10.3. Outbound Links
The "everything in moderation" slogan is really apt when it comes to outbound links. You could also say that the "outbound link giveth and the outbound link taketh." Here's why: you want some respectable outbound links to establish the credibility of your site and pages, and to provide a useful service for visitors. After all, part of the point of the Web is that it is a mechanism for linking information, and it is truly useless to pretend that all good information is on your site. So on-topic outbound links are themselves valuable content.
However, every time your site provides an outbound link, there is a probability that visitors to your site will use it to surf off your site. As a matter of statistics, this probability diminishes the popularity of your site, and Google will subtract points from your ranking if you have too many outbound links. In particular, pages that are essentially lists of outbound links are severely penalized.
If you follow the word-per-page guideline I make in "Words and Keyword Density"a little less than 250 words per pageyou'll get best results if you try to provide at least two or three outbound links on every page and no more than ten or fifteen per page.