Judge John M. Cleland's ruling Friday came after defense lawyer Karl Rominger asked the court for permission to present expert psychological testimony on Sandusky's diagnosis with histrionic personality disorder, the Pittsburgh Tribune-Review reported.
The American Psychiatric Association said individuals with histrionic personality disorder exhibit "an excessive need for approval and inappropriately seductive behavior," among other symptoms.
The Tribune-Review said Cleland said the diagnosis could be used to counter prosecution claims letters Sandusky wrote to one of his alleged victims amounted to grooming the boy for a sexual relationship.
In his petition to the court, Rominger said "the goal of a person suffering from this disorder in writing those letters would not necessarily be to groom or sexually consummate a relationship in a criminal manner, but rather to satisfy the needs of psyche belabored by the needs of such a disorder."
The prosecution has rested its case in the trial and the defense is to begin Monday.
Sandusky, 68, is on trial in Bellefonte, Pa., charged with 52 counts of sexually abusing 10 boys during a span of 15 years.
He has denied the allegations.