New research is providing insight into how stress can shrink your brain cells and prematurely age your immune system.

FORBES: Stress Can Shrink Your Brain

We saw the middle class put under even greater stress and saw its incomes stagnate and shrink while more affluent Americans saw their wealth increase.

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McEwen said his research has found even more signs that repeated stress actually causes neurons in the brain to shrink, at least in rats.

FORBES: Stress Can Shrink Your Brain

But the U.S. argued that parts from polar bears are traded among 70 countries and that trade encourages kills that, coupled with the habitat loss, put stress on populations that will cause them to shrink.

CNN: World rejects new protections for polar bears

Earlier research showed that the neurons shrink in the hippocampus, and that seems to impair memory in response to stress, he said.

FORBES: Stress Can Shrink Your Brain