I n 1986 we began The Netherlands Twin Register (NTR) by recruiting young twins and multiples a few weeks or months after birth. Currently we register around 50% of all newborn multiples in The Netherlands. Their parents receive a questionnaire at registration and afterwards when the children are 2, 3, 5, 7, 10 and 12 years of age. Teachers are asked to rate the behavior of the children at ages 7, 10 and 12 years. Adolescent and young-adult twins were recruited through City Councils in the early 1990s. These twins, their parents and siblings participate in longitudinal survey studies that include items about health, fertility, lifestyle, addiction, personality and psychopathology, religion, socioeconomic status, and educational attainment. The total number of twins and multiples registered with the NTR is currently over 60,000. Subgroups of twins and siblings take part in studies of cognitive development, brain function and neuropsychological indices of attention processes, and molecular genetic studies of classical and behavioral cardiovascular risk factors. DNA samples are currently collected in selected twin families for two large linkage studies, which aim to find QTLs for anxious depression and for nicotine addiction. Sisters who are mothers of DZ twins contribute DNA samples for a linkage study of DZ twinning. Large cohorts of phenotyped family members from the general population are very valuable for genetic epidemiological studies and permit selection of informative families for gene finding studies.

OA and AP are highly heritable at all ages in both genders. The same set of genes appears to be expressed in boys and girls. The size of genetic and environmental contributions remains the same across the ages studied. Stability in OA and AP is accounted for by genetic influences. Children who do not display OA or AP at a given age are unlikely to develop these problems at a subsequent age.

Maternal ratings on internalizing (INT) and externalizing (EXT) behaviors were collected in a large, population-based longitudinal sample. The numbers of participating twin pairs at ages 3, 7, 10, and 12 were 5,602, 5,115, 2,956, and 1,481, respectively. Stability in both behaviors was accounted for by genetic and shared environmental influences. The genetic contribution to stability (INT: 43%; EXT: 60%) resulted from the fact that a subset of genes expressed at an earlier age was still active at the next time point. A common set of shared environmental factors operated at all ages (INT: 47%; EXT: 34%). The modest contribution of nonshared environmental factors (INT: 10%; EXT: 6%) could not be captured by a simple model. Significant age-specific influences were found for all components, indicating that genetic and environmental factors also contributed to changes in problem behavior.

I n order to determine high school entrance level in the Netherlands, nowadays, much value is attached to the results of a national test of educational achievement (CITO), administered around age 12. Surprisingly, up until now, no attention has been paid to the etiology of individual differences in the results of this national test of educational achievement. No attempt has been made to address the question about the nature of a possible association between the results of the CITO and cognitive abilities, as measured by psychometric IQ. The aim of this study is to explore to what extent psychometric IQ and scholastic achievement, as assessed by the CITO high school entrance test, are correlated. In addition, it was investigated whether this expected correlation was due to a common genetic background, shared or nonshared environmental influences common to CITO and intelligence or a combination of these influences. To this end multivariate behavior genetic analyses with CITO and IQ at ages 5, 7, 10 and 12 years have been conducted. The correlations were .41, .50, .60, and .63 between CITO and IQ assessed at age 5, 7, 10, and 12 respectively. The results of the analyses pointed to genetic effects as the main source of variance in CITO and an important source of covariance between CITO and IQ. Additive genetic effects accounted for 60% of the individual differences found in CITO scores in a large sample of Dutch 12-year-olds. This high heritability indicated that the CITO might be a valuable instrument to assess individual differences in cognitive abilities in children but might not be the right instrument to put the effect of education to the test.In the Netherlands, nowadays, much value is attached to the results of a national test of educational achievement (CITO), administered around age 12, in order to determine high school entrance level. The results of the test are often used as an independent judgment, besides the teachers' opinion, in advising the parents on the future educational level of their child. So the CITO is used as an aid in choosing the most appropriate type of high school (e.g., academic versus technical). From a historical perspective, this attention for "independent" testing has to do with the possibilities for selection. The establisher of the CITO (Eindtoets Basisonderwijs, 2002) emphasized that this national test of educational achievement has put the effect of education in a particular school to the test besides measuring possible learning potential or cognitive abilities in children (Geldermans, 2001). It was hypothesized that success in scholastic achievement depended on the quality of the elementary school. A large number of articles in Dutch daily newspapers were dedicated to the influences of the school population and school neighborhood on the test results of the pupils. In these articles the influences of socioeconomic status (SES) and ethnic background of the majority of the children at a certain school were considered important factors to classify the school and the future success...

Multiple twin studies of attention problems (AP) from the Child Behavior Checklist or ADHD from the DSM criteria have reported on the genetic and environmental influences on these behaviors. The majority of these have studied AP and ADHD symptoms in twin samples combined across wide age spans, combined rater information and both genders. Thus, it is possible that the results are complicated by developmental, informant, and gender differences. The purpose of this study was to assess for the genetic and environmental contributions to overactive behavior (a syndrome highly related to AP in 7-, 10-, and 12-years olds) in 3-years olds (3,671 twin pairs), and attention problems in 7-(3,373 twin pairs), 10-(2,485 twin pairs), and 12-years olds (1,305 twin pairs) while controlling for developmental, gender and rater contrast contributions. Using a crosssectional twin design, contributions from genetic additive, genetic dominance, unique environmental and rater contrast effects were estimated for CBCL maternal reports. We found that genetic influences on overactive behavior and attention problems are high across an age span that covers preschool and elementary school age. Although girls display less problem behavior compared to boys, heritability estimates were found equal for both genders at each age. Environmental experiences that are unique to the individual accounted for the remaining influence. At the age of 3 years, a rater contrast effect was detected. We hypothesize that the contrast effect represents a maternal rater bias effect that is dependent on the age of the twins. The implications of these findings are discussed with reference to the clinical setting and in the context of future research. ß