The primary reason that comment spammers post spam comments is to increase their ranking in search engines. Here’s how it works:

1. You post a blog entry on your website, with commenting open to allow your readers to respond to the entry with their own comments.
2. A spammer posts a comment on this entry, containing numerous links to websites they run.
3. These links effectively become part of your site’s content.
4. A search engine visits your site, and sees the links that the comment spammer has posted.
5. The search engine says to itself “Hmm, here’s another site that’s linking to site XYZ. I’m going to bump their popularity up a notch.” (Note: this is an oversimplification, but is generally the right idea.)
6. When someone uses a search engine to search for products or information related to the comment spammer’s company, that company’s website is given a higher page ranking because of the inflated popularity.

So this is great for the spammer, because they’re beefing up their search ranking. But it comes at the blogger’s expense:

• Comment spamming taxes the blogger’s server, as each spam comment posted requires web server and database server resources.
• It creates unnecessary and irrelevant comments on the blogger’s site.

If there were a way to make comment spamming less profitable to the spammer, that might discourage comment spammers from actually posting. That’s what Six Apart ***is working on|http://www.sixapart.com/log/2005/01/support_for_nof.shtml*** with search companies such as ***Google|http://www.google.com/***. They have worked together to create a new standard for use of the ‘nofollow’ attribute in links. This attribute, when seen in a link by a search engine, will decrease the link’s value to the search engine. By using the new ***nofollow|http://www.movabletype.org/news/2005/01/movable_type_nofollow_p.shtml*** plugin for Movable Type, any links posted in comments or in trackback pings will automatically be given this nofollow attribute. This plugin is very simple to install. I implemented it at prwdot.org in just a few minutes.

It’s too soon to say how effective the use of this tag will be, but I’m optimistic. I think it’s a great idea and hopefully it will make comment spamming less attractive to the spammers.

Update
A few concerns have come up that I want to address:

1. Yes, I realize that this will not stop Google from indexing other, non-link content in the comments on our site… but the worst that could do is cause my site to come up for searches on other non-related topics. And that’s only if I don’t remove the offending comments right away.
2. I also realize that it may stop Google from indexing relevant links in comments that other people have posted, but I’m really okay with that. After all, it’s our website, and not a place for other people to promote other websites, as helpful as they may be. If I think a link is useful enough to include it in a blog posting, I will. Or else it might be put in the linklog. Don’t let this stop you from posting legitimate, useful links in our comments if you so desire. Just keep in mind that they won’t help out your page rank at all.
3. I want to acknowledge, as others have, that this is not a silver bullet for comment spam, and as bad people do, spammers will probably find other ways to do their dirty work. But I think it is an important and useful piece of the puzzle, and a step in the right direction.
4. Finally, I want to link to several other posts on the issue by ***Chad Everett|http://jayseae.cxliv.org/2005/01/19/considering_the_nofollow_attribute.html***, ***Jay Allen|http://www.jayallen.org/comment_spam/2005/01/massive_weblog_antispam_initiative_relnofollow***, ***Robert Scoble|http://radio.weblogs.com/0001011/2005/01/18.html#a9229***, and the ***Google/Blogger|http://www.google.com/googleblog/2005/01/preventing-comment-spam.html*** engineers.

Another Update
***Gervase Markham|http://www.gerv.net/*** has posted ***his thoughts|http://weblogs.mozillazine.org/gerv/archives/007350.html*** on the issue, and he makes a statement that describes my exact hope for the nofollow attribute:

qqq|

The only way rel=”nofollow” will ever help Joe Blogger is if so many blogs use it that blogspamming becomes entirely pointless – and then the blogspammer will just stop outright. And, given the number of old and abandoned blogs littering the web, that time is some way off.

|qqq

I agree – it is some way off. But if the blogging and search engine companies can work to bring this functionality to as many blogs as possible as easily as possible, then I think we stand a fighting chance of getting spammers to leave us alone.

Then again, I’m an optimist. 🙂