We all have memories, but how much do we really know about how our mind holds onto the past? Our brains have several separate systems of memory that all work together to form a coherent picture of our past experiences. To help us understand the differences between the two major systems of memory, long-term memory and short-term memory, we will start by looking at the classic recency curve. If I read you a list of words, and then ask you to recite back to me as many as you can remember, you will tend to remember the first and the last words that I read more often than you remember the words in the middle of the list. This is called the serial position effect, and it is illustrated in the recency curve, which shows the percentage of the words correctly recalled, by their position on the list (e.g. first, second, etc.).

The serial position effect can be explained by a system of memory containing two principal types of memory. You remember the first words that I read because those words were entered into long-term memory. Long-term memory is where you store information if you're going to need it later, but you aren't going to be thinking about it between now and then. However, as I read more and more words, the later words found it harder to fit into long-term memory, because the earlier words were interfering with their storage. So, the words in the middle of the list are recalled less than the words at the beginning. Finally, the words that I read last are remembered well because those words are still being stored in short-term memory. Short-term memory is where you store items that you'll need immediately, and you'll be thinking about them the entire time you're holding them in mind.

The modal model of memory illustrates the idea of a multi-part system of memory. Keep in mind that this is a simplified model, although it is useful. The sensory register receives information instantaneously as it enters through the senses. Attention acts as a filter to determine which information in the register is important enough to be considered in short-term memory. In the short-term store, information is processed so that we can respond to our environment. Some information, if it needs to be held onto for later use, is rehearsed in short-term memory. Rehearsal holds an item, like a phone number, in short-term memory for a little while, and it increases the chances that the item will get into long-term memory. Long-term memory is a web-like network of information that acts as a gigantic filing cabinet or library. We can enter items into the system, and we can take them out later to look for a fact or an episode that we need to recall.

The association network of memory is based on how we connect things in our minds. For example, my great grandmother's house always smelled like gingerbread, so I will always associate that smell with her. Whenever I smell gingerbread, the association triggers a memory of my great grandmother, and I think of her. We remember things and search for items in memory by using associations to call up other memories. Mnemonic devices are strategies that people use in an effort to make the search through memory a little easier. They are tricks that you might have already learned in school. We will discuss several mnemonic devices in a later section.

The analogy of memory as a filing cabinet is not exactly correct, because we don't always get out exactly what we put in. Because each memory is connected to so many others, it can be influenced, changed, and distorted by other information. For example, stereotypes can distort memories to make them agree with our bias. In addition, some memories simply disappear. There is some debate over why memories fade. It may be that memories naturally decay over time, like an old house that gradually gets leaks in the roof, then holes in the floor, and finally rots into the ground. Or, it may be that memories get confused with one another and push each other out, like a blackboard that must be erased to be used again. Most likely, both explanations are partly true; we will look at evidence for both sides, and we will also examine seven types of errors that can occur in memory.