One of the interesting things is that they found that you're more likely to notice that the person changed if you are in the same age group as the person that changes. And you're far less likely to notice the person changes if they're wearing something 'uniform-ish' like a hard hat.

So basically you're more likely to notice it if you're seeing that person you're talking to as 'one of you' than 'one of them.'

I love this kind of Psychology.