The halo effect is a cognitive bias that occurs when a person’s positive or negative traits seem to “spill over” from one area of their personality to another in others’ perceptions of them. (As defined by Wikipedia).
How does the halo effect apply to SEO?
Everyone who is concerned with SEO should know, it takes more time to get a top ten ranking with Google, compared against MSN and Yahoo. Yahoo and MSN, allow newer sites to be ranked highly within weeks of creation, however Google makes a new site wait. Google does this so that spammers cannot hog the top rankings through massive illegal link campaigns (such as link farms).
The halo effect takes place, when your site ranks first for a chosen keyword. After your site has starting getting listed as the first site for your chosen keyword, you can use the halo effect to your advantage and try to get the search engines to rank your site in the top ten or even number one for various surrounding keywords.
Now since the SEO work has been undertaken, your primary keyword has already got its first place ranking. So the search engines will also start ranking your site highly in other relevant surrounding keywords therefore creating the halo effect.
Since your keyword is ranking highly, you do not need to undertake the same amount of effort for the other keywords. This may not make sense straight away but it is essentially easier for you to rank your sites for similar and surrounding keywords after you have reached the first ranking position for your primary keyword.
It is imperative that you first chose your primary keyword so that you can rank for that before you try to rank for your surrounding (secondary) keywords. Since the primary keyword should be the most relevant to your site, you should start with that one and bear in mind SEO is an on going task not a monthly task.
So take advantage of the trust you have built up with the search engines for your key words, and ensure that you are always on the look out for similar keywords, which could help you, increase your SEO.