If gender reveal parties were a thing in 1918, you may have cut a blue cake for girls (because blue was “dainty”) and a pink cake for boys (because pink was the “stronger” color). Even then, most people wouldn’t have cared. That’s because we didn’t start seriously color coding our babies until the mid-1940s. 

Genders and the expectations that come with them vary wildly across time and culture. High heels got their start in fashion as a symbol of masculinity and status for wealthy men. In some places, property is passed from mother to daughter, women are the primary hunters, or many genders have been recognized since ancient times. 

While some norms may be rooted in religious beliefs, many are rooted in the latest ad campaign or cultural moment. We constantly look to each other, without realizing it, to figure out how we’re supposed to be. And when enough people like us think something, we often start to think it too.

The result is an ever-changing set of social rules that we instinctively put on ourselves and each other, even if we never knew why we were following them in the first place.