The study of Silberg et al demonstrates one of the possible ways in which two variables may interact.

Variable X (genetic predisposition) interacts with variable Y (stressful life events) to produce the result Z

(depression). In this interaction Y is a mediator between X and Z: even if X is present, Z will only occur if Y occurs.

What other types of interaction between X and Y (to produce Z) can you think of? Think abstractly and then try to nd examples from real life.