Subjects with a history of serious head trauma or health conditions precluding a laboratory visit or the ability to perform the experimental tasks (e.g. severe visual impairment or mental retardation) were excluded. In addition, data from African American participants were not included in the present analyses because preliminary analyses showed substantial ethnic heterogeneity with respect to the studied measure, but due to the small size of the African American group (13.9% of the sample) we did not have sufficient statistical power to model group differences or to conduct separate analyses within this group. Zygosity was determined based on a standard interview administered to the twins’ parents, on the research assistants’ ratings of the twins’ physical similarity, and on a set of seven DNA markers genotyped in 94% of the participants. The reliability of zygosity diagnosis by questionnaire has been demonstrated in previous studies (Kasriel and Eaves 1976), and the agreement between interview and genotyping data was 95.1%.
