According to dailyblogtips:
…linking out is not supposed to directly reduce the PageRank on your pages. In fact there are plenty of pages that contain dozens of links and yet rank in the first positions of Google’s result pages for competitive keywords.
If you ever refrained from linking to an external site because you feared you would lose PageRank (I have been there too), forget that. Linking away to relevant and valuable pages is good for everyone.
Here’s the skinny on what the current stance is with Google. Linking to other websites will neither boost your ranking nor pull it down. In reality, linking out can help your website by turning it into a ‘go-to’ site. Think of it like having everything you need in one place. This will make your website an authoritative resource where website users can find other good resources from which they can benefit. This doesn’t mean you include every resource, but if you can provide the best of the best, you’re on the right track.
It’s not as simple as slapping the links anywhere you want on your page. When linking to other sites, you must create your links carefully so that they are relevant to your website’s content and topic. The right anchor text is important. Not overdoing it with too many links to the same anchor text keyword is something else you need to watch.
You have to choose the right sites as well, but before you can do that you have to know what makes for a good site. Sites you link to must be reputed sites in the industry; should not be blacklisted; and should not contain any malicious software. In other words, visitors to your site should be safe to click on any link to be taken to one of these outside websites. If they encounter embedded malware or spyware, it will affect your website’s reputation and your website will be associated with the harmful website. Think of it this way – when you backlink a site, you are endorsing it. If you endorse bad sites, people will think of your site as bad as well.
The cardinal rule of backlinking is to never, ever allow your website to be associated with websites with bad reputation. Your reputation is everything when it comes to online marketing. If you are perceived as untrustworthy, you are out of business. You should also not be linking to insignificant content sites that may or may not be there tomorrow or the next year. Content farms suck. They exist only to harvest cheap content they can sell off and they have almost no value at all regarding linking. Only spend your time linking to websites that have a long history so your links will not end up being broken links which could affect your website in a very bad way.