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Core Group Evolution Over Time: High-Risk Sexual Behavior in a Birth Cohort Between Sexual Debut and Age 26

Humblet, Olivier BSc; Paul, Charlotte PhD; Dickson, Nigel FRACP, FAFPHM

doi: 10.1097/01.OLQ.0000097102.42149.11

Background and Objective 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.

Goals 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.

Study Design We used a prospective cohort study with a computer-presented questionnaire on sexual behavior.

Results 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.

Conclusion 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.

The sexual behavior of a birth cohort was assessed at ages 18, 21, and 26 years. A substantial fraction reported five or more sexual partners per year at one age, but very few persisted with this behavior through all ages.

From the AIDS Epidemiology Group, Department of Preventive and Social Medicine, University of Otago Medical School, Dunedin, New Zealand

The authors thank Peter Herbison for statistical advice and Dr Julie Fitzjohn for preliminary analyses.

Supported by a grant from the Health Research Council of New Zealand.

Correspondence: Charlotte Paul, PhD, Department of Preventive and Social Medicine, University of Otago Medical School, P.O. Box 913, Dunedin, New Zealand. E-mail:

Received for publication January 31, 2003,

revised June 25, 2003, and accepted July 7, 2003.

© Copyright 2003 American Sexually Transmitted Diseases Association