Short answers, pulled from the story.
A person must experience a low mood or loss of interest in activities for at least two weeks to meet the criteria for major depressive disorder. Symptoms must persist nearly every day during this period to qualify for diagnosis.
Genetic factors account for nearly 40% of the variation in risk for major depressive disorder according to family and twin studies. Researchers identified 102 variants in the genome associated with the condition in a 2019 study.
People most often develop their first depressive episode between ages 30 and 40. A second smaller peak of incidence exists between ages 50 and 60.
The American Psychiatric Association established this specific timeframe within the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders known as DSM-III. This standardization separated clinical depression from temporary sadness.
British Prime Minister Sir Winston Churchill popularized the phrase black dog to describe his own depression after Samuel Johnson used it in the 1780s. He announced he would take a leave of absence to recover from a depressive episode on the 30th of August 1998.