Managers' performance reviews of employees are often unreliable and biased. Unconscious biases can account for 61% of a performance rating reflecting the manager, not the employee. The document discusses six common biases: the halo effect, horns effect, recency bias, similarity bias, contrast bias, and conformity bias. It provides examples of how each bias might influence ratings and recommends practices like using evaluation matrices, frequent check-ins, and customized evaluations to design performance management systems that minimize the effects of these biases.