Robert Balbar, who is 42, is on trial for second-degree murder in the 2003 slaying of Heather Hamill.
Forensic psychologist Hugues Herve testified Balbar underwent two rounds of testing that determined his intellectual functioning was below the first percentile.
That means Balbar scored worse than more than 99 per cent of people would score if tested.
The body of Hamill, who was 31, was found floating in the North Thompson River on Aug. 1, 2003.
Balbar, who is from Kamloops, wasn't arrested until late 2007, after a three-month undercover RCMP Mr. Big operation.
"His ability to read words placed him at a Grade 2 level and his reading comprehension was below the kindergarten level," Herve testified Monday.
"He also showed significantly below average general reasoning capability."
During the elaborate investigation, undercover Mounties posing as gangsters lured Balbar into a fictitious criminal organization that plied him with money and promises of sex.
Jurors have been shown the video from that December 2007 meeting with the leader of the fictitious gang, during which Balbar said he struck Hamill as many as 60 times with three hammers after she started "flailing around" a machete.
He said he then placed her body in a cooler and dumped it in the North Thompson River.
When he took the stand last week, Balbar said he attacked Hamill after she threatened his sleeping son with a machete and he only used one hammer.
He said he was trying to impress the gang boss by toughening up his story.
He said he then placed Hamill's body in the cooler so his son wouldn't see it and later took it to the Tk'emlups Indian Band reserve to give Hamill "a traditional native burial."
Balbar said he became spooked after nearby dogs began to bark and he eventually dumped Hamill's body into the river. (Kamloops This Week)