Arithmetic, reading and writing performance has a strong genetic component: A study in primary school children.

Even children attending the same primary school and taught by the same teacher differ greatly in their performance. In the Netherlands, performance at the end of primary school determines the enrollment in a particular level of secondary education. Identifying the impact of genes and the environment on individual differences in educational achievement between children is important. The Netherlands Twin Register has collected data on scores of tests used in primary school (ages 6 to 12) to monitor a child's educational progress in four domains, i.e. arithmetic, word reading, reading comprehension and spelling (1058 MZ and 1734 DZ twin pairs), and of a final test (2451 MZ and 4569 DZ twin pairs) in a large Dutch cohort. In general, individual differences in educational achievement were to a large extent due to genes and the influence of the family environment was negligible. Moreover, there is no evidence for gender differences in the underlying etiology.

Childhood ODD and ADHD Behavior: The Effect of Classroom Sharing, Gender, Teacher Gender and Their Interactions.

One criterion for a diagnostic and statistical manual of mental disorders (DSM-IV) diagnosis of attention deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) is that symptoms are present in at least two settings, and often teacher ratings are taken into account. The short Conners' Teacher Rating Scales-Revised (CTRS-R) is a widely used standardized instrument measuring ODD and ADHD behavior in a school setting. In the current study CTRS-R data were available for 7, 9 and 12-year-old twins from the Netherlands Twin Register. Measurement invariance (MI) across student gender and teacher gender was established for three of the four scales (Oppositional Behavior, Hyperactivity and ADHD Index) of the CTRS-R. The fourth scale (ATT) showed an unacceptable model fit even without constraints on the data and revision of this scale is recommended. Gene-environment (GxE) interaction models revealed that heritability was larger for children sharing a classroom. There were some gender differences in the heritability of ODD and ADHD behavior and there was a moderating effect of teacher's gender at some of the ages. Taken together, this indicates that there was evidence for GxE interaction for classroom sharing, gender of the student and gender of the teacher.

Polygenic scores associated with educational attainment in adults predict educational achievement and ADHD symptoms in children.

The American Psychiatric Association estimates that 3 to 7 per cent of all school aged children are diagnosed with attention deficit hyperactivity disorder (ADHD). Even after correcting for general cognitive ability, numerous studies report a negative association between ADHD and educational achievement. With polygenic scores we examined whether genetic variants that have a positive influence on educational attainment have a protective effect against ADHD. The effect sizes from a large GWA meta-analysis of educational attainment in adults were used to calculate polygenic scores in an independent sample of 12-year-old children from the Netherlands Twin Register. Linear mixed models showed that the polygenic scores significantly predicted educational achievement, school performance, ADHD symptoms and attention problems in children. These results confirm the genetic overlap between ADHD and educational achievement, indicating that one way to gain insight into genetic variants responsible for variation in ADHD is to include data on educational achievement, which are available at a larger scale.

[Multiple births in the Netherlands]. / Meerlinggeboorten in Nederland.

In the Netherlands, between 2005 and 2011 the twin birth rate decreased from 18.6 to 15.9 per 1000 deliveries. The rise in the number of twin births in the 1980s and 1990 s is not only the consequence of a increase in the number dizygotic twins, but also of a small increase in the number of monozygotic twins. On IVF treatment single embryo transfer is increasingly taking place. This has led to a reduction of dizygotic twins but this does not prevent the development of monozygotic twins. In total, the number of twin pregnancies after IVF treatment has decreased from 803 in 2003 (21.7% of all pregnancies following IVF treatment) to 381 (8.5%) in 2011. Despite the big decline in the perinatal mortality rate over recent years, the risk of still birth or infant death clearly remains much higher in pregnancies with multiples than with singletons. The average gestational age of both monozygotic and dizygotic twins has decreased.