Abstract
Twin studies suggest that both genetic and environmental factors contribute to the development of sexual orientation. Early life conditions suggested by life history theory may constitute one source of these environmental factors. Although scholars often imply multiple influences on sexual orientation, they rarely test biological and psychological influences at the same time. Accordingly, this thesis aims to investigate the biopsychosocial factors that may contribute to sexual orientation differences in human males and females, using a broadly life history framework. Hypotheses will be tested using an extensive meta-analysis, and data from two longitudinal birth cohorts including the Avon Longitudinal Study of Parents and Children (ALSPAC) and the Millennium Cohort Study (MCS).Chapter 3 describes a meta-analysis conducted to determine which early life conditions were associated with reproductive and sexuality-related life history outcomes among men. The results will motivate the choice of early life factors in subsequent studies using two prospective birth cohorts. Chapter 4 and Chapter 6 use the ALSPAC to test whether a range of early life factors were associated with adolescent sexual attraction and sexual behaviour at age 15.5 years in boys and girls. Chapter 5 tests the influence of childhood gender nonconformity on the association between childhood maltreatment and adolescent sexual orientation (in ALSPAC) using propensity score analysis. Finally, Chapter 7 describes a replication attempt of some of these findings using an independent prospective birth cohort, the MCS.
This thesis provides support for the notion that biological theoretical models, including prenatal hormone theory and maternal immunity theory, may better explain the development of sexual orientation; the association between childhood parental maltreatment and adolescent sexual orientation may be at least partly accounted for by greater childhood gender nonconformity among nonheterosexual boys; girls’ sexual orientation appeared to be more associated with psychosocial and environmental factors; while only prenatal and early postnatal life conditions predicted nonheterosexuality among boys. However, the associations between psychosocial factors and adolescent sexual orientation were weak, even for girls. Altogether the findings suggest that psychosocial factors might be less important in the development of sexual orientation, regardless of theoretical approach or proposed mechanism.
| Date of Award | 1 Oct 2019 |
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| Original language | English |
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| Supervisor | Qazi Rahman (Supervisor) & Sam Norton (Supervisor) |