Thees are famous Theories in biological model. Biochemical Theory Very small amounts of certain chemicals in the body can cause profound changes in behavior as proved by researches.   Diathesis Stress Theory Predisposition i.e., diathesis (including genes) and stressful environment combine together to cause abnormal behavior Traits are influenced by genes and by environment   Neurotransmitters The synapse of the neuron releases special chemicals • Existence of neurotransmitters has been known since 1920’s; but the evidence of their relationship with psychological disorders ...