Sexual risk-taking was assessed by girl report using the Adolescent Sexual Activity Index (ASAI, Hansen et al. 1999) administered from age 11 onwards. In this, and every subsequent annual administration of the ASAI, girls who reported having sexual intercourse with a boy in the past year were also asked to report on the number of sexual partners. Three additional items assessed frequency and method of birth control, whether or not they had been pregnant and whether they had been diagnosed or treated for an STI in the past year. Frequency of birth control was assessed by asking: ‘Think of the times you have had sexual intercourse with a boy. How often did you use birth control? By that we mean condoms, birth control pill or patch, withdrawal, the boy pulls out etc.’ Four response choices were given: ‘never,’ ‘sometimes,’ ‘most of the time,’ and ‘always.’ If the girl responded ‘sometimes’ or more often, she was also asked which form(s) of birth control she used. Following prior research (e.g., Locke and Newcomb 2008), a risky sex index was created from four
