What are the plus and minus signs in the model diagram?

[branscombj]

These signs tell the direction of the cause-effect relationship between the variable at the tail of an arrow and the one at the point. A plus sign means they move in the same direction: when the cause variable increases, the effect variable increases – or when the first decreases, the second decreases. A minus sign means they move in opposite directions: when the cause variable increases, the effect variable decreases – or when the first decreases, the second increases.