Schizophrenia, schizoaffective disorder and bipolar disorder can cause a person to hear voices that don't objectively exist, explains Rethink Mental Illness. Not everyone who hears voices has a mental illness; sometimes, the voices are the result of stress, anxiety, street drugs, heavy alcohol consumption or even spiritual beliefs.
Hearing voices is the most common type of hallucination that people with schizophrenia experience, notes the National Institute of Mental Health. Sometimes, the voices speak directly to the individual, such as by instructing him to perform certain actions or warning him that there is impending danger. At other times, the voices may converse with one another. In addition to voices, people with schizophrenia sometimes see, smell or feel things that others cannot sense. Other signs of schizophrenia include disorganized thought patterns, delusions that the person insists are true despite blatant evidence to the contrary, repetitive movements, and talking in a monotonous voice.
Patients with bipolar disorder sometimes hear voices while they are in the middle of a manic episode, explains Psychology Today. Other symptoms of a manic episode include increased energy, rapid speaking, euphoria and grandiosity. These periods of mania alternate with depressive episodes. In the case of schizoaffective disorder, a patient has symptoms of a mood disorder along with schizophrenia symptoms, explains the Mayo Clinic.