Do firstborn children have an increased risk of ADHD?

Magnitude, relationship and determinants of attention deficit hyperactivity disorder and depression among University of Gondar undergraduate students, Northwest Ethiopia, 2022: Non-recursive structural equation modeling

BACKGROUND

Up to 80% of adults with attention deficit hyperactivity disorder (ADHD) may have a concomitant psychiatric disorder. ADHD and depression, combined with the demands of University life, can pose serious challenges for students. However, there are limited studies conducted on this topic in our country. Therefore, the current study assessed the magnitude, relationship, and determinants of ADHD and depressive symptoms among students at the University of Gondar.

METHOD

An institution-based cross-sectional study was employed among 1514 undergraduate students at the University of Gondar from June 1-20, 2022. A stratified, simple random sampling technique was applied. Structural equation modeling was employed. The degree of relationship was interpreted based on the adjusted regression coefficient with 95% confidence interval (CI) and p-value (<0.05).

RESULT

In our study, 37.6% [CI: 35.2-40.1%] and 20.01% [CI: 18.1-22.1%] of the participants had depressive and ADHD symptoms, respectively. Chronic illness [β = 0.19, CI; 0.09, 0.30], alcohol use [β = 0.10, CI; 0.03, 0.17], social support [β = -0.23, CI; -0.29, -0.17], problematic internet use (PIU) [β = 0.23, CI; 0.18, 0.27], and insomnia [β = 0.24, CI; 0.17, 0.30] had a direct relationship with depressive symptoms. Mother education level [β = -0.09, CI; -0.13, -0.06], birth order [β = -0.09, CI; -0.11, -0.07], chat chewing [β = 0.18, CI; 0.06, 0.30], and depressive symptoms [β = 0.73, CI; 0.62, 0.86] had a direct relationship with ADHD. Chronic illness [β = 0.14, CI; 0.06, 0.22], PIU [β = 0.16, CI; 0.13, 0.21], social support [β = -0.16, CI; -0.22, -0.12], and insomnia [β = 0.17, CI; 0.13, 0.23] had an indirect effect on ADHD.

CONCLUSION AND RECOMMENDATION

The prevalence of ADHD and depressive symptoms was high. Chronic disease, alcohol use, social support, PIU, and insomnia had a direct effect on depressive symptoms and an indirect effect on ADHD. Mother education, birth order, chat chewing, and depressive symptoms had a direct effect on ADHD. Our study provided useful clues for clinical treatment and school-based interventions that aim to improve college students' mental well-being. It is better to design school-based intervention and prevention programs to achieve timely diagnosis and treatment of these disorders among university students.

Adult ADHD Positive Screening and Sibship, Birth Order and Academic Achievement: Implications for Epidemiologists and Physicians in the Kingdom of Saudi Arabia

Background

Attention deficit hyperactivity disorder (ADHD) among the youth and university students differs internationally. This study aims to assess the prevalence of ADHD positivity from a validated screening test among medical students across the Kingdom of Saudi Arabia (KSA) and its associated sociodemographic and family-related factors and the impact of ADHD on students' academic performance. Also, to highlight the implications of the current findings on epidemiologists and family physicians in the region.

Methods

This cross-sectional study utilised the Adult ADHD Self-Report Scale (ASRS) symptom checklist along with several questions on sociodemographic, siblings and personal academic achievement. Multivariable logistic regression models were performed to obtain the Odds Ratios (ORs) of risk of ADHD screening positivity and their 95% confidence intervals (CIs) against potential predictors.

Results

The overall prevalence of positively screened medical students was 38.86%. After adjustment, a statistically significant increased risk of ADHD positivity was observed for students who had one sibling (OR = 1.70, 95% CI = 1.09-2.64). Also, upon examination of birth order, a significant increased risk was observed for students who were firstborn (OR = 1.22, 95% CI = 1.02-1.47). With regard to academic achievement, both before and after adjustment, students who screened positive had a 14% increased risk of obtaining an F GPA (OR = 1.14, 95% CI = 0.99-1.30).

Conclusion

A considerable number of medical students are potentially positive for ADHD. Albeit not diagnostically confirmed - this indicates that a substantial number of people have displayed symptoms akin to ADHD, but were not clinically diagnosed. Research into ADHD in the region is still in its infancy, and epidemiologists would benefit from high-quality databases to address this condition. Also, primary care physicians should develop skills in detecting and managing ADHD in children at an earlier age to improve symptoms in adulthood.

In utero antidepressant exposure not associated with ADHD in the offspring: A case control sibling design

Recent studies have reported an association between antidepressant (AD) use during pregnancy and the risk to develop attention-deficit/hyperactivity disorder (ADHD) in the offspring. However, the association might be confounded by risk factors in the pregnant parent. To control for unmeasured factors between pregnancies carried by the same parent, we set up a case-control sibling study using the University of Groningen prescription database IADB.nl. Children receiving medication for ADHD (cases) before the age of 16 years were matched to siblings not receiving such medication (controls). Exposure was defined as at least two prescriptions for any AD during pregnancy, i.e., the period of 39 weeks before the birth date of the offspring. Secondary analyses were performed to assess the effects of the degree of exposure (the amount of Defined Daily Doses) and the type of AD exposed to. Univariate and multivariate logistic regression was used to estimate odds ratios (ORs) with corresponding 95% confidence intervals (CI). In total, 2,833 children (1,304 cases and 1,529 controls) were included in the analysis. Exposure rate to ADs among cases and controls was 2.2% and 2.4%, respectively. After adjusting for the birth date of the child (as a proxy for the date of pregnancy), age of the pregnant parent at birth, use of psychostimulants, opioids, and antiepileptic drugs by the pregnant parent in the 15 months before birth of the child, an adjusted OR of 1.11 (95% CI 0.67–1.83) was found for the risk of ADHD in the offspring when exposed in utero to ADs. This indicates no increased risk of ADHD in offspring following in utero exposure to ADs. The secondary analyses revealed no statistically significant associations either. The present study provides further evidence that an association between in utero AD exposure and ADHD in offspring might not exist. This perceived association may be caused (at least partially) by confounding by indication. The extent to which depression in the pregnant parent could cause mental disorders such as ADHD in offspring, and the mechanisms involved, should be investigated in further studies.

The association between prenatal perfluoroalkyl substance exposure and symptoms of attention-deficit/hyperactivity disorder in 8-year-old children and the mediating role of thyroid hormones in the Hokkaido study

BACKGROUND

Disruption of thyroid hormone (TH) levels during pregnancy contributes to attention deficit hyperactivity disorder (ADHD). Exposure to perfluoroalkyl substances (PFAS) during gestation may affect levels of maternal and neonatal TH; however, little is known about the effect of PFAS on ADHD mediated by TH.

OBJECTIVES

We investigated the impact of maternal PFAS exposure on children's ADHD symptoms with the mediating effect of TH.

METHODS

In a prospective birth cohort (the Hokkaido study), we included 770 mother-child pairs recruited between 2002 and 2005 for whom both prenatal maternal and cord blood samples were available. Eleven PFAS were measured in maternal serum obtained at 28-32 weeks of gestation using ultra-performance liquid chromatography coupled with triple quadrupole tandem mass spectrometry. TH and thyroid antibody, including thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), thyroid peroxidase antibody (TPOAb), and thyroglobulin antibody (TgAb) were measured in maternal blood during early pregnancy (median 11 gestational weeks) and in cord blood at birth. ADHD symptoms in the children at 8 years of age were rated by their parents using the ADHD-Rating Scale (ADHD-RS). The cut-off value was set at the 80th percentile for each sex.

RESULTS

Significant inverse associations were found between some PFAS in maternal serum and ADHD symptoms among first-born children. Assuming causality, we found only one significant association: maternal FT4 mediated 17.6% of the estimated effect of perfluoroundecanoic acid exposure on hyperactivity-impulsivity among first-born children.

DISCUSSION

Higher PFAS levels in maternal serum during pregnancy were associated with lower risks of ADHD symptoms at 8 years of age. The association was stronger among first-born children in relation to hyperactivity-impulsivity than with regard to inattention. There was little mediating role of TH during pregnancy in the association between maternal exposure to PFAS and reduced ADHD symptoms at 8 years of age.

Factors moderating the link between early childhood non-parental care and ADHD symptoms

Introduction

There is little known about the association between non-parental care in early childhood and attention-deficit/hyperactivity disorder (ADHD) symptoms in preschoolers. Therefore, we examined the association between childcare from grandparents/babysitters during infancy/preschool and ADHD symptoms in preschoolers.

Methods

Using stratified random sampling, we developed a sample comprising 1597 parents of children aged 1.6–7 years who were enrolled in preschool in rural and urban areas of China. Parental reports of ADHD symptoms were assessed using the preschool version of the ADHD Rating scale-IV. A regression analysis was used to examine the association between childcare from grandparents/babysitters and ADHD symptoms.

Results

Childcare from grandparents during infancy was associated with ADHD symptoms in childhood (β = 1.03, P < 0.0001). Significant associations between grandparental care and ADHD symptoms were also observed in children from families with more than one child (β = 0.64, P = 0.0035) and children living in rural areas (β = -0.78, P = 0.0032). A babysitter as the primary child caregiver in preschool was especially strongly correlated with ADHD symptoms in girls (β = 7.95, P = 0.0042). Moreover, region was strong factor associated with ADHD symptoms without adjustment for the non-parental caregivers’ age and education, whereas family income was not strongly associated with ADHD symptoms.

Conclusion

Non-parental caregiving (i.e., from grandparents or babysitters) in early childhood was associated with more ADHD symptoms in children. Certain characteristics in children, such as female gender and the existence of siblings, were stronger moderating factors than were family income and region.

Prenatal antibiotic exposure and risk of attention-deficit/hyperactivity disorder: a population-based cohort study

BACKGROUND

Abnormal microbiota composition induced by prenatal exposure to antibiotics has been proposed as a potential contributor to the development of attention-deficit/hyperactivity disorder (ADHD). We examined the association between prenatal antibiotic exposure and risk of ADHD.

METHODS

We conducted a population-based retrospective cohort study of children born in Manitoba, Canada, between 1998 and 2017 and their mothers. We defined exposure as the mother having filled 1 or more antibiotic prescriptions during pregnancy. The outcome was diagnosis of ADHD in the offspring, as identified in administrative databases. We estimated hazard ratios (HRs) using Cox proportional hazards regression in the overall cohort, in a separate cohort matched on high-dimensional propensity scores and in a sibling cohort.

RESULTS

In the overall cohort, consisting of 187 605 children, prenatal antibiotic dispensation was associated with increased risk of ADHD (HR 1.22, 95% confidence interval [CI] 1.18-1.26). Similar results were observed in the matched cohort of 129 674 children (HR 1.20, 95% CI 1.15-1.24) but not in the sibling cohort (HR 1.06, 95% CI 0.99-1.13). Two negative-control analyses indicated a positive association with ADHD despite the lack of a reasonable biological mechanism, which suggested that the observed association between prenatal antibiotic dispensation and risk of ADHD was likely due to confounding.

INTERPRETATION

In our study, prenatal antibiotic exposure was not associated with increased risk of ADHD in children. Although the risk was higher in the overall and matched cohorts, it was likely overestimated because of unmeasured confounding. Future studies are warranted to examine other factors affecting microbiota composition in association with risk of ADHD.

Siblings and Birth Order-Are They Important for the Occurrence of ADHD?

Objective: The associations of birth order, number of siblings, and ADHD was examined. Method: The analysis based on representative, epidemiological data from the German Health Interview and Examination Survey for Children and Adolescents (KiGGS) study (N = 13,488). Results: An increased risk for ADHD in firstborn versus youngest born children (odds ratio [OR] = 1.31, 95% confidence interval [CI] [1.09, 1.58]) and also versus children with no sibling (OR = 1.31, 95% CI [1.03, 1.68]) was revealed, while number of siblings was not associated with ADHD. Results remained stable after controlling for confounders. Conclusion: Firstborn children may receive simultaneously less parental resources and more responsibilities if younger siblings are born. This happens during the vulnerable developmental period of ADHD. In addition, due to higher levels of insecurity, parents are assumed to focus more on potential physical or psychological abnormities in their firstborn children. This may result in a diagnostic bias in firstborn children.

Antibiotic Exposure in the First Year of Life and the Risk of Attention-Deficit/Hyperactivity Disorder: A Population-Based Cohort Study

Early childhood antibiotic exposure induces changes in gut microbiota reportedly associated with the development of attention-deficit/hyperactivity disorder (ADHD). We conducted a population-based cohort study to examine the association between antibiotic use in the first year of life and ADHD risk. We included children born in Manitoba, Canada, between 1998 and 2017. Exposure was defined as having filled 1 or more antibiotic prescriptions during the first year of life. ADHD diagnosis was identified in hospital abstracts, physician visits, or drug dispensations. Risk of developing ADHD was estimated using Cox proportional hazards regression in a high-dimensional propensity score-matched cohort (n = 69,738) and a sibling cohort (n = 67,671). ADHD risk was not associated with antibiotic exposure in the matched-cohort (hazard ratio = 1.02, 95% confidence interval: 0.97, 1.08) or in the sibling cohort (hazard ratio = 0.96, 95% confidence interval: 0.89, 1.03). In secondary analyses of the matched cohort, ADHD risk increase was observed in those exposed to 4 or more antibiotic courses or a duration longer than 3 weeks. These associations were not observed in the sibling cohort. We concluded that antibiotic exposure in the first year of life does not pose an ADHD risk on a population level.

Attention-deficit/hyperactivity disorder and risk for psychiatric and neurodevelopmental disorders in siblings

BACKGROUND

Probands with attention-deficit/hyperactivity disorder (ADHD) are at increased risk for several psychiatric and neurodevelopmental disorders. The risk of these disorders among the siblings of probands has not been thoroughly assessed in a population-based cohort.

METHODS

Every child born in Finland in 1991-2005 and diagnosed with ADHD in 1995-2011 were identified from national registers. Each case was matched with four controls on sex, place, and date of birth. The full siblings of the cases and controls were born in 1981-2007 and diagnosed in 1981-2013. In total, 7369 cases with 12 565 siblings and 23 181 controls with 42 753 siblings were included in the analyses conducted using generalized estimating equations.

RESULTS

44.2% of the cases and 22.2% of the controls had at least one sibling diagnosed with any psychiatric or neurodevelopmental disorder (risk ratio, RR = 2.1; 95% CI 2.0-2.2). The strongest associations were demonstrated for childhood-onset disorders including ADHD (RR = 5.7; 95% CI 5.1-6.3), conduct and oppositional disorders (RR = 4.0; 95% CI 3.5-4.5), autism spectrum disorders (RR = 3.9; 95% CI 3.3-4.6), other emotional and social interaction disorders (RR = 2.7; 95% CI 2.4-3.1), learning and coordination disorders (RR = 2.6; 95% CI 2.4-2.8), and intellectual disability (RR = 2.4; 95% CI 2.0-2.8). Also, bipolar disorder, unipolar mood disorders, schizophrenia spectrum disorders, other neurotic and personality disorders, substance abuse disorders, and anxiety disorders occurred at increased frequency among the siblings of cases.

CONCLUSIONS

The results offer potential utility for early identification of neurodevelopmental and psychiatric disorders in at-risk siblings of ADHD probands and also argue for more studies on common etiologies.

ADHD Prevalence in Spanish Preschoolers: Comorbidity, Socio-Demographic Factors, and Functional Consequences

OBJECTIVE

The object was to examine the prevalence of ADHD among preschoolers, analyzing comorbidity, and the association with socio-demographic factors.

METHOD

We conducted a two-phase epidemiological study of 1,104 preschoolers aged 3 to 6 years in Catalonia, Spain. The Early Childhood Inventory-4 (ECI-4) was administered to parents and teachers. Children at risk of ADHD were assessed using open-ended face-to-face interviews and were observed in a school setting. ADHD diagnoses were based on Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) criteria.

RESULTS

The prevalence of ADHD diagnosis was 5.4%. Male sex and first-born status were risk factors for ADHD. Parents reported more symptoms (12.9%) than teachers (8.7%). Behavioral problems (odds ratio [OR] = 12, p = .001), autism spectrum disorder problems (OR = 9.5, p = .001), and obsessive-compulsive problems and tics (OR = 5.9, p = .001) were specifically related to ADHD diagnosis. Mother's health status and school achievement were lower in ADHD children.

CONCLUSION

Even at early stages of development, ADHD has high rates of comorbidity and a significant impact on school performance and family health.

Parental psychopathology and offspring attention-deficit/hyperactivity disorder in a nationwide sample

OBJECTIVE

To study the associations between a wide range of parental psychiatric disorders and offspring attention-deficit/hyperactivity disorder (ADHD).

METHOD

This study is based on a nested case-control design. The association between parental registered psychiatric diagnoses and offspring ADHD was examined adjusting for socioeconomic and prenatal factors. Data was linked from Finnish nationwide registers. The cases (n = 10,409) were all the children born between years 1991 and 2005 in Finland and diagnosed with ADHD by the end of 2011. Four controls without ADHD (n = 39,124) were matched for every case by sex, age and place of birth. Main outcomes were adjusted odds ratio (aOR) for parental diagnosis of cases vs controls. Analyses were further stratified by sex. Disorders diagnosed before and after birth were analyzed separately.

RESULTS

The odds ratio for ADHD increased when only mother (aOR 2.2, 95% CI 2.0-2.3), only father (aOR 1.7, 95% CI 1.6-1.8) and both parents (aOR 3.6, 95% CI 3.3-4.0) were diagnosed. Maternal diagnosis showed stronger association than paternal. The weight of association between several parental disorders and offspring ADHD were similar. Maternal psychopathology overall showed stronger associations with girls than boys with ADHD. The diagnoses registered after birth did not show stronger association than the diagnoses registered before.

CONCLUSIONS

Maternal psychopathology showing stronger association than paternal implies that environmental factors or their interaction with genetic factors partly mediates the risk of parental psychopathology. Similar associations between several maternal psychiatric disorders and offspring ADHD points towards the need for investigating some common mother-related risk factors.

Maternal Smoking During Pregnancy and the Risk of Psychiatric Morbidity in Singleton Sibling Pairs

Introduction

Maternal smoking during pregnancy has been associated with an increased risk for psychiatric morbidity. We further studied this with Finnish siblings to control for genetic/familial factors.

Methods

From the Finnish Medical Birth Register, sibling pairs were selected as the first two children born 1987-1995 to the same mother (n = 150 168 pairs), along with information on maternal smoking (no smoking/smoking). Information on the children's psychiatric diagnoses related to outpatient care visits (1998-2013) and inpatient care (1987-2013), and the mothers' psychiatric morbidity (1969-2013) was derived from the Finnish Hospital Discharge Register. The first pair analysis compared siblings of mothers who only smoked in the first pregnancy (Quitters, 4.7%) and mothers who smoked in both pregnancies (Smokers, 9.6%); the second analysis included mothers who smoked only in the second pregnancy (Starters, 3.3%) and mothers who did not smoke in either pregnancy (Nonsmokers, 77.5%). Smoking information was missing for 5.0% of pairs. Psychiatric morbidity of the siblings and mother was included in the statistical analyses.

Results

The risk of psychiatric diagnoses was significantly lower for the second child of quitters (adjusted OR 0.77, 95% CI 0.72-0.83) compared to the risk among smokers. A higher risk for psychiatric diagnoses was found for the second child of starters (1.39, 1.30-1.49) compared to the risk among nonsmokers. The effect of smoking was more robust for externalizing diagnoses.

Conclusions

Maternal smoking was independently associated with a higher risk for psychiatric morbidity in children, even when controlling thoroughly for genetic and familial factors.

Implications

Maternal smoking during pregnancy has an independent effect on the risk of psychiatric morbidity in children, even after controlling for non-measurable genetic/familial factors by using a sibling pair design. The effect of maternal smoking was robust for externalizing diagnoses. Maternal smoking during pregnancy had an effect on diagnoses both in outpatient and inpatient care.

Patterns of Early-Life Social and Environmental Exposures and Child Cognitive Development, Rhea Birth Cohort, Crete, Greece

Early-life exposures are critical for later child cognitive development. McCarthy Scales of Children's Abilities (MSCA) were used to assess cognitive development of 700 preschoolers (M_age  = 4.2 years), derived from the "Rhea" birth cohort, in Greece. Principal component analysis (PCA) was applied on prospectively collected exposure data. Six components were extracted; five of them were associated with child cognition. Higher parental social status, preschool attendance and less TV watching, nonsmoking during pregnancy and breastfeeding, and parental involvement in child life were protective factors of child cognition at 4 years. Increased child birth order was negatively associated with child cognition. Offspring's size at birth was not associated with any cognitive outcome. These findings reveal the importance of early-life exposures to child cognitive development.

Identifying Unique Versus Shared Pre- and Perinatal Risk Factors for ASD and ADHD Using a Simplex-Multiplex Stratification

Autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) frequently co-occur. Besides shared genetic factors, pre- and perinatal risk factors (PPFs) may determine if ASD, ADHD, or the combination of both disorders becomes manifest. This study aimed to test shared and unique involvement of PPFs for ASD and ADHD, using an approach that stratifies the sample into affected/unaffected offspring and single-incidence (SPX) versus multi-incidence (MPX) families. Pre- perinatal data based on retrospective parent-report were collected in 288 children (71 % males) from 31 SPX and 59 MPX ASD families, 476 children (65 % males) from 31 SPX and 171 MPX ADHD families, and 408 control children (42 % males). Except for large family size and more firstborns amongst affected offspring, no shared PFFs were identified for ASD and ADHD. PPFs predominantly related to ASD (maternal infections and suboptimal condition at birth) were more often reported in affected than unaffected siblings. PPFs associated with ADHD (low parental age, maternal diseases, smoking and stress) were shared between affected and unaffected siblings. Firstborn-ship was more frequent in SPX than MPX ASD probands. Our results suggest that the co-morbidity of ASD and ADHD is not likely explained by shared PPFs. Instead, PPFs might play a crucial role in the developmental pathways leading up to either disorder. PPFs in ADHD appear to index an increased shared risk, whereas in ASD PPFs possibly have a more determining role in the disorder. SPX-MPX stratification detected possible etiological differences in ASD families, but provided no deeper insight in the role of PPFs in ADHD.

Obstetric and Neonatal Adversities, Parity, and Tourette Syndrome: A Nationwide Registry

OBJECTIVE

To determine the relationships between parity, obstetric adversities, neonatal factors, and Tourette syndrome in a large nationwide cohort.

STUDY DESIGN

This nationwide, register-based, nested case-control study identified all children diagnosed with Tourette syndrome born between 1991 and 2010 from the Finnish Hospital Discharge Register (n = 767). Each case was matched to 4 controls. Information on parity, obstetric, and neonatal factors was obtained from the Finnish Medical Birth Register. Conditional logistic regression was used to determine the relationship between parity, obstetric, and neonatal factors, and Tourette syndrome.

RESULTS

Nulliparity was associated with increased odds for Tourette syndrome (OR 1.7, 95% CI 1.4-2.2), and 3 or more previous births was associated with decreased odds for Tourette syndrome (OR 0.5, 95% CI 0.3-0.9) compared with parity 1-2. Birth weight 4000-4499 g was associated with decreased odds for Tourette syndrome (OR 0.7, 95% CI 0.5-0.9). Low birth weight, gestational age, weight for gestational age, Apgar score at 1 minute, induced labor, birth type or presentation, neonatal treatment, or maternal blood pressure were not associated with Tourette syndrome.

CONCLUSIONS

Increasing parity and high birth weight are associated with decreased odds for Tourette syndrome.

Attention deficit hyperactivity disorder in children is found to be related to the occurrence of ADHD in siblings and the male gender, but not to birth order, when compared to healthy controls

OBJECTIVE

The aim of the present study was to explore the extent to which the prevalence of attention deficit hyperactivity disorder (ADHD) in childhood is associated with birth order and gender, and the prevalence of ADHD and mental retardation (MR) in siblings, as compared to healthy controls. Methods. Data from 200 children diagnosed with ADHD (mean age: 11.13 years; 10.5% females) were compared to data from 200 healthy controls (mean age: 11.0 years; 27.5% females). The data were related to symptoms of ADHD, birth order, gender, family size, and the occurrence of ADHD and MR in siblings.

RESULTS

Compared to controls, the occurrence of ADHD was found to be related to the male gender and to the occurrence of ADHD-related symptoms in siblings (odds ratio: 13.50). Birth order and MR were not associated with the occurrence of ADHD and ADHD-related symptoms. ADHD- related symptoms increased if a further sibling also suffered from ADHD. Conclusions. Among a sample of Iranian children suffering from ADHD, the ADHD and ADHD-related symptoms in childhood were found to be related to the male gender and to the occurrence of ADHD in siblings. Moreover, birth order was found to be unrelated. The fact that symptoms of ADHD-related symptoms increased if a further sibling was suffering from ADHD, and decreased if a further sibling was suffering from MR, is intriguing and needs further explanation.