Stephen Greenspan, a psychologist who did not evaluate Román but reviewed the reports of three other doctors, called Román "the odd man out" in his family, and said he has always needed others' care to survive.
"They are all competent adults," Greenspan said. "Mr. Román on the other hand clearly was not competent at that level."
Doctors said Román, who was born in a home in Mexico, could have been impacted by perinatal anoxia, a shortage of oxygen during birth. That would be a "major risk factor" for mental retardation, Greenspan said.
Doctors also said Román's drug and alcohol use, which started at an early age, could have damaged his brain.
"The damage that you do to your brain [with drugs and alcohol] when you're an adult is severe, but it's nothing compared to what you do to your brain in adolescence," said Dr. Ricardo Weinsten, who also found Román to be mentally retarded.
Doctors said Román struggled with impulse control and decision making. When he was younger, he would walk in front of trucks "and thought he had superhuman abilities" to make them stop, Greenspan said.
But prosecutors and two other psychologists doubted the diagnosis. Assistant Attorney General Patrick Nolan questioned how Román could have then sold drugs and engaged in other criminal activities for which he has been convicted.
According to charging documents, Román, 39, shot and killed the 37-year-old Fox during a traffic stop on Jan. 5, 2010. Fox had stopped the car because Román had allegedly been involved in a drug deal just minutes before.
When Fox walked up to the car and asked for license and registration, Román pointed the barrel of an AK-47 out the driver-side window and fired, according to preliminary hearing testimony.
Another man, Rubén Chávez-Reyes, is serving time in prison for helping Román evade police following the shooting.
A trial for Román is scheduled for August.