Structuralism was established in 1879 at Leipzig by Wilhelm Wundt and focused on analyzing conscious experiences into their basic elements using introspection. Wundt believed mental states could be studied scientifically by asking trained subjects to report their experiences in response to stimuli. He focused on thoughts, images, and feelings. However, critics argue introspection is subjective. The biological perspective relates behavior to the nervous and endocrine systems and how genes, chromosomes, hormones, and the brain influence behavior through natural selection. For example, generally males exhibit more aggression than females, and biological psychologists have sought biological explanations for mental illnesses like explaining schizophrenia in terms of dopamine levels. However, Freud argued this just treats symptoms not underlying causes.