Stereotyping impairs performance. A *LOT*

 Michelle G points out that the simple act of thinking about stereotypes can impair your performance (if you're the one being stereotyped)
To be precise, as you probably have heard of from the "women should know their place" crowd,

A large-scale 1995 meta-analysis found that on average, men outperform women in a cluster of tests related to spatial ability by nearly a full standard deviation, and in attempt to explain this, researchers have hypothesized about the impact of testosterone and differences in brain wiring in capacity for spatial thought.

As you might have guessed, this has - only recently! - turned out to be be bullshit.
How so?
Well, at the University of Utah, they put men and women through pretty much the same test, except they "primed" them beforehand by asking some of the women to imagine themselves as stereotypical men, and some of the men to imagine themselves as stereotypical women.
The result? Well,

As it turns out, there is zero statistically significant gender difference in mental rotation ability after test-takers are asked to imagine themselves as stereotypical men for a few minutes. None. An entire standard deviation of female underperformance is negated on this condition, just as a man’s performance is slightly hindered if he instead imagines himself as a woman. (well then.)

The point being, a level playing field requires that you actively go out of your way to make the playing field level - behavior of the XKCD type below just makes things way worse.

Mind you, it gets worse when you add race to the mixture.  Have black and white kids take the same tests, but before the tests, tell some of the kids - black and white - that the test is going to be diagnostic of their intelligence/abilities.
The result?

Get that?
In both cases, it affects people's performance, but it significantly affects that of the black kids!
This is called Stereotype Threat.  Its a Real Thing, and has consequences in the real world - significant consequences if you are a stereotyped minority.

It turns out that this isn't just socio-babble, it shows up when you look at the brains under an MRI

More recently, neuroscientists have begun to examine the effects of this condition under an fMRI. Under a control (non-stereotype threat) condition, Dartmouth undergrads engaging in mathematical problem-solving showed activation in the typical brain regions associated with problem solving. In the stereotype threat condition ... participants showed no evidence of heightened activation in problem-solving areas and instead showed activation in the ... processing of ... negative thoughts and emotions. In other studies, inducing stereotype threat has been shown to temporarily reduce working memory capacity, which is one of the strongest correlates with general intelligence

Yikes!  You actually get less smart when you are working under these conditions!

The good news here is that this is - potentially - resolvable

...simply learning about how stereotypes unfairly harm performance does ameliorate a portion of its effects. A group of researchers told female math test takers: "It's important to keep in mind that if you are feeling anxious while taking this test, this anxiety could be the result of these negative stereotypes that are widely known in society and have nothing to do with your actual ability to do well on the test.” The girls' scores significantly improved, an effect which I would assume holds for people of color and for anyone else whose brain is taught to expect less from itself by a culture that wants this to be so.

So, the next time you hear somebody (or yourself!) utter a sentence from the Stereotype Threat Playbook ("Math is hard"; "he is doing well despite his background" etc.), you should realize that this is exactly the type of situation that actually adversely affects people!
Its the background radiation that is the killer, and which needs to be eliminated...