Biological markers like finger length ratios and the number of older brothers a man has reveal that sexual orientation is largely determined by prenatal testosterone and maternal immune responses. While the brain remains plastic throughout life, the core neural circuits for romantic attraction are established in the womb, independent of social conditioning or parenting styles. This exploration of the 'fraternal birth order effect' and the 'aversion pathway' suggests that who we love is a fundamental product of our earliest developmental environment.