West Lafayette, Indiana - Americans' memories of 9/11 will continue to shift as time passes, and individuals' memories will become less personal and more collective as a nation, says a Purdue University professor who studies history and memory.

"Memories are real but they are filtered through experiences and information from the time in between," says Caroline E. Janney, professor of history, who studies American history, memory and memorials. "The saying that hindsight is 20/20 is a complete fallacy. Hindsight is blurred and filled with other images. We can't help but look now at any event in the past without knowing what we know now. Memory is not passive. Consciously and unconsciously our experiences since that time change the way we remember and understand what happened.

"Both place and time matter in the way different individuals and groups will remember that day. For those who live in New York, Northern Virginia or Pennsylvania it may feel more personal but the further we move in distance and time from these events, the memory becomes less emotional and more secular. This is true with any historical event."

Every generation has a touchstone event, such as Pearl Harbor, the assassinations of John F. Kennedy or Martin Luther King, Jr., or the Challenger explosion, that was defining for their generation. People who experienced these moments often recount vivid details of where they were on that day. For 12 years, Janney has asked her undergraduate students about their memories from 9/11. Five years ago, students were able to share vivid details about where they were.

"The memories from the students today who were toddlers when 9/11 happened, are shaped by what they've seen on TV or social media or they are pulling from what other people have told them," she says. "This is the natural progression of memory."

This is another significant aspect of the 15th anniversary as there is now a significant part of the population, many of whom will be voting in the presidential 2016 election, who do not have personal memories of 9/11.

"We should watch closely to see how both presidential candidates frame their rhetoric around 9/11," says Janney, who is teaching "Ends of War," a graduate class about how wars end. "We, as a country, can't help but see 9/11 through the lens of the war on terror, and it will be interesting to see how many anniversary speeches reference the war on terror compared to what just happened that day."

Janney is the author of "Remembering the Civil War: Reunion and the Limits of Reconciliation" and "Burying the Dead but Not the Past: Ladies' Memorial Associations and the Lost Cause." She also is past president of the Society of Civil War Historians.