Opposite-sex twins.
D000328:Adult; D002648:Child; D003674:Defense Mechanisms; D005260:Female; D005639:Frustration; D006252:Hate; D006801:Humans; D007398:Interpersonal Relations; D008149:Love; D008297:Male; D010287:Parent-Child Relations; D010551:Personality; D011247:Pregnancy; D011574:Psychoanalytic Theory; D012043:Regression, Psychology; D012649:Self Concept; D012723:Sex; D013850:Thinking; D014427:Twins