Increasing numbers of adolescents are seeking treatment from gender identity services, particularly natal girls. It is known from survey studies some adolescents exaggerate their belonging to minorities, thereby distorting prevalence estimates and findings on related problems. The aim of the present study was to explore the susceptibility of gender identity to mischievous responding, and prevalences of cis-gender, opposite-sex and other/ non-binary gender identities as corrected for likely mischievous responding among Finnish adolescents.

The School Health Promotion Survey 2017 data was used, comprising data on 135,760 adolescents under 21 years (mean 15.73, ds 1.3 years), 50.6% females and 49.4% males. Sex and perceived gender were elicited and gender identities classified based thereon. Likely mischievous responding was analysed using inappropriate responses to biodata and handicaps.

Of the participants, 3.5% had most likely given facetious responses, boys more commonly than girls, and younger adolescents more commonly than older. This particularly concerned reporting of non-binary gender identity. Corrected prevalence of opposite-sex identification was 0.6% and that of non-binary identification was 3.3%. In boys, displaying non-binary gender identity increased from early to late adolescence, while among girls, opposite-sex and non-binary identifications decreased in prevalence from younger to older age groups.

Prevalence of gender identities contrary to one’s natal sex was more common than expected.