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It's a hot summer's day. You're eight years old. And you're excited. It's the first time you've been up in a hot air balloon. You can see fields below. You feel the heat of the flame. It's something you'll never forget. A memory to last a lifetime. It's certainly not something you'd think you've done if it had never happened.

Psychologists at Victoria University in Wellington are finding out just how easy it is to implant false memories using digitally altered photographs. There's nothing sinister about what they're trying to do. They're just trying to get to the bottom of how memory works. It's the first time anyone's investigated the power that digital technology has to change what we remember. And the results are remarkable.

It seems like stuff of science fiction. 30 students are shown pictures of their childhood in a study they believe is about how we reminisce. In fact it's testing how vulnerable their memories are.

This is Jessica. It's day one of the experiment. When she's first shown the fake photo of her hot air balloon ride, unsurprisingly, she has no memory of it.

Tell me everything you can recall about that event.

Absolutely nothing.

By the end of the week she believes she's been in a hot air balloon. But the psychologist knows it's something she's never done.

I think we walked on a platform

In fact she's been deliberately tricked. What we've been doing is showing people a selection of photographs from their childhood. Four photographs. Three of them are true. One of them is fake. So the third photograph in the booklet always depicts the subject and another family member taking a hot air balloon ride.

But they've never actually been on that ride.

No. We know that for a fact. We interview them three times, over the course of a week. And by the time they get to the third interview, just a week later, half of them, 50% of our sample, believes they've been on a hot air balloon ride. 50% is much higher than Maryanne Garry and her team ever expected. Even the students that don't remember the hot air balloon ride believe the photos are real.

We asked family members to give us a photo of the subject that shows the subject clearly from the waist up. And, hopefully, standing next to another family mean member. So we get a photograph that looks something like this. And we remove everything around that we don't want. We just grab them here. Copy them over. And suddenly they're in the picture.

A bit big at the moment. Aren't they? They won't quite fit in the hot air balloon.

They are a bit big. So we have to transform them. Make them smaller. And make them fit. So we end up with something like that.

That looks fantastic. They really look like they're in there.

Over the course of the week subjects are asked not to speak to their family about the study. Kim asks them to think about the photos every night. By the third interview their imaginations are starting to fill in the detail.

Picture myself walking onto the balloon, as if I was actually there. And I end up getting a picture of some kind of platform.

I remember looking up at the gas and the balloon [? proper. ?] So I just remembered stuff like that. And also looking down and saw the patchwork on the ground because we were quite high up.

Contrary to what we believe memory does not work like a tape recorder. Our memories are not recorded and stored faithfully in the back of our heads, and then played back. Like on a video recorder. Memories aren't permanent. They're a reconstruction, a blend, of imagination, and fantasy, and things you might hear about or think about afterwards. And also like in our case, a photograph.

What kind of implications does this have?

One of the implications is that, if you remember something, and you report it with great confidence, great detail, and it's very vivid and clear, got a lot of motion, it doesn't necessarily mean it really happened.

What kind of reaction do you get from people when you tell them this didn't happen at the end of the week?

Surprised. Especially if they believed the event happened. They're also quite scared. Because people, they have a lot of faith in their memory. So I think as an adult they can remember their childhood quite clearly. And it's a big wake up call when suddenly they realize their memory is not that reliable.

One of the four photos we discussed didn't actually occur. So one of them didn't happen. Do you know which one that is?

The balloon one?

Yeah, the balloon one. Yeah,

It's scary really. A hot air balloon ride is not something you'd easily forget.