The biological model assumes that psychological illnesses have a physical cause in the brain due to biological dysfunctions, genetics, toxins, infections, or stress. Possible biological causes of abnormal behavior include imbalances of neurotransmitters or hormones, structural damage or abnormalities in the brain, and genetic factors associated with schizophrenia. While studies have found associations between psychological disturbances and biological changes, it is difficult to determine whether biological changes cause or result from psychological symptoms. The biological model also faces issues regarding evidence, patient roles, and social stigma.