Essentialism, as neuroscientist Lisa Barrett puts it, "is the belief that familiar categories - dogs and cats, space and time, emotions and thought - each have an underlying essence that makes the what they are."
Essentialism leads us to believe that there's some fundamental difference between races when science tells us that we're all a complicated mix of races descended from a couple hundred thousand folks in Africa 70,000 years back - not enough time to evolve differences that are any more than skin deep.
I think essentialism is at the heart of why people try to force all these crazy and arbitrary rules about gender down the throats of children. People dress baby girls up in a dizzying array of pink and purple and sparkles, paint their boys' rooms with blue and sportsballs. We have this notion that pink is essentially feminine, yet a hundred years ago it was the opposite - pink was a "strong" color suitable for boys, and blue was dainty and delicate like girls.
We flip out over a dude wearing panties, but in Victorian times, any kind of bifurcated undergarment was scandalously masculine. In centuries past, it was men who wore the panties, makeup, lace, high heels. Now a man in high heels is an abomination, putting an infant boy in lace will confuse him for life. Though it didn't seem to harm the pretty princess pictured below - on account of the fact that she grew up to be FDR - back in the day, this was a perfectly acceptable outfit for a boy.