# Interference

## The Concept of Interference Explained

Interference theory posits that new memories alter existing memory traces and make it harder to retrieve earlier memories.

Interference is the tendency for new memories (or newly learned information) to impair retrieval of older memories (and vice-versa).

Interference is more likely to occur when information is very similar to other information that was previously learned or stored in memory.

A classic experiment, in which college students learned lists of nonsense syllables, validates the interference theory. After studying, students in one group slept for 8 hours and were then tested for memory of the lists.

A second group stayed awake for 8 hours and went about business as usual. When members of the second group were tested, they remembered less than the group that slept. This difference is based on the fact that new learning can interfere with previous learning.

There are two basic types of interference:

1. Proactive interference, and
2. Retroactive interference.

### Retroactive Interference

Retroactive interference refers to the tendency for new learning to inhibit retrieval of old learning. The sleeping college students remembered more because retroactive interference was held to a minimum.

Retroactive interference is easily demonstrated in the laboratory by this arrangement:

 Experimental group: Learn A Learn B Test A Control group: Learn A Rest Test A

Imagine yourself as a member of the experimental group. In task A, you learn a list of telephone numbers.

In task B, you learn a list of Social Security numbers. How do you score a test of task A (the telephone numbers)? If you do not remember as much as the control group that learns only task A, then retroactive interference has occurred. The second thing learned interfered with memory of the first thing learned; the interference went “backward,” or was “retroactive”.

### Proactive Interference

Proactive interference, the second form of interference, is the tendency for old memories to interfere with the retrieval of newer memories.

This form occurs when prior learning inhibits recall of later learning.

A test for proactive interference would take this form:

 Experimental group: Learn A Learn B Test B Control group: Rest Learn B Test B

Let’s assume that the experimental group remembers less than the control group on a test of task B. In that case, learning task A interfered with memory for task B.

Then proactive interference goes “forward”? Yes. For instance, if you cram for a psychology exam and then later the same night cram for a history exam, your memory for the second subject studied (history) will be less accurate than if you had studied only history. (Because of retroactive interference, your memory for psychology would probably also suffer.)

The greater the similarity in the two subjects studied, the more interference takes place. The moral, of course, is don’t procrastinate in preparing for exams. The more you can avoid competing information, the more likely you are to recall what you want to remember (Anderson & Bell, 2001).

The interference effects described in this entry apply primarily to memories of verbal information, such as the contents of this entry. When you are learning a skill, similarity can sometimes be beneficial, rather than disruptive.