) and the interaction between local experiments and global regulations that direct actor behaviors. Nill and Kemp, (2009) [23] discussed how, although in the past years evolutionary policy approaches have been progressively recommended, seeming ready for supporting sustainable innovation policies, the success of development approaches that comprehend radical or systemic changes is not demonstrated. In the paper, the authors assessed the theoretical principle, instrumental features, and the handling of policy limitations related to SNM, transition management, and time strategies.  Table 4 reports general articles on the role of networks for the niche management.  [24], due to the fact that networking among actors is a fundamental element for the successful development of new technologies [26], tried to shed light on the way such success is related to the characteristics of the network using Social Network Analysis. Hermans et al. (2013) [25], using the perspective of SNM to explore the network practices of a collaborative innovation network, described a method to construct longitudinal two-mode affiliation networks by means studying the network characteristics of an agriculture niche in the Netherlands. Table 5 lists general articles on renewable energy and SNM. Verbong et al. (2008) [27] analyzed long-term innovation policies and development paths of wind energy, biomass, fuel cells and hydrogen, and photovoltaics by means of the utilization of SNM. Al-Sarihi and Cherni (2018) [28] assessed the potentialities of renewable energy initiatives in Oman by using SNM and the analysis of the three internal niche mechanisms.  Table 6 reports other topics dealing with sustainability and SNM. Lovell (2007) [29], discussed how-although SNM regards (also) how governments can spread the introduction of new technologies-the low-energy housing niches built during the 1900s has been leaded by entrepreneurs and not by the Government policy. Hommels et al. (2007) [30], by examining SNM and the PROTEE approach, discussed about the application of intuitions from technology studies to policy decisions with regard  [31], reviewing the literature on transition and niche management, showed the presence of a strong analytical core and the absence of a managerial perspective. For such reason the authors developed a competence kit that provides guidelines to practitioners involved in transition experiments. Coenen et al. (2010) [4] discussed how SNM concerns to closeness benefit in innovation procedures, as recognized in the geography of innovation literature. The authors highlighted how, differently from the latter, SNM does not explicitly claim the locations in which innovations come out as intentional and following specific models. Simulations and policy scenarios for sustainability are paramount [35]. Quitzau et al. (2012) [32] wrote about the necessity to overcome the gap between policy visions and their practice execution, underlining the need of specific forms of strategic work in accordance with new transformative ideas in spatial planning compared to SNM. Kivimaa (2014) [33] examined the role of government-related intermediary organizations in system-level transitions drawing from two theoretical areas: innovation intermediation and sustainability transitions. The analysis indicates how to get from niches to transition, describing systemic intermediaries-such as governmentaffiliated intermediaries-as fundamental in delineating new visions and expectations. Barrie et al. (2017) [34] showed a new decentralized governance structure for sheltered niches in the scenario of transition to circular economy that meets present obstacles of SMN by recognizing the connection with the triple helix innovation system and innovation intermediation. Figure 3 briefly depicts the macro-categories and sub-categories discussed above.  Table 7 reports specific articles of electric vehicles SNM.   [36] discussed how, although previsioning is widely accepted in industry and governments for strategic formulation, its effect on technology development is still vague. The authors proposed the SNM to strength the link between the long-term visions and the short/medium-term actions by developing technological niches. In their view, the improvement of the latter can be fostered by means of field experiments, such as those analyzed related to the application of sustainable technologies on the individual transportation. Temmes [42], joining the SNM and project management literature, examined how learning processes support niche aggregation in the analysis of the fast-charged electric bus system in Europe. Jin and McKelvey (2019) [43], considering intuitions from evolutionary economics and SNM, investigated the rise of an innovation system for new energy vehicles in Hangzhou, China. Table 8 lists specific contributions on biofuel and SNM.  [44] argued about the development of successful policies to promote biofuels by analyzing in depth three experiments in the Netherlands and using the SNM to justify success and failure of these projects. Van Eijck and Romijn (2008) [45], using SNM as analytical tool, presented a study in Tanzania centered on the production of biofuels through a plant called Jatropha Curcas Linnaeus. Caniëls and Romijn (2008c) [46] adopted SNM to study the supply chain design from the point of view of complex dynamics systems analyzing the development of a new biofuels supply chain in East Africa. Kwon (2012) [47], to expand the market share of alternative fuel vehicles, explored the market obstacles and possible policy options, presenting SNM as an effective tool able to strength the policy consequence of financial incentives, especially when is present a strong network effect. Koistinen et al. (2019) [48] explored a case of stakeholder management in sustainability transitions on aviation biofuel to analyze the complications around signaling in SNM processes. Comparing stakeholder management theory and SNM, regarding the illegitimacy of a technology in its initial phase of development, the authors presented a view of the latter as in part given by its reputation. Table 9 gathers specific articles on transport and SNM. Ieromonachou et al. (2004) [49] introduced SNM to discuss the problem related to the congestion, the rising energy use and the pollution of the UK transport system, exploring the transferring of this technique for evaluating travel demand management policy measures. Shah et al. (2009) [50], suggesting a mechanism to generate, expand, and manage a niche, presented an overview on intelligent transport systems in developing countries using an outlook that considers compatibility, cost, technological composure, and institutional preconditions to evaluate the general effectiveness. Turnheim and Geels (2019) [51] utilized the SNM to study the French infrastructure systems, focusing their attention on the modern tramways.  Table 10 reports specific articles on solar power and SNM.  [52], by using insights from both business model and transition studies literature (such as SNM), contributed to the understanding of the increasing number of initiatives started experimenting on photovoltaic in the Netherlands. Elmustapha et al. (2018) [53], to confront the niche development of solar thermal energy and solar photovoltaics in Lebanon, analyzed the success and failure of the development and the spreading of solar energy technologies (for the economic viability at country level see [55]). Mirzania et al. (2020) [54] investigated the reasons for the lag of the concentrated solar power with respect the other renewable technologies, comparing the advanced situation of the US with that of South Africa. Using SNM analysis, the authors identified success factors that could foster the assimilation of concentrated solar power projects in developing countries. Table 11 lists specific articles on energy innovations, local communities and SNM.  [56] analyzed the work of the intermediary actors in reinforcing, increasing, and spreading novel innovations in the UK community energy sector by using SNN for highlighting the important roles played by them. Smith et al. (2016) [57] considering several empirical studies on community energy in the UK and using recent support from national governments as a case study, studied grassroots innovations for sustainability applying three analytical perspectives: SNM, niche policy advocacy, and critical niches. Ruggiero et al. (2018) [58] examined through the lens of SNM how in Finland a small sector such as community energy-conceptualized as a socio-technical niche with the potential to foster a transition to renewable energy-could increase the change in energy production. This is in line with empirical analysis on bioenergy (see [59]). Table 12 deals with topics related to biomass and SNM.  [60] studied the development and implementation of biomass gasifiers in India by using the SNM framework, concluding that contrary to theoretical forecasts, niche development undergoes from regime precariousness rather than regime stability, as investors and customers need more security. Romijn et al. (2010) [61] investigated the sustainability and limitations of the SNM in the scenario of developing economies of South and East Asia. The authors, showing their analogies with SNM, applied different learning-based methods to four biomass energy projects in rural India recognizing decisive factors of success and failure. Table 13 reports other topics dealing with sustainability and SNM.  Tian & Wang, (2020) [77] Chinese green process innovation in automotive painting: the strategic niche management perspective Analysis of the evolution history of Chinese automotive painting by means of a SNM approach

Other
Other (automotive painting) Stiles, (2020) [78] Strategic niche management in transition pathways: telework advocacy as groundwork for an incremental transformation Study of the telework in the US as a case of SNM Other (telework) Smith (2006) [62], studying the case of organic food, examined the relationship between new radical socio-technical practices (belonging to niches) and the incumbent sociotechnical regimes, arguing that mainstream changes are more easily influenced by niches when they show some affinity with the dominant regime. Hegger et al. (2007) [63] examined the processes by means niches can drive to broader changes at the level of sociotechnical regimes analyzing the Western Europe innovation developments in the sanitation. Agnolucci and Ekins (2007) [64] assessed the interest in the possibility of wide-ranging transition to hydrogen as an energy source with regard to technological transition theory and in particular the chance of hydrogen use becoming diffused through SNM. Monaghan (2009) [65], by defining a remodulation of conceptual niche management applied to the UK body disposal, tried to explain how grassroots innovations can be capitalized on and intentionally supported to substitute dominant systems. Schilpzand et al. (2010) [66], reviewing six main outcomes of SNM and a case study on near field communication (NFC) technologies from mobile payment, examined whether SNM is also functional for the study and administration of other kinds of socially beneficial change. Witkamp et al. (2011) [67], identifying theoretical and practical impediments and proposing solutions, examined if and how SNM can be applied to social innovations that do not evolve around a technical artefact, such as social entrepreneurship. Rehman et al. (2012) [68], to develop different cooking technology modalities and using the assumptions of SNM, analyzed two projects applied by The Energy and Resources Institute (TERI) in nine Indian villages. Verbong et al. (2013) [69] examined practices and perceptions of stakeholders on including users in smart grinds experiments in the Netherlands by using the SNM framework. Fam and Mitchell (2013) [70], adopting SNM in view of a sustainable management of wastewater, analyzed how early the experimentation in urine diversion has involved users, observing also whether or not the three internal niche processes have been taken into account. Mlecnik, (2014) [71], describing the rise of a passive house innovation network in the Flemish region, reviewed and discussed the limitations and success elements of SNM with regard the experiences of a company network to boost niche development for integrated design notions. Carvalho (2015) [72], studying the cases of Songdo (South Korea) and Plan IT Valley (Portugal), described how smart city visions centered on information and telecommunication technologies build composite socio-technical challenges that can gain from SNM to promote technological learning and societal enclosure. Seyfang and Longhurst (2016) [1], tested the capability of SNM to be applied to the field of grassroots innovations and movements (see Falcone et al. (2020) [79] for an empirical analysis of grassroots movements and their role towards sustainability) presenting new experimental discoveries from an international study of 12 community currency niches. Ivanov (2017) [73], in a scientific framework that is looking to increase SNM as a tool for politicians and professionals to deal with incremental change in the context of sustainable entrepreneurship, by applying SNM to three Bulgarian universities, revealed some limitation and proposed six precon-ditions for its application. Jain et al. (2017) [74] assessed the governance context for the adoption and diffusion of the net zero-energy buildings through niche formation in India analyzing the state of governance in New Delhi by using the SNM and the governance assessment tool. Susur et al. (2019) [75], in a scenario that sees the industrial park as the mainstream industrial agglomeration model by conducting a SLR and developing a theoretical relying on eco-industrial park literature and SNM, tried to shed light on how can be reached a transition to eco-industrial park, as well as to create a research agenda that would elaborate on sustainability transitions into eco-industrial park growth. Rantala et al. (2020) [76], underlining the existence of autonomous and disconnected niches, analyzed a social media discussion on Facebook related to the reform of national level energy policy in Finland. Tian and Wang, (2020) [77], to analyze the mechanisms underlying green process innovations, conducted a study on the painting procedure of Chinese automotive industry. By utilizing a SNM approach which supports the contributions of stakeholders, the authors investigated, summarized, and analyzed the evolution history of Chinese automotive painting sector from 1986 to 2018. Stiles (2020) [79] presented telework support in the US as a case of SNM that spurred the transformation of office work location practice in the US. Figure 4 briefly depicts the macro-categories and sub-categories discussed above.

Conclusions
Being a focus on socio-technical niches and the related management activities necessary to understand the replacement of the dominant socio-technical regime, the SNM has become a fundamental analytical framework to comprehend the introduction and diffusion of new sustainable innovations by means of social experiments.
The presented SLR, carried out by adopting the PRISMA statement, investigates the focus, development, and implications of the SNM literature by examining the year of publication, geoghraphical distribution, number of citations obtained, and the contents to which SNM have been applied. The first aspect analyzed shows an increase of the number of articles from 2007, with a peak in 2013 and a stabilization from 2018. The second demonstrates that the country/state presenting the greatest number of publications are the Netherlands, followed by the United Kingdom, Finland, Sweden, and Germany. The third illustrates how the vast majority of the articles rank in the span that goes from 0 to 100, presenting a gradual decreasing of the number of titles with the increasing of the considered range value. The fourth feature studied allows for a division of the articles in two macro-categories. The first, "General", gathers all the pubblications that use SNM generically, withouht focusing on a specific subject, recognizing in turn four sub-categories: sustainability transition, networks, renewable energy, and other (which deals with topics such as governance, radical innovations, SNM competence kit, etc.). The second macrocategory, "Specific", puts instead together articles that utilized SNM to study a specific subject. In this case, the sub-categories identified are: electric vehicles, biofuel, transport, solar power, community energy, biomass, and other (concerning topics such as organic food, sanitation, hydrogen economy, etc.).
The increase in publications over the years, the enlargement of the geographical borders interested, the presence of points of reference in the literature, as well as the wide adaptability (both generical than specific) of the theoretical framework to the most diverse energy-related fields, allows us to remark the relevance of the SNM for studying the transition towards sustainability in socio-technical systems.
Future research should look at the development of the SNM research also in light of the current pandemic and try to apply a more refined method of paper selection able to accomplish with different spheres and intersections of scientific diciplines.
Author Contributions: The authors have contributed equally to the study. All authors have read and agreed to the published version of the manuscript.