Happiness is a universal and ultimate goal of life that everybody expects from birth, and it can be considered the psychological state constantly pursued in the course of life [1]. In addition, rather than being abstract, it is defined as the mind feeling sufficient satisfaction and joy in life and the state in which people acquire what they want in everyday life and feel satisfaction and joy for what they have acquired [2]. According to the 8th international comparison report on children and adolescents' happiness index [3], the 2016 subjective happiness index of children and adolescents in South Korea was the lowest among the 22 member countries of the Organization for Economic Cooperation and Development (OECD), with 82 points (out of 100 points), and has maintained its lowest level with scores in the range of 60~70 points every year since the first survey in 2009 until the 2014 survey. Happiness experienced by children is not only an important index to explain the current development and adaptation of children, but also a factor that can predict development and adaptation during adolescence and adulthood, especially because school-age children are prone to experience conflicts and confusions in the process of rapid physical development and mental maturity. Due to the nature of developmental stages, the happiness of school-age children also becomes a developmental task to help them face a smooth adolescence [4].

When children's happiness is closely looked at in conjunction with top-down and bottom-up theories, the explanation of children's happiness with the bottom-up theory is limited because the bottom-up theory is focused on the sociodemographic characteristics related to the happiness perceived by children. The bottom-up theory is the method of evaluating the development and function of the state at the level of people's happiness while the feeling of happiness is an emotion subjectively experienced as individuals live their daily lives. On the other hand, the top-down theory is focused on the factors of the individual's stable tendencies and psychological characteristics that experience the environment in a positive or negative way, and emphasizes children's self-esteem, social activities, and interpersonal relationships as important factors influencing happiness [1]. Specifically, because the first relationship children form with their parents after they are born becomes the source of their intellectual, emotional, social, linguistic, and physical development, the positive relationship between parents and children is explained as the direct predictor variable that influences children's happiness [5]. In addition, from the ecological point of view, the happiness of school-age children is perceived from the activities experienced in various areas such as individual, family, school, and society and, especially for school-age children, a large portion of their happiness is from the experience of school life [6]. The relationship between parents and children is more important than any other interpersonal relationship, as it is the first interpersonal relationship to form when children are born [7] and because school-age children adapt to school life based on the social life learned from parents at home [8]. Additionally, the psychological and emotional propensities formed through interactions with parents and the role of parents in the happiness children perceive has significant meaning [9].

Parents of school-aged children have the obligation and responsibility to provide a good childcare environment for the growth and development of their children, but parents in modern society are complaining of the burden of proper childcare as the time to be with their children decreases due to busy social lives and experiencing the stress of parenting [9]. Parenting stress is defined as distress and uncomfortable experiences occurring in the process of carrying out the role of parenting [10]. It is the cause of emotional distress in marital life [11] and has been reported to be a predictor variable that negatively influences children's development [12]. Moreover, if parenting stress is high and for an extended period of time, the chance of experiencing depression increases, and because parental depression not only makes the relationship with children negative but also increases problem behaviors and negative emotions in children, it can be a conclusive factor that decreases the perceived happiness of children as a result [13].

Research on infants [14] has reported that parenting stress experienced by mothers directly influences depression, and maternal depression influences the negative emotion of the child. Research on children [15] has reported that parenting stress and depression of mothers also influences the development of the child's social behavior. Abidin [16] stated that the high parenting stress negatively influences interactions with children, resulting in inappropriate behavior of the caregiver; and Park & Kwon [17] emphasized that parenting stress is the major variable that influences not only the psychological satisfaction and parenting attitude of parents, but also the development and adaptation of children. In addition, depression of the caregiver is the cause for the caregiver taking a negative parenting style in interactions with children [18], and a study by Marchand & Hock [19], which investigated parental differences in depression, reported that paternal depression has a greater influence on the child's problem behaviors than maternal depression.

Since a husband and wife are significant others, the relationship between them is not only intimate and bidirectionally influencing [20], but also has strong interpersonal dynamics. The stress experienced by one of the parents is very likely to be transferred to the other, and it increases the possibility of transferring depression to the entire family [13]. Accordingly, it is necessary to investigate the actor and partner effects at the same time while treating parenting stress, depression, and family interactions related to children's happiness as a single unit [21]. The majority of previous studies, however, have the limitation of attempting to measure the propensity of children through the characteristics of one parent. In particular, research investigating children's happiness focused on the variables related to parents of school-age children and that are related to development and adaptation in adolescence and adulthood are also lacking. Accordingly, the present study investigated the relationship between parenting stress, depression, parental family interactions, and children's subjective happiness based on the Actor-Partner Interdependence Model (APIM) using the data of the Panel Study on Korean Children. While studies of couple's relationship choose couple an analysis unit, they examine two sets of individual data, assuming that independent variables of an individual not only affect his or her own dependent variables, but also the partner's dependent variables. In such research, the effect of one's own psychological and behavioral aspects on oneself is referred to as ‘actor effect’ while its effect on others is called ‘partner effect’. Presence of partner effect means interdependence of a couple and the greater the partner effect is, the higher level of interdependence becomes [21]. In this sense, APIM, an analysis method for evaluating a couple's effects to each other, serves as a statistical tool for testing the two persons' mutual influence, a partner relationship model that integrates conceptual perspectives on interdependence, and an alternative to avoid type 1 errors.

Specifically, we investigated the relationship among the factors including parental interactions using the Actor-Partner Interdependence Model. The specific purposes of the present study were as follows: 1) Identify actor effect and partner effect of parents' parenting stress and depression on their family interaction as well as the effect of parents' family interaction on children's happiness. 2) Identify the direct and indirect influences of parenting stress, depression, and parental family interactions on children's subjective happiness.

1. Conceptual framework and hypothetical model

As for the conceptual framework of this study, APIM suggested by Kenny et al. [21] was adopted, in an attempt to investigate how parents' parenting stress, depression and family interaction, some of the key variables in the parent-child relationship in the family, affect children's subjective happiness. Considering that emotional status of children in the early childhood is highly influenced by interaction with parents, the relationship of mothers and fathers, who are in charge of childcare in the family system, was examined based on APIM. The effect of one's own psychological and behavioral aspects on oneself is referred to as actor effect while its effect on others is called partner effect. Parents' parenting stress ultimately influences the mutual relationship of the couple, and the stress also affects depression. As such, with an assumption that parenting stress and depression perceived by parents make actor and partner effects on family interaction, the model in Figure 1A as well as research hypotheses were developed as follows.

Figure 1
The path diagram for the hypothetical model.

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Hypothesis 1. Mothers' parenting stress has actor effect on their depression.

Hypothesis 2. Fathers' parenting stress has actor effect on their depression.

Hypothesis 3. Mothers' parenting stress has actor effect on their family interaction.

Hypothesis 4. Fathers' parenting stress has partner effect on maternal family interaction.

Hypothesis 5. Mothers' depression has actor effect on their family interaction.

Hypothesis 6. Fathers' depression has partner effect on maternal family interaction.

Hypothesis 7. Mothers' parenting stress has partner effect on paternal family interaction.

Hypothesis 8. Fathers' parenting stress has actor effect on their family interaction.

Hypothesis 9. Mothers' depression has partner effect on paternal family interaction.

Hypothesis 10. Fathers' depression makes actor effect on their family interaction.

Hypothesis 11. Maternal family interaction affects happiness of their children.

Hypothesis 12. Paternal family interaction affects happiness of their happiness.

The present study investigated the effect of actor and partner effects on children's subjective happiness in the relationship of parenting stress, depression, and family interactions of the parents using the 7th year data of the Panel Study on Korean Children. The following discussion is centered on it.

First, the present study found that parenting stress has an actor effect on depression, and the finding is similar to that of a previous study [14] on infants, which found that the parenting stress mothers experience has a direct effect on depression. Parenting stress is the representative cause of negative emotions, especially depression, and as parents perform the unfamiliar parental role, their skepticism and feeling small in their parenting ability continues to result in negative emotions [27]. Specifically, parenting stress and depression from it have the characteristics of not easily healing with the passage of time, and it is difficult to solve such issues at home. It is one of the important issues that require countermeasures at the community or government level. In addition, since parental depression is closely related to the child's emotional and social development, it appears to be necessary for nurses to pay attention to parenting stress and the depression of parents and offer education and programs related to them when they mediate children's emotional issues.

Second, parenting stress and depression of parents were found to have significant actor and partner effects on maternal family interaction. The finding is similar to the result of a previous study [13] that found the likelihood of parents experiencing depression increases, and the parental depression will create a negative relationship between parents and children, if the level of parenting stress parents experience is high for an extended period of time. It appears that the parenting stress and depression fathers perceive also influences maternal family interaction because, traditionally, the major portion of parenting is shared among mothers in South Korea, and such an excessive role negatively changes interpersonal relationships by causing stress and depression. Additionally, the level of fathers' parenting participation and parenting quality is mostly evaluated by mothers [28]. Therefore, even though it is important to develop educational programs centered on mothers to increase maternal family interaction, it is also necessary to develop contents and programs that help to control paternal parenting stress and depression and to train professionals.

Third, paternal parenting stress was found to have an actor effect on paternal family interaction, and the finding is similar to the result of a previous study [16], which showed that the higher the parenting stress, the higher the negative effect on the interaction with children. The partner effect of maternal parenting stress on paternal family interaction in the present study, however, was found to be statistically nonsignificant. It is difficult to make an accurate comparison due to the lack of research that investigated the relationship of family interactions using the APIM while simultaneously addressing parental stress, but the reason appears to be due to the fact that while the psychological stability and satisfaction of the mother varies depending on the father's behavior in the process of parenting, the emotions the father feels in the process of parenting have greater influence on the father's behavior and satisfaction of parenting [29]. Therefore, it may be effective to find methods of reducing paternal parenting stress to increase paternal family interaction, but it is also necessary to reconfirm these findings through repeated research in the future because of the lack of relevant studies.

Fourth, the depression of parents was found to have actor and partner effects on paternal family interaction. It is similar to the finding of previous studies [18, 19] that showed negative responses in interactions with children if parents were depressed. Because the husband and wife are significant others and the relationship between them has strong interpersonal dynamics, the stress experienced by one of the parents is very likely to be transferred to the other, and it increases the possibility of transferring depression to the entire family [13]. Ultimately, it appears to have a negative influence on the interactions occurring within the family. Therefore, it is necessary to simultaneously nursing approach depression perceived by the husband and wife along with paternal parenting stress to increase the paternal family interaction, and because the process of early diagnosis and treatment by professionals is important for depression, it may be necessary to actively participate in education and promotion in those areas.

Fifth, maternal family interaction has a statistically significant influence on children's subjective happiness, but paternal family interaction was found to be statistically nonsignificant. The results are similar to the finding of a previous study [30], which showed that the family interaction with children perceived by the mother positively influences the child's social relationships. The family serves a key role in the development of the child's sociability, and especially, because the mother's behavior, which becomes the basis for forming social relationships, has a significant influence on children's subjective happiness and plays a positive role in the child's peer relationships, maternal family interaction appears to influence children's subjective happiness. Therefore, to increase children's subjective happiness, it is important to consider not only the characteristics of the child but also various factors that can increase the family interaction the mother perceives. Especially, it appears to be important to simultaneously search for the methods that can reduce the parenting stress and depression of parents identified in the present study. In addition, parents' parenting stress and mother's depression were found to indirectly influence children's subjective happiness with family interaction acting as a mediator. It is difficult to make an accurate comparison due to insufficient research that comprehensively confirms the relationships among such variables, but the parental variables should be accurately identified and addressed to increase children's subjective happiness, and especially, controlling the stress that parents experience in the process of parenting appears to be important.

The significance of this study lies in the verification of the effects of parents' parenting stress, depression and family interaction on children's happiness, by examining the data of the Panel Study on Korean Children, the provision of baseline data for adjustment of the relationship. Furthermore, the study not only provides useful information for developing the contents for parental counseling or education of children and parents in nursing research, but it can also serve as basic data for a nursing intervention for parenting in which parents promote children's happiness. In addition, it is necessary to identify the relationships among cognitive and psychological factors that constantly change according to the flow of time by conducting longitudinal studies rather than cross-sectional studies. As this study utilized data from the Panel Study on Korean Children, there was a limitation for authors to select a variety of variables of children's happiness and check their effects. Also there is a need of additional analysis and replication based on cut-off standards of depression score. Moreover, this study focuses on parental interaction variables of the family unit which are included in the micro-system which has important influence on the children's emotional development based on socio-ecological theory. Thus, there are limitations to generalization because it does not include external environmental variables for children and parents.