title: Core group evolution over time: high-risk sexual behavior in a birth cohort between sexual debut and age 26.
abstract: 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.
  
  
  OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
authors: AIDS Epidemiology Group, Department of Preventive and Social Medicine, University of Otago Medical School, Dunedin, New Zealand.
authors: Olivier
authors: O
authors: Humblet
authors: Charlotte
authors: C
authors: Paul
authors: Nigel
authors: N
authors: Dickson
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009520:New Zealand / Q000453:epidemiology; D011446:Prospective Studies; D012725:Sexual Behavior; D012749:Sexually Transmitted Diseases / Q000453:epidemiology* / Q000209:etiology; D011795:Surveys and Questionnaires