Too many outbound links on a website can deplete a site’s own PageRank (the importance Google assigns to a page based on an automatic calculation that factors in the link structure of the web and many other variables). However, this may be more of a relative than an absolute statement; there are ways to externally link to relevant websites without reducing the importance of your own website.
In its simplest form, PageRank can be explained with this formula:
PR(A)= PR(B) + PR(C) + PR(D)
From the formula above, the sum of the PageRank of pages B, C and D is awarded to page A because all of their links are pointing to A. Basically, pages B, C and D are passing on some of their own “link juice” to Page A.
PageRank is assigned on a page by page basis in that the more outbound links one has on a particular page, the PageRank of the page itself is affected. An intelligent strategy is to create a page on the site that includes a collection of links to websites that your users will find helpful. This is a good idea as externally linking from an internal page on your site as opposed to the homepage will not have that much of an affect on the site as a whole. The homepage typically has the highest PageRank by virtue of the links pointing to it. That PageRank from the homepage flows to the internal pages on the site. So, to keep that healthy internal link structure intact, we’d essentially want to have outbound links coming from a lower level page.
However, all of this can be completely avoided with the nofollow attribute. A few years ago, Google put into action the nofollow attribute so webmasters could link to other websites without losing their own “link juice”. This is useful if a webmaster wants to provide links to other quality websites for their users and not lose their own PageRank status.