Twin similarity of childhood and adult environments was assessed during the 1996-2000 interview. Twin similarity of childhood environment was measured with four items: frequency of sharing the same friends, dressing alike, being in the same classes in primary school, and being in the same classes in high school. Responses to each item were averaged across twins within a pair and then summed to form a composite scale, with higher scores indicating greater similarity of environment. Twin similarity of adult environment was measured with two items assessing frequency of seeing each other and frequency of contacting each other by telephone, e-mail, letter, or fax. Responses to these two items were also averaged across twins within a pair and summed to form a composite scale, with higher scores indicating greater contact.
