https://www.latimes.com/science/la-sci-emotional-stereotypes-about-women-20190530-story.html :
Until recently, most neuroscience labs have conducted their experiments on males only, said essay author Rebecca Shansky, a neuroscientist at Northeastern University in Boston. Scientists justified this exclusion by claiming that fluctuating hormone levels in females had the potential to make test results “messy.”

This was true even for research involving disorders that are more likely to affect women, like depression and PTSD.

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Ironically, she thinks the roots of the disparity can be traced in part to the 1960s and ’70s, when the notion of equality between the sexes began to gain ground.

“There was this idea that men and women are the same and women can do everything a man can do,” she said.

Back then, a male neuroscientist who went looking for differences between a female brain and a male brain would likely have been skewered by his peers, she said.

Later, research showed that hormones can affect cognitive parts of the brain, McCarthy said. That convinced some scientists that if hormones were the variable separating male and female brains, they could easily eliminate that variable by running their experiments exclusively on males.

“It seemed like sound experimental design,” she said.

But it wasn’t. As scientists would later discover, male and female brains do not function exactly the same, either in rodents or in humans.