As we have seen, cognitive psychology describes the mental processes that allow the mind to process information and carry out internal tasks. Because the information the mind needs to process is so complex, there are normally many tasks running at once. If the mind were a computer, it would have many windows open at any one time; for example, it might be running a word-processing program, an internet browser, and a media player all at once. If you have a computer, you may have noticed that programs tend to run slower if there are many open at one time, especially if one of the programs takes up a lot of the processor's resources. Like a computer, the mind has a limited capacity, and tasks will slow down if there are too many running at once. Interference describes the mental slow-down and other problems that result from the mind trying to do too much at once. Interference only occurs when two processes draw on the same limited resource; if a resource were unlimited, it wouldn't matter if one task were using it, because there would always be plenty left over for other tasks. So, if one task interferes with another, we can say that they must both share a mental resource that is only available in limited quantities.

Interference is usually measured in experiments by comparing completion times for a task when performed alone, and the same task when performed at the same time as another, interfering task. For example, if I wanted to know if reading interfered with multiplication, I could ask you to perform a simple multiplication problem (e.g. 7 x 9) and time how fast you came up with the correct answer. Then, I could have you read a short passage, and while you were reading it, I could ask you to do the multiplication problem again, and time your answer again. If your answer was slower when you were reading and multiplying at the same time, then I could deduce that reading must interfere with multiplying. This means that they must share some limited processing resource that both need to use. The Stroop task, also described in in its own section of this SparkNote, is a classic example of reaction-time interference. Interference can also be measured by a decrease in accuracy or quality of performance, as in the shadowing task (described in the dichotic listening section).

Multiple tasks will usually show some interference with each other. In general, more difficult tasks that take up more attention cause more interference, while easier tasks can be performed with little cost. For example, most people can carry on a conversation while washing dishes, but not while trying to untie a complex knot. In addition, those tasks that are more similar to each other tend to cause more interference with each other. This similarity effect occurs because similar tasks tend to draw on many of the same resources, while different ones use skills that seldom overlap. In one experiment, people were asked to shadow one list of words, which they heard over headphones, and to memorize a second list of words. This second list was presented in one of three ways: subjects either heard spoken words, saw written words, or saw pictures. They showed more interference for the spoken words than the written words, and more for the written words than for the pictures. It makes sense that the spoken words would interfere with the shadowed list, since both required verbal and auditory processing; we would also expect less interference between the pictures and the shadowed list, since they had very little in common and so used very different mental processes (i.e. one was auditory and the other visual; one verbal and the other pictorial, etc.).

However, difficult tasks still interfere with each other, no matter how different they may seem. Driving and talking are very dissimilar tasks, yet we know that drivers with cellular phones are more likely to have accidents because their concentration on the road is impaired by their conversations. This suggests the existence of a limited, general processing resource that is used whenever we want to devote attention to a task. Thus, we can say that some general type of attention is needed for many different tasks. Attention is limited, and we use more attention for difficult tasks.

What defines a task as "difficult"? When we use the word difficult in this sense, we mean that we need to concentrate more to carry out the task. Different things may be difficult for different people. In general, if you have practiced a certain task a lot, then that task will seem easier, requiring less concentration. Typing requires much less attention for a professional secretary than it does for an average high school freshman. Therefore, that secretary has more attention left to talk on the phone, carry on a conversation, or perform another task at the same time. Practice reduces the demand on processing resources, so it also reduces interference. Tasks that are highly practiced can become automatic (see Automaticity and Practice). Almost any task can become easy through practice, so over time, you can learn to do almost any two tasks at once. However, it should be emphasized that interference is only reduced, not eliminated; that is, talking on the phone while driving, no matter how often you do it, will still lower the attention you have available for driving.