Adolescence is a period of behavioral, cognitive, emotional and physical development and thus is potentially a period of vulnerability (1). Currently, the prevalence of mental disorders among adolescents is increasing, and more attention is paid to it (2). According to the World Health Organization (WHO) in 2020, the global population aged 0–14 reached 1.976 billion, accounting for 25.49% of the total population (3). However, accidental death caused by self-harm and suicide represent the third leading cause of death for all adolescents, with approximately 67,000 adolescents dying every year (4, 5). China has 247 million people aged 0–14, accounting for 12.57% of the global population aged 0–14 and ranking second in the world (6). The overall pooled prevalence of total mental disorders among children and adolescents aged 6–18 was estimated at 15.6% in China (7). In contemporary China, the rapid development of the social economy, heavy academic pressure and termination of the one-child policy have changed the traditional family structure and social support system (8). The mental health problems of adolescence, such as academic fatigue, anxiety, compulsion and depression are increasing year by year (9).

In 1982, the state implemented the one-child policy to adapt population growth to economic and social development, that is a couple was encouraged to have one child (10). For 34 years after the implementation of the plan, the Chinese economy developed rapidly and achieved great success, but there were problems such as population aging and low fertility (11). To promote a balanced population development, the state has advocated the two-child plan since 2016; that is a couple is encouraged to have two children (12). At present, second children account for 57% of the total birth population (13). Meanwhile, changes in family structure, intensification of economic burden, ways of living, parents' education and emotional neglect, may cause mental health problems for adolescents from two-child families (14). Gu (15) found that the social anxiety of children in grade 4–6 with siblings was significantly higher than that of only children, and depression was positively correlated with the conflict degree of the sibling relationship. American psychologist Caplan first proposed crisis theory, which considers the birth of siblings to be a pressure event faced by the eldest children in the family (16). Rutter's psychological structure development hypothesis proposes that pressure events, such as family structure transition, are usually considered to lead to developmental changes in psychological functioning, especially the risk of psychopathology (17).

Childhood trauma is an important predictor of mental health problems in adulthood (18). It is related both to a propensity for increased violence later in life and behaviors harmful to health, such as weariness of learning, social disorder, alcohol addiction, physical inactivity, anxiety, depression and self-harm, leading to poor health outcomes, including an increased risk of personality impairment and psychiatric disorders (19). Galletly et al. in a 20-year follow-up study showed that psychotic symptoms were more common in subjects who had experienced childhood trauma; and were associated with higher rates of emotional and behavioral disturbance, childhood adversity, dysfunctional parenting, and alcohol and cannabis abuse (20). Guo et al. (21) found that nearly half of patients with major depressive disorder (MDD) have experienced childhood trauma, which may cause more serious depressive symptoms, a risk of suicide, and cognitive impairment. The response to antidepressants is also worse. With the birth of a second child, an elder child may face childhood trauma such as emotional and physical neglect, or the second child may be excluded by siblings, even facing physical and emotional abuse (22). Apart from the dramatic impact of childhood trauma, anxiety and depression on the person experiencing them, there is also a high social cost to be paid as a result of the individual's poor adjustment and dysfunction in the community. Early support and intervention in children from two-child families may significantly minimize the negative effects of mental health.

At present, there is no research on the mental health of adolescents in two-child families. We do not know the psychological characteristics of this special population and its correlation with the two-child families. By 2030, the birth rate of second children will exceed 60%, and this population will reach 170 million in China (23). There may be more mental health problems related to the factors of the two-child families in the future. This multicenter cross-sectional study investigated the prevalence and contributing factors of childhood trauma, anxiety and depression among adolescents from two-child families in contemporary China.

We examined the prevalence of childhood trauma, anxiety and depression, as measured by the CTQ-SF, SAS and SDS, among senior high school students from two-child families and examined contributing factors in a multicentre cross-sectional study. Our study is the first to pay attention to the mental health problems of adolescents from two-child families, available till date, which is an original in the research direction. We found that the overall prevalence of childhood trauma, anxiety, and depression among adolescents from two-child families in China was high. Regarding childhood trauma, there was significant difference in the prevalence of emotional and physical neglect between the two-child and one-child families. They are somewhat lower than those described in the European report on preventing child maltreatment (35), which reported emotional abuse in 29%, physical abuse in 22.9%, and sexual abuse in 9.6% of participants; however emotional neglect and physical neglect were higher in our study than in theirs, accounting for 18.4 and 16.3% of participants. According to the National Comorbidity Survey-Adolescent Supplement (NCS-A) survey in the U.S., the prevalence of anxiety among 10,123 adolescents aged 13–18 was the highest. Nearly one in three adolescents (31.9%) met the criteria for anxiety disorder, while affective disorder was found in 14.3%, which is different from our study and may be related to cultural backgrounds and the educational model (36).

On the other hand, the prevalence of childhood trauma in different subgroups (when participants were male, in senior grade one, lived alone, had parents who frequently quarreled, had parents with low education, were the first child of two-child families, and reported parents disliking them) were significantly higher than those in other subgroups. The prevalence of anxiety and depression was higher in the groups of students: who lived alone, whose parents were separated, who were the second child of two-child families, and who reported their parents disliked them. We further confirmed the research of Assari and Tanaka. High parental education and family income had significant protective effects on childhood trauma and family conflict was related to high levels of anxiety (37–39). It also supports the hypothesis that the family is a very important social ecosystem affecting the psychological development of adolescents (40).

Furthermore, the social and psychological development of children and adolescents will also be affected by changes in family structure and the coexistence mode of family members (41). Our study found that the two-child families and the ranking of siblings had a significant correlation with childhood trauma, and parental preference had an impact on adolescents' anxiety and depression. Some studies have shown that the function and structure of the family are negatively correlated with the occurrence of childhood trauma (42). It is also an important risk factor for anxiety and depression in adolescents, and there is an interactive effect (43). Earlier studies of Trias's with 419 twins had given and believed that those twins who experienced parental preference had the least total depressiveness, while twins in the intermediate situation had the highest self-confidence (44). This is consistent with our findings, but there are still few studies on adolescent mental health in multiple birth families.

Emotional abuse, physical abuse, emotional neglect and physical neglect in only children and children with siblings were positively correlated with anxiety and depression, but sexual abuse in the one-child family group was significantly correlated with anxiety symptoms. Humphreys et al. found that childhood trauma was positively associated with depression diagnosis and scores, with emotional abuse and emotional neglect demonstrating the strongest associations in a meta-analysis of 192 unique samples from 190 studies, consisting of 68,830 individuals (45). Importantly, individuals with greater childhood traumatic experiences had a tendency to use more overall maladaptive cognitive emotion regulation strategies, which mediated the relationship between early-life traumatic experience and current anxiety symptoms (46). In addition, one possibility for no significant correlation with sexual abuse, anxiety and depression is that chastity is highly valued in Chinese culture and there is early sex education in the two-child families (47). Our finding is consistent with theoretical and empirical accounts of childhood trauma and depression/anxiety.

We also note several limitations. First, research has been greatly restricted because of the COVID-19 pandemic. The current sample was limited to high school students aged 17–19 years. Second, we did not expand more measurement tools for the prediction of adolescent mental health problems. Future studies should further explore the correlation between the mental health status of adolescents from two-child families and family dynamics, family structure and system family function and expand the age range of the sample to follow-up the outcomes after 5, 10, and 20 years. It could provide more solutions for psychologists or psychotherapists.

In this study, the overall prevalence of childhood trauma, anxiety, and depression among adolescents from two-child families in China was high. The two-child families and family ranking are important factors associated childhood trauma, while parental preference is related to anxiety and depression. These results highlight an urgent need to be addressed by adolescent' mental health service providers and policy-makers, and provide them with more solutions.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by the Research Ethics Committee of the Shanghai Pudong New Area Mental Health Center and Tongji University Mental Health Center (No: PDJWLL2019008). Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin.

JT and TZ: data analysis and writing—original draft preparation and revising. MH and XZ: conceptualization and writing—review and editing. FC: methodology, analysis, and interpretation. QW: supervision. MH: project administration. JT, TZ, and MH: sample collection. All authors approved the submitted version of the manuscript.

This work was supported by grants from the Foundation of the Shanghai Municipal Commission of Health and Family Planning (Funding No: 201940161) and the Outstanding Clinical Discipline Project of Shanghai Pudong (Funding No: PWYgy2021-02).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.