Core Group Evolution Over Time

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Abstract
Among the limitations of the concept of a sexually transmitted disease core is uncertainty about the stability of sexual behavior over time. The objective was to shed light on characteristics and stability of the core group by assessing sexual behavior longitudinally in a birth cohort. The goals were to describe group size and characteristics of people who report 5 or more heterosexual partners per year (a surrogate for the core group) at ages 18, 21, and 26 years. We used a prospective cohort study with a computer-presented questionnaire on sexual behavior. Of the original cohort members, 991 (97.3% of those believed to be alive) responded at at least one age. A total of 14.7% of women and 26.0% of men were in the core group at either age 18 or 21 or 26, but only 0.5% and 0.9% of women and men, respectively, were in the core group at all ages. Those in the core group were significantly more likely to report concurrent partnerships and higher sexually transmitted disease (STD) rates. Early age at first sex was consistently associated with being in the core group, whereas those with less education were more likely to be in the core group at age 18 but not at later ages. The high degree of variability in sexual behavior over time of individuals adds another degree of complexity to the identification of a core group for STD transmission.