Inclusion Criteria:

1. For all subjects female gender;
2. For subjects assigned to the MDD groups, current DSM-IV diagnosis of MDD;
3. For subjects assigned to the early-life stress group, repeated (once per month or more for at least year) sexual or physical abuse before the age of 12 years by a perpetrator at least 5 years older at the time;
4. For all subjects, age of 18 to 45 years;
5. Regular menstrual cycle and assessment in the early follicular phase as verified by sex steroid measures.

Exclusion Criteria:

1. For all subjects, gender identity disorders;
2. For all subjects assigned to non-MDD groups, DSM-IV diagnosis of current MDD;
3. For all subjects assigned to the group without early-life stress, major stress experiences before the age of 12 years, such as separation from parents, neglect, parental loss, accidents, severe illness or natural disaster;
4. For all subjects, significant medical illness, such as gastrointestinal, neurological, endocrine, cardiovascular, pulmonary, renal, hepatic, immunological or hematological disease, organic brain disease, or cancer as determined by history, physical examination, ECG, and laboratory tests;
5. Pregnancy or nursing;
6. For all subjects, past or current presence of psychotic symptoms or bipolar disorder;
7. For all subjects, current presence of psychoactive substance abuse/dependency or eating disorders;
8. For all subjects, hormonal medication;
9. For all subjects, psychotropic medication in the four weeks prior to study entry;
10. For all subjects, inability to provide informed consent.