Memories are not concrete or accurate. Yes, they often feel that way. But, even outside of degenerative diseases like Alzheimer’s, they’re not photographs, snapshots or recordings. Instead, they are impressions and reconstructions. 

What this means is that they can be altered and they can be wrong. Some evidence suggests that reconstruction happens each time someone recalls a memory. They can add or detract from the information in that memory. Your mind filters the memories and so they are far from perfect. 

Say, for instance, that Person A has no recollection of Person B being at a specific event. They honestly do not remember it, though they can remember the layout of the room and what it was like walking through it. Then they see a surveillance video of the room, and it clearly shows that they walked by Person B as they crossed the room. Over time, that can be filtered into the memory until they “remember” walking past Person B. 

In this case, the memory has grown more accurate with supporting evidence, but it can also work the other way. Outside impressions, news stories, accounts from other people and many other factors can all make someone remember incorrectly an event that they originally remembered properly. 

The issue that this creates for criminal cases is that witnesses may count solely on their memories when giving testimonies. Even when these witnesses sound convincing and authentic, how much can we trust those memories? Those who are accused must know what legal defense options they have, especially when facing accounts that they know are not true.