Depression is one of the diseases with the highest prevalence in the global population, which has caused great disease burden. World Health Organization predicted that major depression would become the primary cause of global disease burden by 2030[1]. According to a global disease survey in 2016, more than 34 million all-age disability-adjusted life-years (DALYs) could be attributed to depression[2]. Compared with that of the world level, the present situation of depression in China is also severe. A meta-analysis showed that the prevalence of depression among Chinese children and adolescents in different regions was about 20%[3]. And the result of a survey on depression among the elderly in China found that the depression prevalence among the study population was 38.5%[4]. It shows that the prevalence of depression is high in different Chinese populations. So, it is of great practical significance to study the prevention of depression.
In order to prevent the occurrence of depressive symptoms more specifically, it is important to understand the occurrence and development of depressive symptoms and to identify the relevant influencing factors during this period[5]. It needs to be clear that the occurrence of depressive symptoms is the accumulation of related events in the whole life cycle[6]. Experiences in childhood and adulthood could affect the development of later depressive symptoms[5, 7]. At present, relevant studies often focused on the influencing factors of adulthood. For the general population, researchers often consider the effects of life events in adulthood on depressive symptoms including stress, burnout et al.[5]. For people with disease, the disease condition, medication and life quality are the focus of attention[5, 8], while the Experiences in childhood are less studied. However, childhood is a critical period for the health development of individual psychology. Related studies have found that childhood life events contributed to adult depressive symptoms[9]. The mental state of long-term contact objects in childhood also affected the occurrence and development of individual depression symptoms[10].
For children, parents might be the people with the highest contact frequency. Parents with depressive symptoms might influence their children in many ways. From the perspective of psychiatry, some mental diseases, such as depression, schizophrenia, anxiety, have certain genetic tendencies[11–13]. From a psychological point of view, people are more susceptible to their parents' influence in childhood. Individual behaviors and thoughts reflected in adulthood have something in common with their parents to a certain extent. When parents existed mental problems, it meaned that they might have difficulties on taking care of their children, such as lower enthusiasm, over-participation, disgust, abuse and over-supervision[14–16]. These may directly or indirectly affect the mental health of their children in adulthood. Based on lifelong epidemiological studies, parents with depressive symptoms might have poor financial conditions and living habits, and they shared such a living environment with their offspring. All of these factors might be associated with adult depression[7, 17]. Related study showed that the people contacting with parents with depressive symptoms in childhood had a higher risk of adult depressive symptoms than the non-contacts[17]. Many studies supported this conclusion[18, 19]. In addition, parents' depressive symptoms in individuals’ childhood not only increases the risk of adult depressive symptoms in offspring, but also makes the individual's treatment effect on depressive symptoms worse[20]. Overall, childhood parental depressive symptoms is a relatively clear risk factor for adult depressive symptoms.
However, there are still some points to discuss about the childhood parents' depressive symptoms and individual adult depressive symptoms. Firstly, what are the factors that may moderate or mediate between the two? Secondly, whether parents' depressive symptoms in childhood are still the influencing factors of adult depressive symptoms after adjusting the relevant factors in adulthood? Thirdly, the existing studies tend to focus on the general population, specific age groups (children, adolescents, the elderly) and people with chronic diseases. In certain disease groups, is childhood parental depressive symptoms still a risk factor for adult depressive symptoms?
People with cardiovascular diseases(CVD) are a high-risk group suffering from depression. Moreover, depressive symptoms and CVD often coexist[21–23]. Studies have shown that the prevalence of CVD in different groups was from 20–45%, which is higher than that of the general population of the same age[24]. In patients with some kinds of heart diseases, depressive symptoms is even more serious. Related study found that 71.4% of women and 60.4% of men had symptoms of depression or anxiety in people with myocardial infarction(MI)[25]. However, it is different from the general population. The studies on depressive symptoms in patients with CVD often focus on the related factors after the occurrence of the disease[8]. At present, there is few studies exploring on the influence of childhood-related factors on adult depressive symptoms in patients with CVD.
We want to explore a protective mediator between childhood parental depressive symptoms and depressive symptoms in adulthood. However, in recent years, studies are more likely to consider some risk factors as mediator s, such as rumination, emotion regulation and negative life events[10, 26, 27]. In fact, protective and risk factors are equally important. In some depression-related studies on the mediating role, friendship is often regarded as a protective mediator. A longitudinal survey on child abuse and neglect found that depression and friendship were related to suicidal ideation in late adolescence, and friendship was the protective factor of the other two[28]. The result of an adolescent study showed that adolescent friendship /family support mediated between boys' and girls' early life stress and late adolescent depressive symptoms[29]. Therefore, friendship might also be a mediator between childhood parents' depressive symptoms and offspring's depressive symptoms in adulthood. The purpose of our study is to clarify the possible relationship among childhood parents' depressive symptoms, friendship and adult depressive symptoms.