Some family therapy is based on behavioural principles, but they are mostly based on family systems theory which considers the whole family as the unit of treatment. Key emphasis are more towards relationships and communication patterns, rather than symptoms or traits which individual members possessed.
The fact of the matter is that family therapy is fast becoming a common form of treatment in many parts of the world, including in the US.
So, when is it used? Well, it is often used when a family member has