Scientists have long been puzzled about why women are twice as likely as men to suffer from depression and eight times more likely to suffer from such autoimmune diseases as rheumatoid arthritis and lupus.

Now researchers at the National Institute of Child Health and Human Development believe they have found the answer-estrogen, the primary female sex hormone.

In susceptible women, estrogen appears to increase levels of a key stress hormone called corticotropin releasing hormone, Dr. George Chrousos reported in the Journal of Clinical Investigation.