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Objective: To examine gender differences in attention-deficit/hyperactivity disorder ("ADHD") symptom comorbidity with "oppositional defiant disorder", "conduct disorder", "separation anxiety disorder", "generalized anxiety disorder", speech therapy, and remedial reading in children. Method: From 1994 to 1995, data from a large sample (N = 4,371) of twins and siblings studied in the Australian Twin ADHD Project were obtained by mailed DSM-IV--based questionnaires, investigating patterns of comorbidity in the three subtypes of "ADHD": "inattentive", "hyperactive/impulsive", and "combined". A total of 1,550 questionnaires were returned (87%) over the next 12 to 18 months. Results: Analysis of variance showed significant between-group differences in males and females for inattention and hyperactive/impulsive symptom counts with higher rates of "oppositional defiant disorder" and "conduct disorder" in males, and higher rates of "separation anxiety disorder" in females indicating internalizing disorders are more common in females and externalizing disorders are occurring more often in males. Differences were found between the "ADHD" subtypes and the no ADHD category for all comorbid conditions, for both males and females. Children without ADHD consistently had fewer symptoms, while children with the combined subtype showed consistently more comorbid symptoms indicating a strong relationship between high rates of externalizing symptoms and high rates of internalizing symptoms. Gender differences in speech therapy were significant only for the children without ADHD. The rates of "separation anxiety disorder" were higher in females with the "inattention" subtype and the rate of "generalized anxiety disorder" higher for females with the "combined" subtype, indicating that the subtypes of ADHD were associated with these internalizing disorders in different ways. Conclusions: Although comorbidity differs among ADHD subtypes, there were no significant gender differences in comorbidity for externalizing disorders. Inattentive girls may present with anxiety. Clinical approaches for both males and females should be sensitive to possible language and reading problems.

There are a growing number of large-scale initiatives for twin registers of children. The Australian Twin ADHD Project (ATAP) is used to illustrate two key limitations which may arise with such studies, namely (1) the importance of including or possibly excluding families in which one or both twins have significant developmental disability, and (2) the selective failure to recruit and/or the selective attrition of families in which parents and children share behavioral difficulties. Initially ATAP excluded 1 in 6 of families whose twins were enrolled in the volunteer-based Australian Twin Registry (ATR), and as more children with significant problems were identified, these families were sequentially excluded. With longitudinal data over ten years, two points about retention were identified: the difficulty of retaining the twins in late adolescence, and the loss of the families whose twins had more ADHD symptoms. We discuss strategies for limiting the loss of families and for ensuring comparability of data across registers with similar interests but different methods of recruitment and exclusion.

Objective: The thalamus and caudate nucleus are key subcortical structures in the fronto-striato-thalamic pathways that have been implicated in the pathogenesis of schizophrenia. Previous studies have been inconsistent in identifying structural and functional abnormalities in these structures. However, methodologies in these studies have been unreliable and some have not adequately matched patients and controls., Methods: Using algebraically-manipulated double-echomagnetic resonance (MR) images, we developed a reliable method to estimate caudate and thalamic volumes in a group of 13 monozygotic(MZ) twins; eight discordant for schizophrenia and five normal.Initially, volumes were measured on four image types: proton density(PD), T2-weighted, summed (PD + T2) and subtracted (PD-T2) to determine the most reliable method., Results: There was a significant method by region interaction, where caudate volumes measured on PD images were significantly larger than those measured on T2-weighted images, while the opposite was found for thalamic volumes. However, there was no interaction of method by diagnosis. Test-retest reliability was highest for the summed images. Using summed images to measure the volumes of the caudate and thalamus in each twin, we found significantly increased caudate volumes in affected twins compared to their unaffected cotwins,but no significant difference in thalamic volume., Conclusions: Our results in a small sample of MZ twins discordant for schizophrenia do not support the presence of structural abnormalities in the thalamus. The findings in the caudate are consistent with previously reported effects of antipsychotic medication. We also report a reliable method for assessing the volumes of subcortical structures. However, volumetric estimates of brain structures may be dependent on which method is used and the structure being assessed. Such interactions need to be considered in future studies investigating brain structural abnormalities in schizophrenia and other disorders.