Should teachers post their lecture slides for students to download? I think there's a better way. Allow me to elaborate.
The good thing about posting slides (powerpoint, keynote, etc.) is that the students have full access to the information. If having more information helps them learn more, then by all means, let's give them more information.
There's another benefit. Note-taking is the bane of many teachers' existence. Some students are really bad at taking notes. Like me, when I was a student. And not just beause of my handwriting.
Sometimes I would write down too little because it all made sense (so how could I forget it?). Then other times, when I was confused, I'd try to write everything down so I could "figure it out later." The idea that you'll figure it out later is a much, much bigger myth than Bigfoot and Nessie combined. But it's just as fanciful, and it can prevent you from actually thinking about what the teacher is saying.
Dear students: Next time you are totally confused and taking copious notes so you can figure it out later, please PLEASE put aside your pen or laptop and raise your hand!! Immediately and often!!
Thus, another advantage of posting slides, for the students, is that they know they can go check the slides later if they didn't get everything down. This gives them license to pay more attention to you. (Quick side-note for teachers: This year, when I would show a graph I put the numerical values above the bars, even though it was redundant, because it makes taking notes easier.)
The downsides of posting notes? First, students don't always feel they need to come to class. So they don't. Withholding the slides is a good way of forcing people to come to class. Although to me it feels a little bit like holding knowledge hostage.
For me, the other big downside is that my slides are full of private notes to myself, hidden slides, and stuff that I didn't have time to cover. I have to edit them before I can post them. What a pain.
Enter the Wiki
This year I tired a new system. I asked my TA to take notes during class. I gave her full access to my Powerpoint slides so she could grab images, graphs, etc., and put them in the notes. (It took almost no work on my part, especially because I gave the TA access to the slides via a shared Dropbox folder so I didn't even have to remember to pass them along to her.)
After class, she posted her notes to a class Wiki: http://cog221.wikispaces.com/ This way, the students in the class could edit it if they wanted to improve upon it. That didn't really matter much, because none of them did. But it was handy because my TA and I could both easily make changes, see what changes had been made, and so on. Oh, the problems the wiki solved.
I was very curious about whether my class would stop taking notes because they knew quality notes were available online. So after a good 15 lectures, I asked them. Only one person had stopped taking notes (although he was using his laptop during class, not a good sign). Many students said taking notes kept them attentive and helped them organize their thoughts. I responded by observing that sometimes note taking prevents people from listening and understanding. In the end, we agreed there are pros and cons.
Making it an offer they couldn't refuse
There was one more key feature that really made the wiki useful. Each lecture had its own page. But we would aggregate all of the questions I'd asked on a given day and put them on an additional "questions" page. I would often add to the questions page after a lecture was done as well. When it came time to study, the students could go to a single page that asked 8-10 questions about every lecture and start by making sure they knew the answers. By the time of the cummulative final exam there were probably over 150 questions on there.
The questions did a lot more than give the students a reason to visit. In fact, I would guess they had a lot of benefits.
Did They Use It?
One last thing: the wiki, which was free to create at http://www.wikispaces.com/, provided free usage statistics. I pulled two interesting ones out for this post. The first is page views. And yes, there was a midterm in mid March and another on May 1st.
The peak studying actually happened in the hours between midnight the night before the exam and the exam itself, which started at 1:10 pm the next day. So no, students don't cram the day before; technically speaking, they cram the same day as the exam. There's also a chunk of views in mid-May. Because the students could take the final exam any time that week, the studying was kind of spread out. (By the way, the questions page has been viewed almost 800 times by a 50 person class, more than twice as much as any other page.)
I also looked at unique visitors. I doubt my class is still looking at these pages, but it seems that other people are stumbling upon them now and then. So maybe the wiki can continue to serve an educational purpose, even if a very small one, even after the class is over.
The bottom line
Instead of posting your slides, or holding them hostage, get an amazing TA, give her access to your slides, and her post notes. And add questions to the notes so students have a reason to use them.