A charity has explained some of the most common symptoms of head injuries after the PSNI said this may explain unusual behaviour by missing teenager Noah Donohoe.
Official advice published by Headway said that behavioural changes after a brain injury are many and varied.
This includes some that appear to cause an exaggeration of existing personality traits, while others may seem completely out of character for that person.
A common early change is disinhibition, resulting in a loss of control over behaviour that could be socially inappropriate.
This can include a tendency to divulge personal information too freely and unpredictable outbursts of anger.
Although most gradually regain control over their behaviour, symptoms may remain in the cases of a severe injury.
Another common trait associated with brain injury is impulsiveness, causing a person to think or act without thinking of the consequences.
Obsessive behaviour, or becoming fixated on certain thoughts, are also possible - such as a mistaken belief that possessions will be stolen, or insisting things are done in a certain way.
Headway has said that the most common behavioural change is increased irritability, causing those with a brain injury to be impatient or intolerant of others' mistakes.
Dr Gemma Elliot is a clinical neuropsychologist and trustee of Headway Lincolnshire.
"Developing an understanding of what is going on for each individual is essential in order to direct the type of support and intervention required," she said.
"Whilst hints and tips can certainly be useful, there is no one strategy which will work for everyone."
Further possible symptoms of brain injury include apathy and a loss of initiative. This may cause some people to become passive and appear unconcerned or even unaware of their difficulties.
The advice states: "Others may appear interested and have good intentions to carry out activities, but are unable to organise themselves and initiate action.
"This may happen to any of us when feeling depressed, but for the person with a brain injury this can result directly from the injury itself rather than solely as a result of depression."
The final common symptom the charity discusses is egocentricity.
This may cause a tendency to act in a self-centred manner and not consider the feelings or needs of their family and friends.
While this can often be the case for anyone facing adversity, a brain injury can cause this to be greatly exaggerated.
Further information is available at headway.org.uk