Darker in print?
 

Okay so admittedly I've not worked with printing my photos that often. I've done most of my work on my Mac and haven't printed many of the photos that I've taken. I'm more likely to post them to my flickr account than to print them.

However, since doing some commissions I've been noticing that the prints are often slightly darker than the screen images. I know this seems fairly obvious, and I feel silly for asking but I thought I would at least try to get an official answer on this to help improve my knowledge for future commissions. Do images normally come out darker when printed? I've printed a few of my photos recently and they seem dark compared to the screen version.
It feels really silly to ask this, and I recognize that my monitor settings might "lie" to me in terms of how bright an image really is, but I'm just wondering if this is expected behaviour.
Thanks