Maybe -G-d forbid-useful for ageing parents. I have seen this.

Often useful in older people: suspicious, paranoid everybody deceives him/her/, jealous [partner is unfaithful]
Delusions of being poisoned, being followed/watched.
May lose feeling of decorum [undressing!]
Fumbles all the time with his hands, picks at face,lips,fingers clothes. This is very easy to recognize if you pay attention to the patient
And sometimes very useful for a dry, spastic cough which is worse when lying down,has to sit up and the cough is worse when talking or drinking.

In children there may be a strong jealousy because they always feel [without reason] that the other children are favoured by the parents.