留守青少年因长期亲子分离，面临巨大的成长挑战，近年来引起了学术界的极大关注，同时，留守青少年的发展，也是我国乡村振兴和留守青少年关爱政策的重要关切点。

以往研究更多是基于“问题”或“缺陷”视角，聚焦在留守青少年的身心健康问题和发展风险上，这种“问题/缺陷”视角聚焦在留守青少年是“如何变差的”而不是“何以变好的”，容易给留守青少年贴上负面标签，但鲜有研究者从积极发展(Positive Youth Development)视角出发更全面地看待留守青少年的发展。积极青少年发展视角是上世纪90年代发展心理学领域研究范式的一个重要转变，它强调每个青少年拥有潜在可塑性和个体优势，获得积极发展的青少年进而可发展出较好的终极发展结果。尽管留守青少年身处不利处境，存在一些发展问题，但他们身上也并非全是问题，并非所有留守青少年都发展出严重问题，重“缺陷/问题”不利于看到他们自身存在的资源和优势，不利于促进他们的全面发展。那么，留守青少年积极发展的状况是怎么样的，积极发展随时间变化的轨迹呈现怎样的趋势？为什么有些留守青少年发展较好可以逃脱不利处境的影响，其背后的个体与环境是如何发挥作用的呢？以上的问题目前均尚不清楚。

基于这些问题，本研究聚焦留守青少年，以流动青少年、农村普通和城市普通青少年为参照，首先考察留守青少年与对照组青少年的积极发展现状，并验证了积极发展作为阶段性发展结果的效标作用。其次，从留守青少年积极发展的两种轨迹特点（包括揭示一般规律的总体轨迹和注重个体差异性的轨迹类型）出发，以关系发展系统元理论为总领，以发展资源理论和发展情境为理论指导，基于累积效应和个体与环境交互效应，考察关键性个体因素（即意向性自我调节）与环境因素（即亲子沟通、学校心理环境）对留守青少年积极发展轨迹特点（包括总体轨迹和轨迹类型）的影响机制。最后，为更深入、全面地揭示留守青少年积极发展轨迹的影响机制，采用混合研究方法(Mixed  research methods)，通过有机整合量化和质性研究结果，在更大的生态情境中对量化研究结果进行解释与细化、丰富与拓展，在此基础上构建了留守青少年积极发展轨迹影响机制的理论模型。本论文包括四个实证研究。

研究一通过全国十省市大样本横断数据(N = 16317)，考察了留守青少年与对照组青少年（流动青少年、农村普通与城市普通青少年）的积极发展现状，并开展了积极发展作为阶段性发展结果的效标验证，即考察了积极发展与问题行为、亲社会行为之间的关系，以此为后续研究做好铺垫。结果显示留守青少年积极发展水平总体上良好，但落后于对照组。与以往理论和实证研究结果一致，留守青少年与对照组的积极发展水平均负向预测内外化问题、正向预测亲社会行为，并且积极发展采用总分和分维度计分时上述结果是一致的，从而为后续研究中采用积极发展总分进行分析考察提供了铺垫与支持。

研究二通过追踪设计，进行了2年4轮调查，整班施测获得3267名青少年（其中留守青少年766名），考察了留守青少年与对照组青少年积极发展的总体轨迹。并聚焦留守青少年，以发展资源理论和发展情境理论为理论指导，基于累积效应和个体与环境交互效应，考察了留守青少年积极发展总体轨迹的影响机制。结果显示，留守青少年积极发展随时间变化呈现上升趋势，与流动、农村普通和城市普通青少年呈现的发展趋势一致，但初始水平落后于流动和城市普通青少年。此外，聚焦留守青少年，将意向性自我调节（目标选择、过程优化和目标补偿）、亲子沟通、学校心理环境（安全与秩序、教师支持与同伴接纳、鼓励自主与合作氛围、公平与公正氛围）共8种发展资源进行编码后加和，考察其对积极发展轨迹特点的截距（初始水平）和斜率（发展速度）的作用，结果支持了累积效应，即发展资源数量越多，留守青少年积极发展轨迹的初始水平越高、发展速度越快。本研究还考察了个体（意向性自我调节）与环境的交互（亲子沟通、学校心理环境）对留守青少年积极发展轨迹特点的作用，结果部分支持了个体与环境交互效应的影响机制，在轨迹的初始水平上，存在意向性自我调节与亲子沟通、学校心理环境的交互效应。具体而言，当亲子沟通不良时，丰富的学校心理环境增大了意向性自我调节对轨迹截距的影响；而亲子沟通良好时，学校心理环境并不能调节意向性自我调节对轨迹截距的作用。

研究三采用个体为中心视角，通过对5019名青少年（其中留守青少年861名）开展2年4轮的调查，分析了留守青少年与三类对照组青少年的积极发展轨迹类型，并考察了留守青少年在轨迹亚类型上的分布特点。随后聚焦留守青少年，基于累积效应和个体与环境交互效应，考察了留守青少年积极发展轨迹类型的影响机制。结果显示，青少年积极发展轨迹呈现多样性、可分为4种亚类型，即低起点稳定组(7.40%)、低起点上升组(38.10%)、中起点上升组(25.50%)和高起点上升组(29.10%)，且与参照组相比，留守青少年在低起点稳定组(13.90%)和中起点上升组(43.20%)上分布比例较高、在低起点上升组(32.60%)和高起点上升组(10.30%)分布比例较低。并聚焦留守青少年，考察了其积极发展轨迹类型的影响机制，结果支持了累积效应，并部分支持了个体与环境交互效应。具体而言，逻辑回归分析发现，发展资源数量越多，留守青少年越容易被归入更积极的亚类型组别（即更高起点上升组别）。其次，采用逻辑回归进一步考察个体与环境的互动如何影响个体归入积极发展轨迹类型亚组的概率。结果显示，以低起点稳定组为参照组，意向性自我调节、亲子沟通和学校心理环境的三重交互项显著预测个体归入中起点上升组、高起点上升组的概率。具体而言，当亲子沟通和学校心理环境两种资源都充足或匮乏时，个体归入中起点上升和高起点上升组的概率均会随着意向性自我调节水平的上升而升高，且在两种资源都充足时达到最高水平；当亲子沟通和学校心理环境两种资源呈现出一多一少时，两种资源可以通过相互补偿的模式来提升个体归入中起点上升组的概率，而通过单方面补偿的模式（即学校心理环境资源充足能补偿亲子沟通不良，反之则不然）来提升个体归入高起点上升组的概率。

研究四采用混合研究方法解释性序列设计，基于量化研究结果，开展整合的质性研究，探究了留守青少年积极发展轨迹的个体优势与环境资源及其作用方式，在整合量化研究结果与质性研究发现的基础上，建构了留守青少年积极发展轨迹影响机制的理论模型。通过对38名教师、学生和家长一对一的深度访谈与文本编码、归纳分析，发现了留守青少年个体优势包括意向性自我调节、留守意义感、学业投入、自律与坚持、未来控制感和留守特征共6个方面，环境资源包括家庭和学校两方面，家庭资源包括父母期望与规范、亲子沟通、祖辈抚养人教育方式、同胞支持与陪伴和家庭氛围共5个方面，学校资源包括教师支持、同伴接纳、班级氛围和家校沟通共4个方面。本研究还发现了五种留守青少年个体与家庭、学校环境交互作用的模式，将中国传统文化融入此5种模式的内涵，分别命名为：“锦上添花”、“雪中送炭”、“相得益彰”、“近朱者赤”和“自强不息”，生动地揭示了个体与环境是如何相互作用从而促进留守青少年积极发展的。并基于以上结果和已有理论，建构了留守青少年积极发展轨迹影响机制的理论模型，将对未来指导留守青少年的积极发展起到重要的作用。

综上，本论文首次将积极发展视角引入留守青少年发展轨迹的实证研究，采用追踪研究设计，基于累积效应和个体与环境交互效应，考察了留守青少年积极发展轨迹的特点及影响机制。在此基础上创新性地采用混合研究方法整合量化和质性研究结果，建构了留守青少年积极发展轨迹作用机制的理论模型，为留守青少年关爱保护政策的制订提供了理论框架，有助于未来留守青少年相关的实证研究与精准干预向纵深推进。

Due to prolonged parent-child separation, left-behind youths usually encounter various challenges when growing up, which has caught great attention in the research field of developmental psychology. Moreover, Chinese rural revitalization and left-behind youth care policies have placed particular emphasis on the development of left-behind youths.

Previous studies have been mainly based on the “problem-oriented” or “deficiency-focused” perspectives to investigate the developmental risks or physical and mental health problems among left-behind youths. However, these perspectives emphasize how left-behind youths are “worse off” rather than “better off”, which may result in negative labels for left-behind youths. Yet, limited studies have taken the Positive Youth Development perspective to understand the development of left-behind youths in a comprehensive way. The Positive Youth Development perspective represents critical paradigm shifts in the research field of developmental psychology in the 1990s. This perspective offers glimpses of the potentials and strengths of youths, and points that positive youth development may facilitate better developmental outcomes in youths. Although adverse environments may lead to developmental problems in left-behind youths, not all left-behind youths are equally influenced or  exhibit serious problems. Moreover, only focusing on the problems and deficits among left-behind youths may conceal the strengths and personal assets, which may hinder the investigation of positivity in youth development.So far, answers to the following research questions remain unclear: (1) What is the level of positive development among left-behind youths? (2) How does positive development change across time? (3) Why some left-behind youths can “escape from” disadvantageous conditions to gain positive development, and how these processes are related to the interplay between individual and environmental factors?

To fill these gaps, the present study aims to investigate the following research questions based on large samples of migrant youths, rural local youths, and urban local youths (as reference group): (1) Examining the differences in the positive development between left-behind youths and control groups, and verify the roles of positive development as a criterion of validity for outcomes of staged development; (2) Testing the effects of key individual (i.e., intentional self-regulation) and environmental factors (i.e., parent-child communication, school psychological environment) on the characteristics of two positive developmental trajectories (i.e., general trends over time and trajectory patterns focusing on individual differences) among left-behind youths. This research question is guided by the relational developmental system metatheory, developmental resource theory and developmental context theory, with a particular interest in revealing cumulative effect and/or individual-environment interaction effect; (3) Conducting mixed-method research (i.e., combine the quantitative and qualitative design) to reveal the mechanisms behind above associations deeply and comprehensively. Results of the mixed-method research may explain and refine, enrich and expand the quantitative research results in a larger ecological context by organically integrating the quantitative and qualitative research results. We are also interested in constructing a theoretical model to describe mechanisms of the positive developmental trajectory among left-behind youths. Specifically, this dissertation is consisted of four empirical studies.

Study 1: Using cross-sectional data from a large sample of youths from ten provinces and cities in China (N = 16,317), this study examined the differences in positive development between left-behind youths and control groups (i.e., migrant youths, rural local youths, and urban local youths). This study also revealed that positive development was a criterion of validity for outcomes of staged development (i.e., testing the relation between positive development and internalizing problem, externalizing problem, and prosocial behavior), which paved the way for follow-up studies. The results showed that left-behind youths had a good level of positive development, but scored lower than that of the control groups. Consistent with previous theoretical and empirical literature, positive development negatively predicted internalizing and externalizing problem and positively predicted the prosocial behavior both among left-behind youths and control groups. The results remained the same regardless of using total score and subscale scores of positive development, supporting using the total scores of positive development for further analyses.

Study 2: Using a four-wave longitudinal data spanning 2 years (n  = 3,267, of which 766 were left-behind youths), this study preliminarily examined the development trajectories of positive development among left-behind youths and youths in control groups. Guided by developmental resource theory and developmental context theory, this study placed a particular focus on the left-behind youths, and examined the underlying mechanisms of positive development trajectories based on the cumulative effect and the interaction effect between individuals and the environment. The results showed that the positive development of left-behind youths significantly increased across time, which was similar to the control groups; however, the initial level of positive development of left-behind youths was significantly lower than that of migrant and urban youths. In addition, for left-behind youths, a total of eight developmental resources, including intentional self-regulation (selection, optimization, and compensation), parent-child communication, and school psychological environment (safety and order, teacher support and peer acceptance, encouragement for student autonomy and cooperation, and equality and fairness), were coded (0 = low, 1 = high) and summed to examine their effects on the intercept (initial level) and slope (developmental speed) of positive developmental. Results supported the cumulative effect that the number of developmental resources was associated with higher initial level and faster rate of change in the positive development trajectory among left-behind youths. This study also examined the role of individual strength (intentional self-regulation) and environmental assets (parent-child communication and school psychological environment) on the trajectories of positive development among left-behind youths. Results partially supported the individual-environment interaction effect, such that the three-way interaction among intentional self-regulation, parent-child communication and school psychological environment significantly predicted the initial level (intercept) of the positive development. Specifically, at lower (compared to higher) levels of parent-child communication, good school psychological environment amplified the effect of intentional self-regulation on the intercept; at higher (compared to lower) levels of parent-child communication, school psychological environment did not moderate the effect of intentional self-regulation on the intercept.

Study 3: Using a four-wave longitudinal design spanning 2 years (n  = 5,019, of which 861 were left-behind youths), this study adopted person-centered approach to depict the developmental patterns of change in youths’ (i.e., left-behind youths, migrant youths, rural local and urban local youths), and examine the distribution characteristics of the developmental pattern among the left-behind youths. Moreover, the current study also examined the underlying mechanisms of positive development trajectory profiles among left-behind youths based on cumulative effect and individual-environment interaction effect. The results showed that in development of youths’ positive development exhibited heterogeneity and could be categorized into four classes: low-stable group (7.40%), low-increasing group (38.10%), middle-increasing group (25.50%) and high-increasing group (29.10%). Compared with the reference groups, the proportion of left-behind youths was higher in the low-stable group (13.90%) and the middle-increasing group (43.20%), while lower in the low-increasing group (32.60%) and the high-increasing group (10.30%). In addition, the current study examined the underlying mechanism of heterogeneity in development of left-behind youths’ positive development. Results supported the cumulative effect and partially supported the individual-environment interaction effect. Specifically, Logistic regression analysis showed that the greater the number of developmental resources, the more likely the left-behind youths were classified into a more positive trajectory subtype (i.e., the high-increasing group). Results also showed that the three-way interaction term of intentional self-regulation, parent-child communication and school psychological environment significantly predicted the probability of individuals being placed in the middle-increasing group and the high-increasing group, using the low-stable group as the reference group. Specifically, when both parent-child communication and school environment resources were sufficient or scarce, the probability of being in the middle-increasing group and the high-increasing group increased with the increase of intentional self-regulation, and reached the highest level when both resources were sufficient; when parent-child communication and school environment resources were one more and one less, the two resources could compensate each other to increase the probability of being in the middle-increasing group; while the one-sided compensation model (i.e., adequate school psychological environment can compensate for poor parent-child communication, but the inverse is not) can increase the probability of being in the high-increasing group.

Study 4: This study used mixed methods sequential explanatory design to conduct an integrated qualitative study to further investigate how personal strengths and environmental resources jointly contribute to the positive development trajectory of left-behind youths, which may extend above quantitative findings. This study would contribute to constructing a theoretical framework depicting how the positive development trajectories of left-behind youths are influenced based on the integration of quantitative and qualitative findings, especially in the context of Chinese culture. Through one-on-one in-depth interviews with 38 teachers, we found that the the left-behind youths’ personal strengths included six aspects: intentional self-regulation, meaningfulness of staying behind, academic commitment, self-discipline and persistence, future control and characteristics of staying behind, and environmental assets included both family and school resources, with family assets including parental expectations and norms, parent-child communication, grandparental caregiver’s parenting style, and school assets including four aspects: teacher support, peer acceptance, classroom climate, and home-school communication. Moreover, this study also identified five patterns of interaction between personal strengths and family and school environments assets, and the connotations of these five patterns were incorporated into traditional Chinese culture and named as “icing on the cake (Jin Shang Tian Hua)”, “sending charcoal in snow (Xue Zhong Song Tan)”, “complimenting each other (Xiang De Yi Zhang)”, “close to the vermilion (Jin Zhu Zhe Chi)”, and “self-improvement (Zi Qiang Bu Xi)”, which vividly revealed how individuals and the environment interact to promote the positive development of left-behind youths. Based on above findings and existing theories, a theoretical model of how positive development trajectories of left-behind youths were influenced was constructed based on Chinese cultures, which would play an important role in guiding research on positive development of left-behind youth in the future.

To sum up, the present study is one of the first to adopt Positive Youth Development perspective to study developmental trajectories and underlying mechanisms among left-behind youths. The present study places particular emphasis on testing the cumulative effect and the interaction effect between individual and the environmental factors and use the longitudinal design. To go further, this study innovatively uses a mixed research method to integrate findings of quantitative and qualitative findings, contributing to a theoretical model of the mechanisms of the positive development trajectories among left-behind youths, which played a role in attracting new ideas, for the implementation of the rural education revitalization national policy and the left-behind youth. This dissertation could provide a theoretical framework for Chinese left-behind youth care and protection policy and guide future empirical research and practical interventions for left-behind youths with rural-to-urban parents.