One of the non-genetic arguments used to explain racial IQ gaps is evnironmental stress. The idea being that blacks and hispanics are more stressed than whites, and this releases stress hormones, which depresses IQ scores.
Race differences in stress hormone levels
So the first obvious thing to do would be to figure out how much “stress hormones” people actually have. One study did this looking at 3 stress hormones in male, female, white, black and hispanic urine samples, controlled for age:
Another study looked at cortisol levels in whites, blacks and hispanic adolescents throughout the day:
Another did the same, but for people aged 48-90:
And so the studies show that whites have higher cortisol levels in general, but lower norepinephrine and epinephrine levels. Now this makes a claim that these differences are due to environment implausible – because environmental stressors always raise all three of these hormones.
And so for whites to be higher in one but lower in two heavily suggests that these differences are primarily due to genetic differences.
But the big takeaway is that these differences are very minor, and moreover, the effect of different levels of these “stress hormones” may be different on average for the races, with blacks and whites responding differently to different levels of cortisol and norepinephrine / epinephrine.
The effect of stress on IQ
Another thing to consider is that a certain amount of stress, at least in the short term, probably raises IQ.
There have been dozens of animal studies all confirming that increasing cortisol improves memory performance. This formed the basis of the Yerkes–Dodson law, which posits that cognitive function is improved by arousal up to a point, after which function declines.
I only know of one study on this in humans, and in it men and women responded positively to a memory test by spiking their cortisol levels up to an extra 400ng/dl, after which additional cortisol made them have worse memory.
Of course the negative effects on IQ are said to be from chronic stress, not short-term stress; but as shown above, the races probably don’t differ to any significant degree in their chronic stress levels. And if they do, it’s even possible that whites have more negative IQ impact from stress than hispanics or blacks do.
One study took men and had them perform public speaking and math in front of an audience, and afterwards measured their urine cortisol levels. One group was given sugar (glucose) before the speech, one was given fat, one was given protein, and one was given water. These there the results:
Compare this to the first chart in this article: the glucose group had cortisol levels that peaked at around 2,300 ng/dl – more than twice that of the white group. This is what “high stress” is like – a situation that elicits stress, plus a high-carb diet.
A bit of a tangent, but it does but the race differences in stress hormones in context.