Example:
In total you have 100 incoming links.
80 links of those 100 are reciprocated by you, someone links to your site and promptly you link back to them or vice versa.
Google recognizes this as reciprocal linking and would not count those links as much as it could have if the ratio - the amount of reciprocal links versus natural incoming links was smaller.
Google would recognize artificial votes for your site/page as that is what they are, artificial, they did not come naturally and are not counted as genuine votes, each incoming link is a vote for your page.
Don't focus to much on reciprocal linking and only reciprocate a link if you feel it ads value to your visitor.
Would you be tempted to link back if you didn't get the link in the first place?
If not then it wouldn't be a good idea to link to site/page.