How one person in the system acts effects the behavior of the next and so on. Members of a system are like the moving pieces of a mobile. The behavior of one person in the system effects the others just as touching one object of a mobile sets the whole system moving. One family member's mood and behavior influence the others. System thinking goes beyond blaming others to seeing how things are for the whole group.
All systems have characteristics of being open systems or closed ones.
Systems can be placed on a continuum from open to closed. Most family systems fall somewhere in the middle between open and closed. Systems vary from time to time in their ability to be open and closed. When more stressors come down on the system, they become more closed and rigid. The quality of life in your family, job or spiritual group system is generally based on the overall mental health of the most dominant member.