A problem with lineups may be that memory is too good. There is an experiment that has been done several times that shows how surprisingly good our memories can be in some ways.

The experiment is to show someone a very large number of images or normal things, showing them each image for a short time. The images might be things like 5 apples on a table, a man petting a dog, a couple of people talking, and that kind of thing...just normal things you might see every day.

Then later, sometimes days later in some versions of the experiment, you show those people pairs of images, where one of the images was from the set they were shown earlier, and one is not. You then ask them which of the two images they have seen before.

This has been done with sets as large as 10000 images, and people get 80% or more correct two days later [1]. There was a version where in the identification portion the two images shown were one from the set they had seen earlier and one that was similar. Examples might be a stack of $1 bills and a stack of $5 bills, or a bell with a narrow handle and a bell with a wide handle. Even with these similar images people did well. The one I've seen discussed used 2500 images and 2500 similar images, and people got 90% correct [2]. That particular one tested shortly after the people were shown the images.

I wonder if this works when instead of the recall test being one image you have seen before against one that you have not, it is one you have seen against several you have not?

And I wonder if it works when it is something live you are seeing instead of images?

And how important is it that when shown the initial set of images people know they are being shown images and are going to later undergo some kind of test involving them?

If the answers to these three questions are "it still works", "it still works", and "not important", then it suggests lineups may be inherently problematic. In the course of an ordinary day in city most people are exposed at least briefly to a large number of strangers.

If a witness only got a brief look at a criminal, and then is shown a lineup that contains a suspect that is not the correct criminal but is someone that the witness happened to see a few days ago out and about in the city without thinking about it, is there a high chance that person will be picked in the lineup?

It seems that with human memory, three things are true. (1) We actually remember a lot more than we think we do, (2) we are not very good at accurately consciously recalling it, and (3) when we recall it we may unconsciously update the memory to better match the inaccurate version we recalled.

Unfortunately most use of memory as evidence depends on accurate conscious recall, and requires the witness to recall it multiple times between the events witnessed and their testimony at trial, giving many opportunities to mutate the memory to match the prosecutor's narrative.

If I am ever on a jury I am going to greatly discount eyewitness testimony unless the side the testimony is for explains what steps they took to address these problems.

[1] Lionel Standing (1973), "Learning 10,000 Pictures", Quarterly Journal of Experimental Psychology 25, 207–22.

[2] Timothy F. Brady, Talia Konkle, et al. (2008), “Visual Long-Term Memory Has a Massive Storage Capacity for Object Details,” Proceedings of the National Academy of Sciences 105, no. 38, 14325–29.