In the context of twin studies, what do 'unique environmental factors' and 'shared environmental factors' refer to? I was reading this study on sexual behavior and they refer to both terms without clarification. Does 'unique environmental factors' refer only to the prenatal environment or does it include individual experiences with, say, illness as well? Would friends be considered a 'shared environment factor' or an 'individual environment factor?'

Shared environmental factors are those shared by the twins: uterine/prenatal environment and then the home environment and other environments both twins are raised in or remain in together. Unique environments are any environment not shared by the twins - they generally appear later in age.