Table 3 Previous studies examining gender identity-related outcomes in twin and non-twin siblings.
From: Gender dysphoria in twins: a register-based population study
Study | Method | N | Outcome | Findings | Comments |
|---|---|---|---|---|---|
Bailey et al.8 | Heritability of childhood gender nonconformity and adult masculinity–femininity in twins | 1341 assigned male (aM) and 2441 assigned female (aF) twin pairs | Childhood gender non-conformity; continuous gender identity | Heritability for aM 0.50 and for aF 0.37 | Retroactive assessment in adults |
Coolidge et al.10 | Heritability of gender identity disorder (GID) in twins | 157 twin pairs | Six GID items based on DSM-IV criteria | Heritability of clinically significant GID 0.62 | Children and adolescents |
Knafo et al.13 | Heritability of atypical gender role development in twins | 5799 twin pairs | Masculinity and femininity; 24 items of pre-school activities inventory | Group heritability for aM 0.26–0.27 and for aF 0.42–0.50 | Children 3–4 years |
van Beijsterveldt et al.15 | Heritability of cross-gender behavior during childhood in twins | 4530 aF twin pairs | Cross gender behavior & cross gender identity | Genetic factors accounted for 70% of cross-gender behavior | Children 7 and 10 years |
Alanko et al.7 | Heritability of cross-gender behavior during childhood in twins | 3261 twins | Shortened version of the Recalled Childhood Gender Identity/Gender Role Questionnaire | Heritability estimates for childhood gender atypical behavior 51% for aF and 29% for aM | Adults, recalled childhood atypical gender behavior before the age of 12 years |
Gómez-Gil et al.17 | Concordance for GID in twin and non-twin siblings | 995 patients | GID according to DSM-IV and ICD-10, assessed with semi-structured clinical interviews and case discussion | 12 non-twin and 2 twin pairs concordant for GID | Patients from gender identity clinics |
Burri et al.9 | Heritability for childhood gender typicality in twins | 4426 twins | Childhood gender typicality; Four items assessing childhood sex-typed behavior and gender identity | Heritability for childhood gender typicality was 32% and for adult gender identity 11% | Adults, same-sex pairs (aF) |
Heylens et al.12 | Concordance for GID in twins | 51 twin pairs | GID as reported in previous case reports or own assessment | Concordance among monozygotic twins 39.1% and among same and different-sex DZ twins 0% | Previous case reports and own unpublished cases |
Diamond11 | Concordance for GID in twins | 112 twin pairs | Transitioning from living in the gender assigned at birth to that of the opposite gender, self-reported or clinical confirmed | Concordance among same-sex DZ pairs 33% for aM and 23% for aF | Age > 10 years, combined from bibliography and survey |
Sasaki et al.14 | Heritability of GID in twins | 1961 aM and 2333 aF twin pairs | GID trait score, assessed with questionnaire items, based on DSM-IV | Heritability among aF in adults was 11% | Children, adolescents, and adults |
