Researchers are making strides in understanding how genes work with the environment to shape behavior, adding a new twist to the age-old debate over whether nature or nurture is mostly responsible for how people develop.

They are finding that sensitivity to the environment resides in the biology of the nervous system. And some people, because of their genetic makeup and life experiences, are more sensitive to outside influences than others. Scientists point to a type they call orchids—people who wilt under poor conditions...