Early first sexual intercourse: A critical examination of the relationships between age at first sexual intercourse and later sexual outcomes among Australian men and women

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Australian Research Centre in Sex, Health and Society, School of Psychology and Public Health, College of Science, Health and Engineering, La Trobe University, Bundoora.

The assumption that early first sexual intercourse leads to adverse sexual outcomes has been used to justify interventions and policies aimed at delaying sexual initiation. There is, however, no consensus on what constitutes early first sexual intercourse. This study examined the relationships between early first sexual intercourse and four sexual outcomes by systematically reviewing current research and conducting secondary data analysis of the Australian Longitudinal Study of Health and Relationships. The systematic review found varying measures of early first sex, and weak relationships with later sexual outcomes. Regression models were used to compare three common definitions of early first sexual intercourse (before the age of 16 years, early/late relative to age group and age at first sexual intercourse). Measures of diagnostic classification and model calibration explored which categorisation of early first sexual intercourse best predicted each of the sexual outcomes. Data analysis found early first sexual intercourse was associated with having more lifetime and recent sexual partners but not with sexual or relationship satisfaction. Findings were consistent across the three definitions of early first sexual intercourse for three out of four outcomes, yet each definition examined and conceptualised early first sexual intercourse differently. The categorisation of early first sexual intercourse that best predicts number of lifetime and recent sexual partners, according to measures of classification and calibration, was less than 18 years; later than common conceptualisations of early intercourse. Classification indices also found age at first sexual intercourse was poor at discriminating between those who experienced a sexual outcome and those who did not. The findings from this study are discussed with reference to implications for the measurement and conceptualisation of early first sexual intercourse, using early first intercourse as a marker for later sexual health, and the usefulness of programs aimed at delaying first intercourse.