During the transition period from childhood to adolescence, the brain undergoes a dynamic maturation process, and due to the protracted developing periods, children and adolescents may be especially vulnerable to mental disorders, such as emotional and behavioral disorders, attention deficit/hyperactivity disorder (ADHD), and peer communication disorder (Gregory & Sadeh, 2012; Merikangas, Nakamura, & Kessler, 2009; Paus, 2005). Childhood psychological problems are often the early manifestation of mental disorders, thus childhood mental disorders and the corresponding symptoms may result in more extensive and sustained social function damage, and may also influence personality formation and lifetime achievement of children and adolescents (Belfer, 2008). It is well known that twins include monozygotic (MZ) twins with identical genotypes and dizygotic (DZ) twins with 50% shared genotypes. Due to the unique genetic characteristics in MZ and DZ twins, some twin studies on mental disorders have been widely reported (Mosing, Zietsch, Shekar, Wright, & Martin, 2009; Nordenbæk, Jørgensen, Kyvik, & Bilenberg, 2014); however, timely detection and early intervention are still limited. Therefore, a thorough understanding of factors that can affect the mental health of twins is critical for effective mental health services.
In general, the etiological factors of most mental disorders include genetic heredity and postnatal family environment (Bagot & Meaney, 2010; Fergusson, Horwood, Miller, & Kennedy, 2011; van Winkel, Stefanis, & Myin-Germeys, 2008). Previous studies have shown that postnatal family environments, such as parental mental health symptoms (Kahn, Brandt, & Whitaker, 2004), parenting style (Dwairy, 2004b), and stressful life events (Mezuk et al., 2010), are associated with the mental health of children. A twin study suggests that during childhood, twins may have different internalizing and externalizing behaviors due to gene–environment interaction (Lamb, Middeldorp, Van Beijsterveldt, & Boomsma, 2012). Although genetic and environmental factors have been extensively investigated in individual psychiatric diseases, their contributions to the general mental health status in twin children have yet to be completely understood. Therefore, the current study recruited twins between ages 6 and 16 who underwent psychological assessment as well as environmental assessment performed by psychiatrists and nurses. The current authors aimed to investigate the influences of genes and environment on the general mental health status in these twins.
The current study was performed in accordance with the Declaration of Helsinki and approved by the Institutional Review Broad of Chongqing Medical School. First, school enrollment information regarding twins was obtained through the Chongqing City Board of Education Committee, and the study was supported by its schools and parents. As a result, a total of 76 pairs of twins (ages 6 to 16) who lived with their families (i.e., parents or guardians), as well as their parents and teachers, were recruited.
All participants received psychological assessment using the Strengths and Difficulties Questionnaire (SDQ), as well as environmental assessments using the Parenting Styles and Dimensions Questionnaire (PSDQ; Robinson, Mandleco, & Olsen, 2001), General Health Questionnaire-12 (GHQ-12), the general functioning scale of the McMaster Family Activity Device (FAD-GFS), and Stressful Life Events (SLE) Scale.
All twins were divided into MZ and DZ groups according to zygosity identification. The zygosity of same-sex twins was identified by the Promega Power-Plex® 16 system according to a previous study (Yang, Tzeng, Tseng, & Huang, 2006). Based on DNA detection, same-sex twins with identical 15 multiplex short tandem repeat loci and sex gene were considered MZ, whereas others were deemed DZ. Opposite-sex twins were also deemed DZ. All participants were ethnic Han Chinese. After a complete description of the study, written informed consent was obtained from all participants and/or their guardians.
Strengths and Difficulties Questionnaire. The SDQ, first developed by Goodman in 1997, is a brief and well-established psychological screening tool with high reliability and validity (Dickey & Blumberg, 2004; Goodman, Ford, Simmons, Gatward, & Meltzer, 2003). It has been widely used for evaluating the emotional and behavioral difficulties of children and adolescents in 40 countries and regions. In addition, the Chinese version of the SDQ has been well established (Kou, Du, & Xia, 2005).
The SDQ has three versions: parent, teacher, and self-report. It comprises 25 items that include five factors: emotion, conduct, hyperactivity/inattention, peer, and pro-social. Each item has three levels: full conformity, partial conformity, and complete non-conformity. The three versions of SDQ data were integrated to predict four diagnoses: any psychiatric disorders, emotional disorders, oppositional/conduct disorders, and ADHD/hyperkinesis based on computerized algorithms. Each diagnosis was predicted to be impossible, possible, probable, and very probable.
Parenting Styles and Dimensions Questionnaire. The PSDQ is a recognized questionnaire to evaluate parenting styles of fathers and mothers (Robinson et al., 2001). This survey is divided into three types of rearing patterns, including authoritative parenting style, authoritarian parenting style, and permissive parenting style. The questionnaire includes 62 items with a five-level evaluation ranging from never to always. The authoritative parenting style includes 27 items with four factors: enthusiasm and concentration, rationality/guidance, democratic participation, and gentleness/amiability; the authoritarian parenting style includes 20 items with four factors: verbal antagonism, physical punishment, irrational/punitive strategies, and orders; and the permissive parenting style includes 15 items with three factors: giving up halfway, ignoring impertinent behaviors, and confidence.
General Health Questionnaire-12. The GHQ-12 was established by Goldberg (Golderberg & Williams, 1988) and is a self-reporting questionnaire to screen and evaluate non-psychotic disorders for non-psychiatric patients and community groups (Quek, Low, Razack, & Loh, 2001). Quek et al. (2001) have shown good internal consistency of the GHQ-12 with high reliability and validity (Cronbach's coefficient = 0.79). They have also shown that the GHQ-12 possesses high sensitivity and specificity (Quek et al., 2001). The stability and repeatability of the GHQ-12 in China have also been confirmed, with a test– retest reliability score of 0.67 (Xiao, Bi, & Bu, 1993). The total score of the questionnaire was used to evaluate parental mental health status in the current study.
General Functioning Scale of the McMaster Family Activity Device. This scale is mainly used to collect information of general family function, including activity plans, crisis support, and mutual trust. The scale includes 12 items (six items are graded in reverse) with a four-level evaluation (i.e., agree totally, agree, disagree, disagree completely) (Byles, Byrne, Boyle, & Offord, 1988). The higher the score, the worse family function.
Stressful Life Events Scale. This scale is mainly used to understand the important life events that children experience (Goodyer, 1990). The scale includes 10 items with a two-level evaluation (1 = positive and 0 = negative). A higher score indicates children have experienced more stressful events.
All evaluations were undertaken by two trained and professional psychiatrists (Y.F., Q.L.) with unified statement and communication methods.
All data were analyzed using SPSS 13.0. Count data were analyzed by chi-square test and measurement data were analyzed by t test. Pearson correlation coefficients of the SDQ diagnostic results for each diagnostic category between twins were separately estimated in the DZ and MZ groups. The correlations of the SDQ diagnoses with the parenting styles and SLE were analyzed by Spearman rank correlation. Level of significance was p < 0.05.
Average participant age was 10.5 years (SD = 2.7 years). There were 69 boys and 83 girls, with 26 boy pairs (34.2%), 32 girl pairs (42.1%), and 18 boy/girl pairs (23.7%). The zygosity identification test showed that in all 76 pairs of twins, 41 were MZ twins and 35 were DZ twins. No significant difference was found in gender and age between the MZ and DZ groups.
Genetic Influence on Children's Mental Health
In the MZ group, 82 parent, 34 teacher, and 38 self-report versions of the SDQ scale were collected. In the DZ group, 70 parent, 46 teacher, and 28 self-report versions of the SDQ scale were collected. Data from the three versions were integrated and the probability of four diagnoses are shown in Table 1. The diagnosis generated based on the SDQ showed that 4.6% to 34.9% of all study participants were possibly diagnosed as having any psychiatric disorders, 1.3% to 7.9% possibly had emotional disorders, 2% to 25.7% possibly had oppositional/conduct disorders, and 1.3% to 13.2% possibly had ADHD/hyperkinesis.
Strengths and Difficulties Questionnaire Diagnoses (N = 152)
Pearson correlation analysis showed a statistically significant correlation in the diagnostic probability of any psychiatric disorders and ADHD/hyperkinesis between twins in the MZ group. By contrast, no significant correlation was found in these two diagnostic categories in the DZ group (Table 2). In addition, neither MZ nor DZ groups showed any significant correlation between twins in the diagnostic categories of emotional disorders and oppositional/conduct disorders (Table 2).
Correlational Analysis of Strengths and Difficulties Questionnaire (SDQ) Diagnoses in Monozygotic (MZ) and Dizygotic (DZ) Twin Groups
Family Environment Influence on Children's Mental Health
Spearman rank correlation (Table 3) showed that the probability of any psychiatric disorders based on the SDQ was positively correlated with fathers' authoritarian parenting style score, whereas it was negatively correlated with mothers' authoritative parenting style score. In addition, the probability of conduct disorders was positively correlated with fathers' authoritarian parenting style score. Furthermore, the probability of ADHD/hyperkinesis was negatively correlated with mothers' authoritative parenting style score. Finally, the probability of emotional disorders was negatively correlated with SLE score. However, no SDQ items showed any significant correlation with GHQ-12 or FAD-GFS scores (data not shown).
Spearman Rank Correlations Between Family Environment and Children's Mental Health
The current study investigated the influences of genetic and environmental factors on the mental health of twins between ages 6 and 16. The results showed that based on the SDQ, 4.6% to 34.9%, 1.3% to 7.9%, 2% to 25.7%, and 1.3% to 13.2% of all participants, respectively, were possibly diagnosed as having any psychiatric disorders, emotional disorders, oppositional/conduct disorders, and ADHD/hyperkinesis. In addition, the diagnostic probability of any psychiatric disorders had significant correlation with ADHD/hyperkinesis between MZ twins but not DZ twins. The probabilities of any psychiatric disorders, conduct disorders, and ADHD/hyperkinesis were closely associated with parenting styles and SLE, but not the scores of the GHQ-12 and FADGFS.
The SDQ had been recommended as one of the screening instruments for mental health and emotional behavior assessment in children and adolescents (Kou et al., 2005). The current study evaluated mental health status of twins using the SDQ, and its three key components from parents, teachers, and children themselves were collected to conduct a comprehensive evaluation of children's mental health status. The prevalence rates of any psychiatric disorders, emotional disorders, oppositional/conduct disorders, and ADHD/hyperkinesis were consistent with the prevalence rate of children's mental disorders in China (7.03% to 14.89%) (Li, 2002). These results confirmed that the SDQ may be a sensitive and reliable tool for evaluating the mental health status in twin children.
In addition, significant correlations in any psychiatric disorders and ADHD/hyperkinesis were found between MZ twins but not DZ twins. Kato, Iwamoto, Kakiuchi, Kuratomi, and Okazaki (2005) summarized and showed that mental disorders might be influenced by genetic or epigenetic difference in MZ twins, which might be caused by DNA methylation status or point mutation between MZ twins. In the current study, all studied twins shared common living environments, suggesting that the differences in the correlations of SDQ diagnosed mental health status between MZ and DZ twins might originate from the genetic basis. Therefore, the current study findings indicate that genetic factors may play an important role in the mental health status (e.g., ADHD/hyperkinesis) in twins, but not in emotional disorders and oppositional/conduct disorders.
Furthermore, family environment is also an important factor for the psychological development of children (Repetti, Taylor, & Seeman, 2002). The current study obtained a battery of scales related to children's growing environment: the GHQ-12 was used to assess the mental health status of parents; the PSDQ was used to understand parenting styles; the FAD-GFS was used to collect information regarding general family function, including activity plans, crisis support, and mutual trust; and the SLE scale was used to evaluate important stressors that children experienced during their growth and development. The data showed that fathers' authoritarian parenting style was positively correlated with the probability of any psychiatric disorders and conduct disorders based on the SDQ, whereas mothers' authoritative parenting style was negatively correlated with the probability of any psychiatric disorders and ADHD/hyperkinesis.
Dwairy (2004a) suggested that authoritative parental style was positively correlated with the mental health of adolescents. McKinney and Renk (2008) demonstrated that adolescents who had authoritative parenting style showed better emotional adjustment. These results indicated important guidance for proper parenting styles, which might help lower the chance of mental illness in children. Furthermore, the probability of emotional disorders was negatively correlated with SLE score, which was consistent with the study by Ge, Conger, and Elder (2001). All of these results indicated that environmental factors, such as parenting styles and stressful events, might be closely associated with the mental health status in twins.
There are several limitations in the current study. First, children's psychology is a developmental process. Therefore, a long-term follow-up study is needed. Second, a larger sample may reveal subtler relationships. For instance, it will be feasible to estimate the interaction of environmental and genetic factors on the mental health of twins within a larger sample.
The current study found that both genetic and environmental factors may be important for twins' mental health. Psychiatrists and nurses should pay more attention to proper parenting styles and reduced stress in the diagnosis and treatment of twins' mental health. Mothers should give children rational guidance and encourage them to participate in some family decisions in a democratic manner, whereas fathers should avoid scolding, beating, and punishing children. The current study not only provided important information for further investigations regarding the etiology of children's mental disorders, but also provided the theoretical basis for clinical practice.
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Strengths and Difficulties Questionnaire Diagnosesa (N = 152)
|Any psychiatric disorders||92 (60.5)||53 (34.9)||7 (4.6)||0 (0)|
|Emotional disorders||138 (90.8)||12 (7.9)||2 (1.3)||0 (0)|
|Oppositional/conduct disorders||110 (72.4)||39 (25.7)||3 (2)||0 (0)|
|ADHD/hyperkinesis||130 (85.5)||20 (13.2)||2 (1.3)||0 (0)|
Correlational Analysis of Strengths and Difficulties Questionnaire (SDQ) Diagnoses in Monozygotic (MZ) and Dizygotic (DZ) Twin Groups
|SDQ Diagnoses||MZ Group||DZ Group|
|r||p Value||r||p Value|
|Any psychiatric disorders||0.401*||0.009||−0.126||0.477|
Spearman Rank Correlations Between Family Environment and Children's Mental Health
|SDQ Diagnoses||Fathers' Parenting Pattern||Mothers' Parenting Pattern|
|Any psychiatric disorders||−0.061||−0.070||0.288*||−0.080||−0.280*||0.137||0.151|