Fairness is a major factor that tends to color how employees view the workplace. If a happy, engaged employee learns that another employee doing the same job is getting 15% more than he or she is getting, all the other things that made the job seem great turn to ash.
Employers may or may not even notice the ill effects when employees feel that they are being treated unfairly. The consequences can range from catastrophically expensive lawsuits, causing public relations nightmares, generating retention/hiring problems, or more likely, all three.
However, there are often instances of unfair treatment, especially unfair compensation that are often not detected by employers, such as diminished engagement, reduced commitment to work, and ultimately, costly turnover. This tends to occur more frequently with female employees.