Also, by law, the mental condition must have been present before the defendant's 18th birthday.
A person of average intelligence will make from 90 to 110 on IQ tests, according to testimony.
Durcho scored a 72 on an IQ test given him last year by the psychologist used by the prosecution. He also scored a 72 on the same test given him in 2009 by a psychologist hired by his defense attorneys.
The defense psychologist, Antoinette McGarrahan, testified Thursday that Durcho's IQ could be as low as 68 because of the margin of error in the test. But the prosecution's psychologist disagreed, saying Durcho's IQ could only be as low as 69.88 because of the margin of error.
Durcho scored a 78 when he was 13 and a 75 when he was 9 on a different intelligence test given him by El Reno school officials. Defense attorneys contend the test he took in school is not a legitimate IQ test.
Prosecutors allege Durcho strangled Summer Rust, 25, when she picked up the phone to call police on him. Prosecutors say he had vowed he would never go back to prison.
They allege he strangled Rust's three daughters, Kirsten Rust, 7, Autumn Rust, 7, and Evynn Garas, 3, and her son, Teagin Rust, 4, to eliminate them as witnesses. The victims' bodies were found Jan. 12, 2009, in the apartment. The children's bodies had been stacked in a bathtub partially filled with water. Durcho was arrested after a car wreck in Texas.
A half brother, Dallas Due, testified last week that Durcho admitted choking Summer Rust.
The defense expert, Keyes, disclosed during his testimony Monday that Durcho said he had lived in the apartment, often helped get the children ready in the morning and that they called him, “Daddy.”
Many who deal with or care for the intellectually disabled consider describing them as retarded extremely offensive. During the hearing, attorneys at times have referred to the sensitivity surrounding the words the law requires them to use.