The COVID-19 pandemic has greatly changed people's way of life and had a profound impact on people's psychology and behavior. In order to avoid the further spread of COVID-19, many countries, including China, have taken strict containment measures such as quarantine, closure of schools and public entertainment places, and social distancing (1, 2). During the normalization of the COVID-19 pandemic prevention and control in China, although the children have returned to school, they no longer go to public playgrounds and interest classes after school and during holidays as usual. Thus, they spend more time with their families.

Benefiting from the newly revised family planning policy, many Chinese families are transforming from one-child families into two-child families. And in the coming future, even more and more families with three children will appear. Due to the changes in family structure, sibling competition and sibling jealousy have become high-frequency issues of social concern, as well as practical problems faced by many young parents. The birth of the second child often causes some physical and psychological problems compared to that of the first (3, 4). Compared to the parents of two or more children, parents of only one child need to pay special attention to a new type of relationship that emerges after the birth of the second child, the sibling relationship, which was found in Previous studies can be influenced by the factors including sibling factors themselves (e.g., structural characteristics, temperament) and parent factors (parent-child relations, differential treatment, marital relationship, etc.) (5).

Sibling relationships might influence children's development through the mechanism of attachment, social learning, and social comparison (5). Siblings often develop a range of problematic behaviors due to jealousy (6–8). In the period of the normalization of the COVID-19 pandemic prevention and control, Children of two-child families are spending more time getting along with each other at home than before. Parents not only need to work, but also spend more time taking care of their children. They bear greater pressure and are more prone to anxiety and irritability, resulting in inappropriate treatment of their children (9). According to Volling et al. (8), first-born children may be jealous of their younger siblings because their parents spend more time with or take more care of the latter. Therefore, studying sibling jealousy in Chinese two-child families currently has special significance for the healthy development of children.

From a relationship perspective, jealousy refers to the emotion that an individual experiences when his or her important relationship with someone is threatened or faced with loss due to the intervention of a third party. Jealousy usually exists in the context of a social triangle formed by the jealous, beloved, and rival (10–12). As an emotion with a specific function, the purpose of jealousy is to prevent others from taking away the intimacy formed within relations, and the jealous person needs to take certain actions to protect this relationship (13). In family relationships, sibling jealousy refers to the complex social emotions that arise when the intimate relationship between a child and their parents is threatened by another child (12, 14). To restore intimacy, children may approach and seek the attention of their loved ones, become hostile to their siblings, or even prevent their siblings from having contact with their parents (15). At the psychological level, first-born children usually show emotions such as sadness, anxiety, and anger, and on the behavioral level, there will be manifestations such as seeking closeness, yelling at someone, degradation of behaviors, sleep changes, violent behavior, and decreased appetite (16–18). In families with multiple children, sibling jealousy is more likely to occur because of the relative reduction in the interaction time between the mother and one of the children, the unstable quality of attachment security, and the differentiated treatment of parents (18–20). Transactional models hold that the degree of sibling jealousy is primarily influenced by several factors, including individual characteristics, parental characteristics, family relations, and social environment (21). In this study, we focused on the influence of temperament and emotion regulation on sibling jealousy.

Researchers have found that girls' siblings are more jealous than boys' (22–24), and the age of the first child is negatively correlated with jealousy (25). As far as siblings are concerned, different gender combinations and age differences provide a unique family background for children's social adaptation process in China (5) and are also an important factor for siblings to compete for the limited resources of their parents (26). However, studies have found that the age gap and gender combination are not associated with sibling jealousy (24, 27). Therefore, it is necessary to study the influence of age, gender, gender combination and age gap on the jealousy of first-born children in the Chinese test group.

The temperament of children refers to the individual differences in the emotion, activity level, and attention of children in the early stage, and it is the external manifestation of children's response to the surrounding stimulation. It is more of an innate factor that is less affected by the environment. The temperament of young children seems to be the key factor in understanding the difficulties children face in the transition to sibling relationships and whether sibling jealousy appears in early childhood (28). Bad-tempered children are more likely to protest their mothers taking care of the younger siblings after their birth (29). Children with difficult temperaments tend to be less self-soothing, show more negative emotions such as anger, and are reluctant to take care of younger siblings (30, 31). First-born children with difficult temperaments show clinginess, withdrawal, and problems with eating and sleeping during the transition to sibling relationships (31–35).

Emotion regulation is the internal and external process of regulating, evaluating, and managing emotional responses to achieve expected goals, such as controlling emotional expression and effectively managing negative emotions (36). It is also a process in which individuals exert influence on the occurrence, experience, and expression of emotions (37). Children's emotion regulation includes the process of emotion regulation and emotional instability (38). The effect of emotion regulation on jealousy has been confirmed by many researchers (39–41). For example, researchers have found that the use of diversion strategies can reduce the level of pain after jealousy, which shows that effective emotion regulation strategies can reduce negative emotional experiences associated with jealousy (42). In the “More Fun with Sisters and Brothers (MFWSB)” program, researchers have asked children aged 4-8 to learn strategies (e.g., recognition, monitoring, evaluation, and correction) to improve their emotional and social abilities. The results showed that the emotion regulation ability and sibling relationship of the experimental group improved (43). Some researchers also believe that the first child's emotional regulation ability is higher than that of the second child. At the same time, jealousy is more implicit and flexible (27, 40).

There are some commonalities in the development of children's emotion regulation, but there are still some significant individual differences. One of these inherent differences is temperament, defined by Rothbart and Bates (44). Rothbart and Bates believe that temperament is a biological basis that leads to differences in individual behavior, and it will be affected by heredity, maturity, and experience (44). Murphy's research has found that children with more negative emotions and low control do not show stronger jealousy, which he believes is related to the individual's good emotion regulation ability (45). In early childhood, as a part of the temperament, children's emotions and emotion regulation are still affected by individual inherent genetic factors (46). Therefore, the abnormal development of different levels of emotion regulation displayed by children can be traced back to differences in children's inherent temperament (47). Temperament can predict and hinder the development of young children's emotion regulation (48).

Based on the above research, we hypothesize that temperament and emotion regulation can jointly predict sibling jealousy and that emotion regulation plays a mediating role in temperament and sibling jealousy (see Figure 1).

The present research was framed during the normalization of the COVID-19 pandemic, providing unique empirical evidence regarding sibling jealousy and the relationships among first-born children's sibling jealousy, temperament, and emotion regulation. Major results showed that, for first-born children between the ages of 1.17 and 7, the older they are and the greater the age difference between them and the second children, the lower their sibling jealousy. There were no significant main effects for the sex difference in sibling jealousy, which is consistent with previous studies (5, 25, 27). The sibling jealousy of male second-born children was significantly higher than that of females, which was inconsistent with the results of previous studies (22–24). A possible reason was that, in the socio-cultural context of China, older brothers or sisters should give way to younger ones, so the sex difference in the jealousy of the first-born children was not significant, while the male second children had a stronger attachment to their mothers, so their jealousy was higher than that of the females.

Furthermore, this study found that difficult temperament could positively predict sibling jealousy, and emotion regulation was a negative predictor. This result was consistent with previous studies that showed that first-born children with difficult temperaments exhibited more jealous behaviors such as negative emotions, attachment issues, withdrawal, as well as eating and sleeping problems during the transition to sibling relationships (30–35, 56, 57). Children with low emotion regulation who were in a jealous situation would report a higher level of sibling jealousy because of their inability to regulate their own jealousy response (57). Effective emotion regulation could reduce sibling jealousy (27, 42, 43). Although the triadic laboratory paradigm was mainly used to measure sibling jealousy, that is, to design a jealous situation in which a mother or father interacted with one child while ignoring the other, the experimental object was the neglected children, and researchers observed the jealousy emotion and behavior dissonance that were shown in that respective context. In this study, we used a questionnaire to survey the sibling jealousy of first-born children, which involved the first child's anxiety, distress, sadness, anger, bad sleep patterns and eating habits, degradation of behaviors, and attacking the sibling after the birth of the second child. These emotional expressions and behavior performances were consistent with the measurement of sibling jealousy in experimental research. Therefore, the conclusion drawn from the questionnaire survey on Chinese samples was consistent with Western research.

Moreover, this study was the first to explore the mediating role of emotion regulation between temperament and sibling jealousy. As a complex social emotion (12, 14), jealousy is closely related to emotion regulation, while in early childhood, the emotion regulation of children is a part of temperament (46, 47), which, in turn, predicts their emotion regulation (48). Compared with children with intermediate temperament, first-born children with difficult temperament had a weaker emotion regulation ability and higher sibling jealousy. out of 4 domains (i.e., rhythm, avoidance, adaptability, and emotional essence) that were evaluated, difficult children revealed low-scores in at least three domains. They had irregular daily routines, including issues with eating, drinking, sleeping, urinating, and defecating; therefore, children with low temperament rhythms were more likely to be in a changing environment, which was detrimental to their emotional stability. First-born children with an avoidance tendency and poor adaptability to the new environment might be afraid of unfamiliar people and the changes in the environment caused by the new event of the second sibling's birth. Lower scores on emotional nature indicated that first-born children were often in a negative mood, which aggravated the frequency and degree of sibling jealousy occurrence. Therefore, during the normalization of the pandemic, we need to pay special attention to difficult children to help them improve their emotion regulation ability. This will have the effect of reducing sibling jealousy and maintaining a good level of mental health.

This study statically examined the relationships among temperament, emotion regulation, and sibling jealousy during COVID-19 pandemic, but failed to longitudinally investigate the interaction mechanism between siblings in the formation of sibling jealousy and also lacked research on the changes in parenting styles before and after the birth of the second child. Therefore, future research needs to further carry out big data tracking research and in-depth investigations of the interaction mechanism between siblings and that of the parental rearing style and sibling jealousy in different periods. This study used a questionnaire to assess the jealousy toward siblings in first-born children. In the future, it will be necessary to use the triad laboratory paradigm, interviews, and questionnaires to evaluate sibling jealousy more comprehensively and objectively from the perspective of both parents and children.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Scientific Research Ethics Committee of School of preschool education, Capital Normal University. The patients/participants provided their written informed consent to participate in this study.

GQ designed the project and supervised the data collection. WY, GQ, and GD collected and analyzed the data. GQ and GD wrote the manuscript with input from all other authors. All authors contributed to the article and approved the submitted version.

This study was supported by the Project of Municipal Science and Technology Commission of Beijing (20530290022), Capacity Building for Sci-Tech Innovation-Fundamental Scientific Research Funds (20530290062) and Scientific Research Cultivation Fund of Hubei University of Arts and Science (2017kypy025).

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.

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