From an evolutionary standpoint, we were designed to cope with the sudden rush of a big cat. That is acute stress and the main chemical is adrenaline. Adrenaline comes on fast, does its job, and quickly clears out.

Chronic stress is a different matter and there are few situations in primitive lives that qualify. The main chemical is cortisol and it is long-lived.

As ol cs points out, stress is every where in modern society - chronic stress. The kind that never stops. Rushing, the job, the boss, the spouse, the television, etc. An elevated level of cortisol.

Pregnant women are not spared and the placenta sometimes fails. If it fails during the short time period when a particular group of neurons are forming they will be changed. The result is that the baby's control system is overly sensitized and the system is constantly struggling to maintain balance.