An assessment of the strengths and limitations of the Children of Twins design and a comparison with other research strategies suggest that the design plays a unique role in the study of developmental psychology and psychopathology. Finally, the authors describe how methodological advances and future applications of the design will provide additional insight into the causal processes underlying children's adjustment to environmental stimuli.

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Many workers assume that genetically determined differences in intellectual ability will be influenced little by changes in educational policy or other environmental interventions. Others, however, have suggested that increasing equality of educational opportunity will lead to an increase in the heritability of educational attainment. The resolution of this issue has been delayed until now because of the extremely large sample sizes which would be required. Education data on twins and their parents, from the Norwegian twin panel, provide a unique opportunity to determine the impact on the heritability of educational attainment of the more liberal social and educational policies introduced in Norway after the Second World War. As reported here, for individuals born before 1940 there is a strong effect of family background on educational attainment, accounting for 47% of the variance, though genetic factors account for an additional 41% of the variance. For females born after 1940 and before 1961, the relative importance of genetic (38-45%) and familial environmental (41-50%) differences changes very little. For males born during the same period, the broad heritability of educational attainment has increased substantially (67-74%), and the environmental impact of family background has correspondingly decreased (8-10%). For males, at least, having well-educated parents no longer predicts educational success, as measured by duration of education, independent of the individual's own innate abilities.

Twin studies provide an efficient method for examining the importance of genetic and environmental factors in the etiology of disorders such as epilepsy. Population-based twin registries are especially valuable for studies of this type since effects of reporting and self-selection biases on the resulting data are minimized. Among 14,352 twin pairs contained in the Virginia and Norwegian twin panels for whom questionnaire information was available, there was a history of epilepsy in one or both members of 286 pairs; febrile seizures were reported in 257 pairs. Analyses of questionnaire data revealed no significant differences in concordance rates between Virginian and Norwegian twins for either epilepsy or febrile seizures. Probandwise concordance rates for epilepsy were 0.19 in monozygotic twins and 0.07 in dizygotic twins. Analogous rates for febrile seizures were 0.33 (monozygotic) and 0.11 (dizygotic). These results provide further evidence that genetic factors do have a role in the expression of epilepsy and febrile seizures.

Summary:Purpose: Benign rolandic epilepsy (BRE) is considered a genetically determined idiopathic partial epilepsy. We analyzed a large sample of twins from four international twin registers to probe the genetics of BRE. We also aim to synthesize the apparently conflicting family and twin data into a model of BRE etiology.Methods: Large population-based twin registries of epilepsies from Odense (Denmark), Richmond, Virginia (United States), and Oslo (Norway) were reviewed for BRE cases and added to our Australian twin data. Diagnosis of classic BRE was based on electroclinical criteria with normal neurologic development. Cases with a compatible electroclinical picture but abnormal neurologic development were termed nonclassic BRE.Results: Eighteen twin pairs were identified (10 monozygous; eight dizygous) of whom at least one twin was diagnosed with classic BRE among a total sample of 1,952 twin pairs validated for seizures, and all were discordant for BRE. The estimated monozygous pairwise concordance for BRE in this sample was 0.0 [95% confidence interval (CI), 0.0-0.3). Four twin pairs (one monozygous, three dizygous) had nonclassic BRE, and all cotwins had seizures.Conclusions: The twin data showing an absence of any concordant twin pairs with classic BRE suggest that noninherited factors are of major importance in BRE. Modelling the data shows that the familial occurrence of centrotemporal spikes makes only a minor contribution to the familial aggregation of BRE. Genetic factors are probably more important in nonclassic BRE. The etiology and mode(s) of inheritance of BRE are much more complicated than initially conceptualized. Key Words: Benign rolandic epilepsy-Inheritance-Twin.The most common epilepsy syndrome in childhood is benign rolandic epilepsy (BRE) (1). The EEG finding of centrotemporal spikes (CTSs) is an obligatory part of the electroclinical diagnosis of BRE, but CTSs are found in many asymptomatic children (2,3). The genetics of BRE has long been perplexing. Early studies suggested autosomal dominant inheritance, based on EEG findings in siblings, and later studies suggested multifactorial inheritance (4-13). A genetic basis also was suggested by twin studies on individual pairs or small case series (7,9,11,(14)(15)(16). The dilemma, however, is that family studies of Accepted October 13, 2005. Address corresponding and reprint requests to Dr. S.F. Berkovic at Epilepsy Research Centre, First Floor, Neurosciences Building, Repatriation Campus, Austin Health, Banksia Street, Heidelberg West, Victoria, Australia 3081. E-mail: s.berkovic@unimelb.edu.au BRE probands have found that few relatives have BRE (7,17).We recently challenged the established hypothesis that BRE is a genetic disorder in an Australian twin study, which found no concordance in six monozygous (MZ) and two dizygous (DZ) twin pairs for BRE (18). This showed a strikingly different concordance compared with idiopathic generalized epilepsy, in which major genetic influences are well established. We postulated that traditional geneti...