The main symptoms of paraphrenia are paranoid delusions and hallucinations. The delusions often involve the individual being the subject of persecution, although they can also be erotic, hypochondriacal, or grandiose in nature. The majority of hallucinations associated with paraphrenia are auditory, with 75% of patients reporting such an experience; however, visual, tactile, and olfactory hallucinations have also been reported. The paranoia and hallucinations can combine in the form of “threatening or accusatory voices coming from neighbouring houses [and] are frequently reported by the patients as disturbing and undeserved". Patients also present with a lack of symptoms commonly found in other mental disorders similar to paraphrenia. There is no significant deterioration of intellect, personality, or habits and patients often remain clean and mostly self-sufficient. Patients also remain oriented well in time and space.

Paraphrenia is different from schizophrenia because, while both disorders result in delusions and hallucinations, individuals with schizophrenia exhibit changes and deterioration of personality whereas individuals with paraphrenia maintain a well-preserved personality and affective response.

Paraphrenia is often associated with a physical change in the brain, such as a tumor, stroke, ventricular enlargement, or neurodegenerative process. Research that reviewed the relationship between organic brain lesions and the development of delusions suggested that "brain lesions which lead to subcortical dysfunction could produce delusions when elaborated by an intact cortex".

A popular symptom of paranoia is the attribution bias. These individuals typically have a biased perception of reality, often exhibiting more hostile beliefs. A paranoid person may view someone else's accidental behavior as though it is with intent or threatening.

An investigation of a non-clinical paranoid population found that feeling powerless and depressed, isolating oneself, and relinquishing activities are characteristics that could be associated with those exhibiting more frequent paranoia.

Some scientists have created different subtypes for the various symptoms of paranoia including erotic, persecutory, litigious, and exalted.

Due to the suspicious and troublesome personality traits of paranoia, it is unlikely that someone with paranoia will thrive in interpersonal relationships. Most commonly paranoid individuals tend to be of a single status. According to some research there is a hierarchy for paranoia. The least common types of paranoia at the very top of the hierarchy would be those involving more serious threats. Social anxiety is at the bottom of this hierarchy as the most frequently exhibited level of paranoia.

Although non-specific concepts of madness have been around for several thousand years, the psychiatrist and philosopher Karl Jaspers was the first to define the three main criteria for a belief to be considered delusional in his 1913 book "General Psychopathology". These criteria are:

- certainty (held with absolute conviction)

- incorrigibility (not changeable by compelling counterargument or proof to the contrary)

- impossibility or falsity of content (implausible, bizarre, or patently untrue)

Furthermore, when a false belief involves a value judgment, it is only considered a delusion if it is so extreme that it cannot be, or never can be proven true. For example: a man claiming that he flew into the sun and flew back home. This would be considered a delusion, unless he were speaking figuratively, or if the belief had a cultural or religious source.

Paranoia is an instinct or thought process believed to be heavily influenced by anxiety or fear, often to the point of delusion and irrationality. Paranoid thinking typically includes persecutory, or beliefs of concerning a perceived threat towards oneself (e.g. ""Everyone is out to get me"", which is an American parochial phrase). Paranoia is distinct from phobias, which also involve irrational fear, but usually no blame. Making false accusations and the general distrust of others also frequently accompany paranoia. For example, an incident most people would view as an accident or coincidence, a paranoid person might believe was intentional. Paranoia is a central symptom of psychosis. It is also a matter of personal tolerance for the individual that might be in conflict with psychiatric diagnoses.

A delusion is a mistaken belief that is held with strong conviction even when presented with superior evidence to the contrary. As a pathology, it is distinct from a belief based on false or incomplete information, confabulation, dogma, illusion, or some other misleading effects of perception.

Delusions typically occur in the context of neurological or psychiatric disease, although they are not tied to any particular disorder and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders including schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.

In the first stages of dementia, the signs and symptoms of the disorder may be subtle. Often, the early signs of dementia only become apparent when looking back in time. The earliest stage of dementia is called mild cognitive impairment (MCI). 70% of those diagnosed with MCI progress to dementia at some point. In MCI, changes in the person's brain have been happening for a long time, but the symptoms of the disorder are just beginning to show. These problems, however, are not yet severe enough to affect the person’s daily function. If they do, it is considered dementia. A person with MCI scores between 27 and 30 on the Mini-Mental State Examination (MMSE), which is a normal score. They may have some memory trouble and trouble finding words, but they solve everyday problems and handle their own life affairs well.

The symptoms of dementia vary across types and stages of the diagnosis. The most common affected areas include memory, visual-spatial, language, attention and problem solving. Most types of dementia are slow and progressive. By the time the person shows signs of the disorder, the process in the brain has been happening for a long time. It is possible for a patient to have two types of dementia at the same time. About 10% of people with dementia have what is known as "mixed dementia", which is usually a combination of Alzheimer's disease and another type of dementia such as frontotemporal dementia or vascular dementia. Additional psychological and behavioral problems that often affect people who have dementia include:

- Balance problems

- Tremor

- Speech and language difficulty

- Trouble eating or swallowing

- Memory distortions (believing that a memory has already happened when it has not, thinking an old memory is a new one, combining two memories, or confusing the people in a memory)

- Wandering or restlessness

- Perception and visual problems

- "Behavioral and psychological symptoms of dementia" (BPSD) almost always occur in all types of dementia. BPSDs may manifest as:

When people with dementia are put in circumstances beyond their abilities, there may be a sudden change to crying or anger (a ""catastrophic reaction"").

Psychosis (often delusions of persecution) and agitation/aggression also often accompany dementia.