Design & Procedure: Extended twin design, intelligence test results of MZ and DZ twins, full biological siblings and biological parents, data from parents and siblings meaning effects of heredity and cultural transmission estimated at same time. Both parents and children gave consent, compensated for travel costs. Questionnaire or DNA cheek swabs at home prior to study to see if identical twins or not. Children- completed cognitive test separately, given verbal instructions and complete test at own pace. 60 problems divided into 5 sets of 12Qs, harder as they go on, missing puzzle pieces to complete analogies. Adults did same but more difficult test, given written instructions and work at own pace. No. and arrangement of Q's different but able to be compared. Analysis, researches use 2 diff models to determine spousal resemblance better explained with phenotypic assortment or social homogamy.