Introduction Stress directly affects physical and mental health, as well as the quality of relationships and relationship satisfaction. According to the Vulnerability-Stress-Adaptation Model (VSA; Karney & Bradbury, 1995), stressful events and circumstances, as well as everyday stressors, influence individuals' behaviors in relationships. High perceived stress reduces the capacity for constructive and adaptive responses to events within the relationship and increases the likelihood of maladaptive reactions. Relationships characterized by hostility, negative behavior, poor communication, or physical violence increase levels of perceived stress. Stress is associated with negative communication within the relationship, reduced abilities to maintain the relationship and selfregulation. It is also linked to a negative global perception of relationships, and higher frequencies of emotional and physical aggression, as well as divorce or breakups. In stressful situations, partners tend to show less affection and spend less time together. Higher stress levels are associated with blaming the partner and focusing on the partner's negative behaviors. Stress also leads to negative attributions about the partner. Antagonistic behaviors, which include negative, hostile, or conflict-driven interactions like yelling, insulting, impatience, and hostility, are linked to lower relationship satisfaction because they create a negative emotional climate that undermines trust and intimacy. Conversely, relationship-enhancing behaviors reduce the perception of stress. Affectionate and positive behaviors, such as expressions of love, positive emotional expressions, tenderness, and support, are associated with greater relationship satisfaction. Such behaviors contribute to the development of intimacy and a sense of security within the partnership, which are crucial for the well-being of the relationship. Affection, mutual recognition, and empathy are protective factors that foster closeness and support in partnerships, ultimately leading to greater relationship satisfaction. Partners’ behaviors in a relationship, or adaptive processes, have the strongest effect on relationship satisfaction and quality, and the perceived quality of the partnership reciprocally shapes individual behavior within the relationship. Individual characteristics also affect one's behavior in a relationship. These characteristics can be strengths or enduring vulnerabilities that increase the likelihood of adaptive or maladaptive outcomes in a relationship. These strengths or vulnerabilities influence how individuals interpret daily events within the relationship, their partner's behavior, and the intensity of conflicts within the relationship. A crucial individual characteristic impacting close relationships is the adult attachment style. While the VSA model encompasses relevant aspects of the partner relationship, unfortunately, it does not consider the fact that partners strongly and enduringly influence each other in various life domains, especially in coping with stress. One partner's experience of stress is linked to the other partner's experience of stress. Simultaneously, one partner's resources can either enhance or diminish the resources of the other partner, affecting not only the quality of their relationship but also their partner's (Bodenmann, 1997). Models focused on studying the effects of daily stress or stressful events on partner relationships should view stress in partner relationships as an interpersonal rather than an intrapersonal phenomenon (Falconier et al., 2015). In other words, they should incorporate dyadic coping with stress into adaptive processes, as the SystemicTransactional Model of Dyadic Coping does (Bodenmann, 1997). This model suggests that for relationship quality, it's essential to focus on the perceived positive or negative responses to one's stress and the reciprocity in stress coping. While the Systemic-Transactional Model encompasses the communication of stress, individual responses to a partner's stress signals, and collaborative coping with shared difficulties, it overlooks the socio-cultural context in which the relationship unfolds, as well as other adaptive processes within the romantic relationship. In sum, understanding how daily stressors affect couples' lives, identifying vulnerability factors and resilience to stress, and combining the Vulnerability-Stress-Adaptation Model and the Systemic-Transactional Model can provide a more comprehensive view of how stress impacts relationships. Therefore, with the goal of better understanding the impact of daily stress on couples' lives and identifying factors of vulnerability and resilience to stress, it is necessary to integrate the Vulnerability-Stress-Adaptation Model, which explains the processes that maintain or disrupt relationships, and the SystemicTransactional Model, which explores the impact of perceived stress on behavior within relationships and relationship quality. Study aim The goal of this research was to contribute to a better understanding of the impact of daily stress on the lives of couples and to identify individual and relational vulnerability and resilience factors to stress. Specifically, this research aimed to examine how daily stress manifests in the behaviors of partners in an intimate relationship and how they perceive their relationship. Methodology In research on romantic relationships, including those related to the effects of stress on relationship outcomes, it is essential to use a dyadic methodology, considering that partners are not two entirely independent individuals. By belonging to the same social group, or dyad, they share something in common, which is referred to as non-independence (Kenny et al., 2006). The ActorPartner Interdependence Model (APIM), developed by Cook & Kenny (2005), is one of the most common dyadic approaches in data analysis. It allows for modeling the interdependence of the results between dyad members. Since both members of the dyad respond to the same set of questions, this statistical model makes it possible to distinguish actor effects from partner effects. The actor effect, or the actor's influence, is an intrapersonal effect in which an individual's values on the predictor variable are associated with their values on the criterion variable. The partner effect, on the other hand, is an interpersonal effect where an individual's values on the predictor variable are associated with their partner's values on the criterion variable (Kenny et al., 2006; Laurenceau & Bolger, 2012). In daily diary studies, participants respond to a set of questions every day, akin to keeping a diary. In such studies, we have two levels of data: the betweenparticipants level and the within-participant level. For instance, at the between-participant level, we can say that individuals who experience more stress are less satisfied with their romantic relationships. At the within-participant level, on the days when individuals experience more stress than they typically do, they are less satisfied with their romantic relationships compared to their usual level of satisfaction. Dyadic intensive longitudinal designs expand on intensive longitudinal designs by collecting data for both members of the dyad. These designs are characterized by two types of nonindependence: non-independence of dyad members within dyads and non-independence associated with repeated measurements (Bolger & Laurenceau, 2013; Savord et al., 2023). However, up to 80% of research studies that collect dyadic intensive longitudinal data overlook these sources of non-independence by estimating the results for dyad members separately or averaging the results for each dyad (Planalp et al., 2017). Neglecting non-independence occurs due to the exceptional complexity of preparing dyadic intensive longitudinal data for analysis and the demanding nature of conducting appropriate statistical analyses (Savord et al., 2023). Given the complexity of performing these analyses, dyadic intensive longitudinal research often assesses only actor effects, disregarding partner effects (Bolger & Laurenceau, 2013). Procedure Couples were recruited for the study through personal contacts, mailing lists via social media platforms like Facebook, Instagram, and Reddit, and with the help of psychology and phonetics students in exchange for experimental hours. Before conducting the study, the couples who agreed to participate were informed about the mandatory introductory meeting where the research's purpose was explained to them collectively. The introductory meeting took place five times at the Faculty of Humanities and Social Sciences in Zagreb, lasting one hour. During this meeting, the research's purpose was explained, and participants were provided with informed consent forms with dyadic codes. A presentation on how to use the mobile application was shown, and the couples completed the baseline through the mobile app. The questionnaire took between 15 and 20 minutes to complete. The daily data collection started the evening after the introductory meeting. For this purpose, a mobile application was used. Data in this study were collected depending on an intervalcontingent schedule with fixed times for participants to answer questions, which was every evening between 7:00 PM and 11:59 PM. Participants received reminders via the application. To reduce participant burden, as no fluctuations in measured constructs were expected within a single day, data were collected only once daily for 14 days. Participants were rewarded with gift vouchers for their participation and had the opportunity to enter a lottery with a prize of a spa weekend package at thermal spas. They were also offered materials aimed at improving their relationship based on the research results. Sample The criteria for participating in the study were that couples, at the time of the research, were older than 22 years, in a serious, other-sex relationship lasting at least two years, and the couples needed to have lived together for at least three months. At least one member of the couple had to be employed or financially independent. In the initial data collection phases, participation was limited to couples living in Zagreb or the surrounding area. In the later stages of data collection, this requirement was removed. Therefore, the analytical sample consisted of 140 couples in diverse relationships (n = 280 individuals) with an average age of 35.9 years (SD = 12.05, range = 21 - 81 years) and an average relationship duration of 11.9 years (SD = 10.75) ranging from 1 year and 1 month to 54 years. On average, couples had lived together for nearly nine years (M = 8.9, SD = 10.38, range = 3 months – 52 years). Just under half of the couples were married (n = 64, 45.7%), and slightly more than a third of the couples had children (n = 56, 40%). The age range of children living in the household was from 4 months to 33 years (M = 10.8, SD = 8.56). The sample was heterogeneous in terms of participants' education, with the majority completing higher education levels (n = 128, 45.7%). Most participants were employed (n = 244, 87.1%) and lived in large cities (n = 234, 83.6%). Nearly half of the sample was born in one of the major cities of the Republic of Croatia (n = 122, 43.6%). Approximately the same number of participants assessed their socioeconomic status as average (n = 120, 42.9%). Finally, participants participated an average of 89.9% in the study (SD = 13.28, median = 92.86, range 21.43% - 100%), with women completing a slightly larger proportion of the study than men (Mwomen = 91.7%; Mmen = 88%). Instruments All questionnaires were developed in two parallel versions for women and men to ensure the clarity of question phrasing. In the first step of the research, when participants completed the Basic Survey and Initial Survey, they answered sociodemographic questions, relationship-related questions, and the following instruments in their entirety: Modified Inventory of Experiences in Close Relationships (Kamenov & Jelić, 2003), Dyadic Coping Inventory (Bodenmann, 2008), Socioemotional Climate in Relationships (Huston et al., 2010), Love Scale (Braiker & Kelley, 1979), and a combined measure of relationship satisfaction consisting of one item related to the assessment of the quality of the relationship and the Couples Satisfaction Index (Funk & Rogge, 2007). For this doctoral dissertation, only the results from the Modified Inventory of Experiences in Close Relationships (Kamenov & Jelić, 2003) from the Initial Survey were used because the focus was on investigating daily partner behaviors. In other words, although data on general tendencies of partner behavior in the relationship were collected through the Initial Survey, only data from the Daily Surveys were used in the analysis. Daily, couples responded to ten items from the Socioemotional Climate in Relationships instrument (Huston et al., 2010), one item from the Love Scale (Braiker & Kelley, 1979), four items from the combined measure of relationship satisfaction, and three questions from the Perceived Stress Scale (Cohen et al., 1983). Depending on the result of the Perceived Stress Scale selected questions from the Dyadic Coping Inventory (Bodenmann, 2008) were presented. Specifically, if participants reported experiencing stress, they were asked three questions from the stress communication subscale and six questions related to their partner's dyadic coping with stress. Once a week, joint dyadic coping with stress was assessed from the same inventory. Results To address the research questions and hypotheses, dynamic structural equation modeling (DSEM; Asparouhov et al., 2018) with latent centering was employed. Dynamic structural equation modeling integrates techniques for time series analysis, multilevel modeling, structural equation modeling, and modeling time-varying effects (Asparouhov et al., 2018; Hamaker et al., 2018; McNeish & Hamaker, 2020; Savord et al., 2023). Dynamic structural equation modeling was developed to adequately investigate intensive longitudinal data, which is not achievable by each of these analyses separately (Hamaker et al., 2023). There was a positive covariance among the members of the dyad in their levels of perceived stress, satisfaction with the relationship, perception of partner antagonism, perception of partner affection, experience of partner's dyadic coping with stress, and experience of shared dyadic coping with stress. At the individual level, there was covariation in daily stress levels between the dyad members. Stress spilled over from the previous day to the next for both men and women. Women and men reported higher levels of perceived stress with a unit increase in anxiety compared to the average of all women and men in the sample. Perceived stress was positively associated with individual stress communication for both men and women. On days when their partners experienced more stress, women communicated less of their own stress. Men in relationships with women who were more anxious than the average woman in the study had a stronger association between their own experienced stress and their communication of stress to their partner. Satisfaction with the relationship and love for the partner also spilled over from the previous day to the next for both men and women. Perceiving the partner as antagonistic mediated the relationship between one's own experienced stress and one's own satisfaction with the relationship, as well as the relationship between one's own stress and the partner's satisfaction with the relationship, for both men and women. Additionally, the partner's perception of the actor's affection (both men and women) mediated the relationship between the actor's stress and the actor's satisfaction with the relationship, as well as the relationship between the actor's stress and the partner's satisfaction with the relationship. Dyadic coping with stress did not mediate the relationship between the actor's perceived stress and the actor's and partner's satisfaction with the relationship. However, it was negatively associated with the perception of partner antagonism for both men and women and positively associated with the perception of partner affection for both men and women. In general, both men and women with higher levels of avoidance reported less experienced dyadic coping with stress. Women with higher levels of anxiety experienced less dyadic coping with stress compared to women with typical levels of anxiety. Shared dyadic coping with stress was positively associated with satisfaction in the relationship. Participants who reported higher levels of shared dyadic coping with stress were in relationships with partners who were more satisfied with their relationship. When actors were under more stress, they reported less shared dyadic coping with stress. The anxiety and avoidance of men explained part of the variability of the random intercepts of DCI. Methodological limitations and contributions In this study, the participants were couples who were mostly in satisfying romantic relationships and who experienced high levels of affection and supportive stress coping from their partners, along with low levels of antagonism. Such a phenomenon, where couples in wellfunctioning relationships participate in dyadic research, is not uncommon. However, while this is common, it is questionable whether the results of this research can be applied to couples in less satisfying or even maladaptive relationships with significantly higher levels of antagonism or violence. Additionally, the sample consisted of individuals who reported relatively low levels of daily experienced stress. According to Neff & Karney (2009), one would expect that higher levels of stress would have an even greater impact on partner relationships. Moreover, Laurenceau & Bolger (2005) pointed out that participating in a daily study may have positive or negative effects on the perception and evaluations of partners or partner relationships due to the daily focus on specific relationship characteristics and experiences. The contribution of this study lies in enhancing and deepening the understanding of the dynamics of changes in the perception of daily stress and the relationship between these changes and the perception of a partner's behavior and global satisfaction in romantic relationships and love for the partner. This study provided insight not only into how daily changes in the perception of stress from day to day affect one's own relationship outcomes, but also into how they affect the adaptive processes of both members of the couple, how partners experience these changes, and the mechanism through which perceived stress affects the satisfaction with the romantic relationship and love for the partner for both members of the dyad. Methodological contributions are related to methodological improvements enabled by the dyadic intensive longitudinal design. This approach allows for the examination of the interaction of stress perception and behavior from both partners, providing a more realistic picture of the relationship among the variables under investigation and insights into the dynamics of the connections of particular variables that are significant for maintaining quality romantic relationships in stressful contexts. This doctoral dissertation was the first such study in the Republic of Croatia, opening the way for further dyadic intensive longitudinal research. The data were processed using dynamic structural equation modeling (DSEM), a new analytical strategy that combines the advantages of time series analysis, multilevel modeling, and structural equation modeling within the Actor-Partner Interdependence Model (APIM). Results of this research have practical implications for counseling and therapy work with couples who have difficulties in mutual interaction, coping with their and their partner's stress, and who are dissatisfied with the relationship, as well as for those with risky attachment patterns. Conclusion By including dyadic stress coping as an adaptive process that occurs in a romantic relationship and using the Actor-Partner Interdependence Model (APIM) within which the data were processed, the mechanisms that occur in romantic relationships due to stress were described more accurately. The study aimed to examine how everyday stress affects the behaviors of partners in an intimate relationship and their perception of that relationship. To achieve this, a VulnerabilityStress-Adaptation Model, which explains the processes that either maintain or undermine partner relationships, and a Systemic-Transactional Model of Dyadic Coping, which explores the impact of perceived stress on relationship behavior and quality, were combined. The focus on daily mechanisms of stress on behaviors in romantic relationships, relationship outcomes, and the examination of how the experiences of the previous day affect the following day for both members of the dyad provide a better insight into how daily interactions shape romantic relationships. Mediation models were tested to understand the effects of stress on satisfaction with the partner relationship through specific behaviors, as well as mediation models for the impact of dyadic coping with stress on the perception of stress the next day through perceptions of partner antagonism and affection. In conclusion, this research contributes to a more comprehensive understanding of the interaction between everyday stress, one's own behavior, and the perception of the partner's behavior in the relationship.