When two things are associated, such as a condition and an outcome, researchers often seek to find out whether one causes the other. If the relationship is bidirectional, then they contribute to one another, much as in a feedback loop.
Deeper dive
For example, in investigating the relationship between smoking and lung cancer, researchers eventually determined that smoking causes lung cancer; the causation runs in one direction, or unidirectional. But what if behavior A contributes to outcome B and outcome B also contributes to behavior A? That would be a bidirectional relationship.
Consider the example of spanking. Research has shown that physical punishment is linked to increased aggression in children and teens. But the question remained for many years whether it was physical punishment that caused aggression or whether more aggressive children simply got spanked more often because they acted out more. Or, is it a bit of column A and a bit of column B?
In the case of spanking, researchers measured baseline aggression in children and then compared children who were and were not physically punished but started out with the same level of aggression. Longitudinal studies eventually revealed that children who were spanked became more aggressive, even compared to non-spanked children who started out with a similar level of aggression. Still, however, it is likely that the relationship is partly bidirectional over time: the more aggressive a child becomes, the more often the child may be physically punished.