according to the DSM-5, Major depressive disorder (MDD) is a serious mood disorder, characterised by at least 5 symptoms present, with common examples being lowered mood, anhedonia, suicidal ideation and disturbed sleep. causes of depression are still unclear, with several models proposing it is caused through genetics, the nature debate, and others suggesting the environment that a person is in and events that happen during their lifetime, especially in childhood and adolescence cause depression aka nurture.