“Circumstances can also force the trait to disappear. Many children born very sensitive are pushed hard by parents, schools, or friends to be bolder. Living in a noisy or crowded environment, growing up in a large family, or being made to be more physically active may sometimes reduce sensitivity, just as sensitive animals that are handled a great deal will sometimes lose some of their natural caution, at least with certain people or in specific situations. That the underlying trait is entirely gone, however, seems unlikely.”