Which of the following is a defining difference between major depressive disorder and bipolar disorder?
1) Major depressive disorder involves only depressive symptoms, but bipolar disorder involves both manic and depressive symptoms.
2) People with major depressive disorder suffer from intense anxiety, but those with bipolar disorder do not.
3) Bipolar disorder can be treated with medications, but major depressive disorder cannot.
4) Psychotic features can occur in bipolar disorder but not in major depressive disorder.
5) Genetic causes have been established for bipolar disorder but not for major depressive disorder.