Can [edit: or should] a scientist's h-index decrease in the case highly cited publications are retracted? I am at the moment preparing a workshop unit dealing with 'digital publication'. While writing down my script I noticed that I usually tell the participants that a scientist's h-index cannot decrease. Now I am wondering what happens in the hypothetical case that highly cited papers are retracted due to scientific misconduct or something similar. So my question is: Does or can a scientist's h-index decrease in the case highly cited publications are retracted?

Just to clarify my motivation to pose this question a little bit. The class/workshop I am conductiong deals with information literacy in the sciences. My participants haven't usually dealt with research(er) evaluation and the like. They may have heard about the existence of the JIF or the h-index but that is where things normally end. So in my presentation I explain how these indices are calculated and that they are often used to evaluate scientific effort. I also address the issues and shortcomings of this latter use. 

I just checked some of retraction watch's most highly cited retracted papers and noticed that they are still listed in Web of Science and also being cited to this day. I see that this fact defeats my question as retractions are still being counted, at least in this data source. So maybe I should have posed it more philosophically like "As the h-index is used in researcher evaluation, shouldn't it decrease if highly cited papers are retracted?" ...  

Thanks to everyone answering and commenting!
The h-index is defined mathematically based on the number of publications and citations. So the only question is what data source you are using to calculate the h-index. If that source removes publications and citation counts upon retraction, the answer is yes, the h-index can decrease.

However, since this is source-based, you could even get occasional decreases due to other factors. Since many of these systems are automatic, you sometimes see incorrect publication associations and citation counts that can be later corrected, causing the h-index to decrease (I've seen this happening in Google Scholar).

I fail to see why this is interesting, though. Isn't this just like saying the number of publications and citations an author has can only increase, except in special circumstances such as retractions etc.?