This thesis examined biologically active estrogen and androgen composite levels in umbilical cord blood at birth and investigated their relationship to autistic-like traits, verbal, visuospatial and nonverbal abilities in a population cohort and a high-risk infant sibling cohort. Sex steroid levels were not significantly related to autistic-like traits or verbal, nonverbal or visuospatial abilities. There findings do not support the extreme male brain theory, but do not invalidate existing findings. Future research should consider other factors that may influence this relationship including sex differences in outcome measures, developmental time periods of sampling and genetic and biological factors (e.g. androgen receptors).