The associations between public spaces and mental health have been well-documented. Scholars have reached a consensus that well-established public spaces in neighborhoods featuring highly accessible or high-quality sidewalks [23], parks [24], public transport [25], restaurants [26], and leisure facilities [27] contribute to positive mental health outcomes. According to the degree of publicness, public spaces can be classified as typical, semi-, and privately owned [28]. Typical public spaces are openly accessible to everyone, including streets, parks, and public transport [29]. Semi-public spaces are commonly consumed areas, including but not limited to bars, clubs, and restaurants [10]. These facilities often target specific demographic groups depending on their interest preferences or socioeconomic status. Privately owned public spaces include home courtyards, terraces, gardens, small corners in front of and behind the house, and sidewalks in residential areas with restricted access for outsiders, creating a gated environment [30]. These spatial typologies highlight space’s ‘liminality’ nature and the spatial challenges faced by economically or identity-based disadvantaged groups, such as rural migrants. These challenges include inequality, exclusion, segregation, and monopoly [31], resulting in varied access to geographical opportunities and culminating spatial injustice among migrant groups, thereby exacerbating their mental health disparities. Under the national strategic goal of optimizing the overall health of all citizens, it is imperative to give in-depth attention to this issue. Therefore, it is necessary to investigate the impact of different public space types on the mental health of rural migrants. Upon reviewing the existing literature, we propose the following hypothesis:

H1: Public spaces, categorized as typical, semi-, and privately owned, exert significant impacts on the mental health of rural migrants, with different effects depending on their type.

The nexus between public spaces and mental health goes beyond a simple correlation. Numerous nuanced pathways contribute to this relationship, including physical activity, residential satisfaction, stress reduction, and spiritual restoration [9]. Notably, social processes have received considerable attention in recent years as a critical mechanism, reflecting the social capacity-building function of public spaces [32]. The ‘Third Place’ theory provides a crucial lens for interpreting the relationship [33]. It emphasizes the social benefits of third places (public spaces) apart from primary places (homes) and secondary places (workplaces), arguing that third places (1) facilitate social interactions and neighborly relations, (2) provide a sense of belonging and unity and make people feel anchored in their community, and (3) involvement in third places conduces to positive mental status.

A series of social effects arising from the aforementioned social processes, including social capital, social cohesion, and sense of place, are imperative for an individual’s mental well-being [9]. Among these factors, social interaction and perceived integration are particularly important to the mental health of rural migrants [34]. Being culturally and environmentally marginalized in the host cities, migrants need to establish new networks through social interaction and reconstruct their identity and sense of belonging through perceived integration [31]. Social interaction provides essential support for them by improving intercultural communication, while perceived integration diminishes the isolation among outsiders and enhances their involvement in community activities [35]. These two social elements, fundamental to other social and mental outcomes, play a crucial role in determining migrants’ ability to adjust to their new surroundings.

Specifically, social interaction refers to an individual’s explicit social behavior, measured by the frequency of greeting, chatting, playing, eating, and reciprocating with others [36, 37]. Research suggests that increased public access encourages social engagement and face-to-face encounters [38]. Regular interaction with neighbors enhances self-efficacy in coping with mental health problems [39, 40]. Another essential aspect to consider is perceived integration, which refers to an individual’s implicit perception of how well migrants get along with their neighbors. It is generally measured by social trust, emotional connection, and discrimination [10, 41, 42]. For instance, street greenery has been shown to improve mental health by strengthening place attachment [16]. Local restaurants provide unique sites for acquiring emotional support and are advantageous for preserving mental well-being [26].

Furthermore, it should be recognized that public spaces with varying spatial attributes, such as publicness, accessibility, safety, comfort, and social diversity, may construct social capacities differently, leading to inconsistent mental health outcomes [13]. According to the ‘Spaces of Encounter’ theory, the publicness of spaces determines their accessibility and use by different groups [29]. For instance, typical public spaces are intentionally designed to encourage social interaction across a broad spectrum of individuals, promoting inclusivity and diversity. In contrast, semi- and privately owned public spaces often aim to serve particular demographics, leading to a lack of diversity in terms of ethnic/migrant status, potentially resulting in social exclusion among minorities and impeding their integration [13]. Furthermore, people may feel more comfortable and included in open spaces with less surveillance and management than in private spaces with relatively tight control [28]. To summarize, encounters in typical, semi-, and privately owned public spaces can facilitate intergroup interaction and integration to varying degrees [43], which are widely regarded as the critical social determinants of health [44]. Therefore, the following hypotheses are proposed:

H2: Social factors critically mediate the relationship between public spaces and rural migrants’ mental health.

H2a: Social interaction specifically serves as an explicit mediating mechanism and varies across three types of public spaces.

H2b: The influence of perceived integration, as an implicit mediating mechanism, on mental health also varies among different types of public spaces.

It should be noted that while prior empirical investigation acknowledges that well-established public spaces promote intergroup encounters differently [43], it is still controversial whether mere interaction inevitably brings psychological benefits. On the one hand, previous studies have posited the ‘potentially naïve assumption’ that intergroup contact invariably results in inclusive attitudes of urban locals towards migrants [29]. However, contemporary studies have challenged this viewpoint, contending that in certain instances, locals and migrants may coexist ephemerally in the same spaces without engaging in meaningful interactions [45]. Additionally, migrants may experience prejudice, discrimination, and oppression in interactive environments, which can negatively impact their mental health [5].

On the other hand, social interactions can occasionally be disconcerting or negative. As intergroup interactions intensify, rural migrants perceive increasing social and economic disparities with natives in social comparisons, which upsets their psychological equilibrium [46]. Likewise, social interaction may have negative externalities that increase mental health risks, such as unhealthy behavior contagions, like smoking and overdrinking, peer pressure, and excessive rule-based control [47].

These pieces of evidence suggest that the favorable health outcomes of social interaction depend on its ability to yield positive social benefits, particularly through achieving perceived integration. Furthermore, in examining public spaces and the mental health of rural migrants, both explicit (interaction) and implicit (integration) mechanisms operate not only independently but also in interconnected serial dynamics. Dai and He’s study in Shenzhen investigated the influence of neighborhood social environment (social mix) on subjective well-being, verifying a serial mechanism akin to domino effects between social interaction and social capital [15]. However, it is unclear whether this mechanism is applicable in public spaces. Therefore, we propose the following hypothesis:

H2c: In the relationship between public spaces and rural migrants’ mental health, social interaction can also affect mental health through perceived integration, showing a serial mediation mechanism.

Under the guidance of the ‘Healthy China’ strategy, urban public spaces are increasingly seen as an essential component of urban infrastructure to enhance the quality of life and promote social harmony [24]. For rural migrants, public spaces serve as a platform for interacting with urban locals, beyond providing necessary daily services [10]. The social significance of public spaces crucially enhances social adaptation and mental well-being among rural migrants [5].

Rural migrants in urban China without official urban hukou [11] face social and health inequalities due to spatial injustice [12]. In particular, rural migrants residing in deprived neighborhoods have limited access to high-quality typical public spaces, such as parks and greenways [48], which is linked to poorer mental health compared to urban locals [7]. Additionally, rural migrants have challenges in accessing semi-public spaces, such as pubs and upscale restaurants, potentially causing psychosocial stress [34, 49]. Although privately owned public spaces like courtyards are highly accessible [10], they frequently suffer from poor conditions in disadvantaged neighborhoods, including urban villages and old-dilapidated communities [4]. This paradox of high accessibility coupled with low quality may further deteriorate mental health. For instance, Liu et al.’s research in Guangzhou failed to support the influence of neighborhood cleanliness and public space provision on migrants’ subjective well-being [3]. Recently, the social pathways of public spaces affecting migrants’ mental health have also been widely discussed. The social construction of public spaces, such as green spaces, parks, sidewalks, and squares, has also been advocated [43]. A recent study in Shenzhen demonstrated that migrants’ perceptions of green space significantly and indirectly influenced their mental health by enhancing social cohesion [34].

Despite considerable evidence for the positive impact of typical public spaces on the mental health of rural migrants, the influence of semi- and privately owned public spaces, as well as the differences among these types, is not well-understood. Moreover, current research often consolidates social factors into a single mediator, neglecting the nuanced effects and sequential mechanisms within the social process. Further research is necessary to refine the operationalized forms of social mediators and unpack the linkage from explicit behavior to implicit perception underlying the associations between public spaces and mental health among rural migrants.

Addressing this gap, our study proposes a theoretical framework to examine the impact of diversified public spaces on the mental health of rural migrants in urban China. It incorporates both explicit (social interaction) and implicit (perceived integration) mediators, detailing their sequential roles in the progressive social process to enrich our understanding of the intrinsic link between public spaces and mental health (Fig. 1). This adds to the knowledge needed to construct socially friendly public spaces essential for promoting inclusive urbanization.