There is a pervasive notion that men and women are quite different when it comes to behaviour and temperament—and those extend way beyond the physical differences seen in our nether regions. In fact, neuroscientists have discovered that there are quite noticeable variances between male and female brains, in terms of both structure, volume, and function. And sex hormones, like estrogens and testosterone, can target regions of the brain, affecting many aspects of signalling and function at the epigenetic, cellular, and behavioural levels. Many of us think of hormones as the gender-specific molecules we learned about in middle school health class—the chemical messengers that arrive during puberty to govern our reproductive development. But sex steroids like testosterone and estrogens also play a critical role in brain development even before adolescence: shaping, activating, and fuelling sexually dimorphic brain circuits. These circuits are not limited to those involved with romantic and sexual entanglements. Many have been implicated in complex behaviours including the stress response, learning, and memory—as well as linked to the development of several psychiatric disorders.