There is a continuing debate about the importance of social versus biological factors in the expression of same-sex attraction. Investigation of prevalence, continuities, and changes over time among young adults growing up in a country with a relatively accepting climate to homosexuality is likely to illuminate this debate. Analyses were therefore undertaken of self-reported same-sex attraction at age 21 and 26, in a cohort of about 1000 people born in 1972/3 in one New Zealand city. Participants were also asked about same-sex behaviour and attitudes to same-sex relationships. By age 26, 10.7% of men and 24.5% of women reported being attracted to their own sex at some time. This dropped to 5.6% of men and 16.4% of women who reported some current same-sex attraction. Current attraction predominantly to their own sex or equally to both sexes (major attraction) was reported by 1.6% of men and 2.1% of women. Occasional same-sex attraction, but not major attraction, was more common among the most educated. Between age 21 and 26, slightly more men moved away from an exclusive heterosexual attraction (1.9% of all men) than moved towards it (1.0%), while for women, many more moved away (9.5%) than towards (1.3%) exclusive heterosexual attraction. These findings show that much same-sex attraction is not exclusive and is unstable in early adulthood, especially among women. The proportion of women reporting some same-sex attraction in New Zealand is high compared both to men, and to women in the UK and US. These observations, along with the variation with education, are consistent with a large role for the social environment in the acknowledgement of same-sex attraction. The smaller group with major same-sex attraction, which changed less over time, and did not differ by education, is consistent with a basic biological dimension to sexual attraction. Overall these findings argue against any single explanation for homosexual attraction.