Outline of Personality & Behaviour Changes


Healthy individuals differ significantly in their overall personality, mood, and behaviour, and each individual also varies from day to day, depending on the circumstances.

However, a sudden, major change in personality and/or behaviour, particularly one that is not related to an obvious event (such as taking a drug or losing a loved one), often indicates a problem.

Changes in personality and behaviour can be roughly categorised as one of the following:

  • Confusion or delirium;
  • Delusions;
  • Disorganised speech or behaviour;
  • Hallucinations; or
  • Mood extremes (such as depression).

These categories are not disorders, they are just one way medical professional can organise different types of abnormal thought, speech, and behaviour.

These changes in personality and behaviour can be caused by physical or mental health problems.

An individual may have more than one type of change. For example, an individual with confusion due to a drug interaction sometimes have hallucinations, and individuals with mood extremes may have delusions.

Confusion and Delirium

Confusion and delirium refer to a disturbance of consciousness. This means that the individual is less aware of their environment and, depending on the cause, may be excessively agitated and belligerent or drowsy and sluggish.

Some will alternate between being less alert and being overly alert. Their thinking appears cloudy and slow or inappropriate and they have trouble focusing on simple questions and are slow to respond. Speech may be slurred. Often, the individual will not know what day it is, and they may not be able to say where they are. Some cannot give their name.


Delusions are fixed false beliefs that an individual holds despite evidence against those beliefs. Some delusions are based on a misinterpretation of actual perceptions and experiences. For example, an individual may feel persecuted, thinking that a person behind them on the street is following them or that an ordinary accident is purposeful sabotage. Others think that song lyrics or newspaper articles contain messages that refer specifically to them.

Some beliefs seem more plausible and can be difficult to identify as delusions because they could occur or have occurred in real life. For example, individuals are (occasionally) followed by government investigators or have their work sabotaged by coworkers. In such cases, a belief can be identified as a delusion by how strongly the individual holds the belief despite evidence against it.

Other delusions are easier to identify. For example, in religious or grandiose delusions, the individual may believe they are Jesus or the president of the country. Some delusions are quite bizarre. For example, an individual may think that their organs have all been replaced by machine parts or that their head contains a radio that receives messages from the government.

Disorganised Speech

Disorganised speech refers to speech that does not contain the expected logical connections between:

  • Thoughts; or
  • Questions and answers.

For example, an individual may jump from one topic to another without ever finishing a thought. The topics may be slightly related or entirely unrelated. In other cases, the individual responds to simple questions with long, rambling answers, full of irrelevant detail. Answers may be illogical or completely incoherent.

Occasionally misspeaking or intentionally being evasive, rude, or humorous is not considered disorganised speech.

Disorganised Behaviour

Disorganised behaviour refers to doing quite unusual things (such as undressing or masturbating in public or shouting and swearing for no apparent reason) or to being unable to behave normally.

Individuals with disorganised behaviour typically have trouble doing normal daily activities (such as maintaining good personal hygiene or obtaining food).


Hallucinations refers to hearing, seeing, smelling, tasting, or feeling things that are not actually there.

That is, the individual perceives things, seemingly through their senses, that are not caused by an outside stimulus. And, any sense can be involved.

The most common hallucinations involve hearing things (auditory hallucinations), usually voices. The voices often make derogatory comments about the individual or command the person to do something.

Not all hallucinations are caused by a mental disorder. Some types of hallucinations are more likely to be caused by a neurologic disorder. For example, before a seizure occurs, the individual may smell something when there is no smell (an olfactory hallucination).

Mood Extremes

Mood extremes include outbursts of rage, periods of extreme elation (mania) or depression, and, conversely, constant expression of little or no emotion (appearing unresponsive or apathetic).