How do you know that depressed people aren’t born with smaller hippocampi?
We don’t. And there’s some evidence that hippocampal volume can be inherited. One monkey study showed it’s 54 percent heritable. On the other hand, we know definitively that the hippocampus is a brain area that makes new neurons throughout life. And that ability can vary. It decreases with age, for example. So you’re leaning toward shrinkage. What does it? Stress. Stress sets off a cascade of steroid hormones that likely alters the hippocampus. The hormones also appear to retard its new growth. So the hypothesis … … is that stress encourages depression. Brain and endocrine sites for our response to stress—circuitry called the HPA axis—become overactive in depression. And while the subsequent wash of steroid hormones doesn’t destroy hippocampal neurons, it likely prunes them back, changing their activity in brain circuits and perhaps producing symptoms of depression. Or maybe it’s the hormones’ slowing new growth that’s the cause. Perhaps it’s both! Can depression therapy restore th
