Appeal from the Circuit Court of Edwards County. No. 01-CF-49 Honorable David K. Frankland, Judge, presiding.
 The opinion of the court was delivered by: Justice Goldenhersh
 After a bench trial in the circuit court of Edwards County, the defendant, Kenneth R. Lemke, was found guilty of first-degree murder (720 ILCS 5/9-1(a)(2) (West 2000)). On appeal, the defendant raises the issue of whether his trial counsel's failure to present the defense that the defendant had committed the lesser offense of involuntary manslaughter constituted the ineffective assistance of counsel. We reverse and remand for a new trial.
 The defendant was charged with two counts of first-degree murder and one count of domestic battery arising from the shooting death of his stepson, Lance Albertson. The matter proceeded to a bench trial.
 The defendant testified in his own defense. The defendant testified that he had been married to Albertson's mother for eight years until their divorce in 1988. After the divorce, he remained close with Albertson. Occasionally, Albertson would come to the defendant's property to ride four-wheelers and play ball. On the date of the incident, Albertson and a friend, Rusty Heindselman, stopped by the defendant's property and borrowed a four-wheeler in order to go deer hunting. While the others were gone, the defendant cooked some food and drank some beer. Upon Albertson and Heindselman's return, the defendant served them some food and then left for a bar in Grayville, drinking on the way. The defendant testified that he had "quite a few" beers before he left and also picked up some beer at a Huck's store on his way to Grayville. At the bar, the defendant drank Jack Daniels and Coke and met with his girlfriend.
 When the defendant returned home, Albertson and Heindselman were still there. After a while, Albertson left the house. The defendant thought Albertson was upset about a girl. The defendant then heard what he believed was Albertson banging things around in the barn. The defendant went outside and told Albertson to settle down. According to the defendant, Albertson "got real violent" and started kicking on an old vehicle that the defendant kept covered up. The defendant told Albertson that he could not act like that and to settle down. Albertson then went wild and asked the defendant, "Do you want a piece of me?" The defendant and Albertson wrestled. Albertson took away a walking stick that the defendant had been carrying, and the defendant crawled away. The defendant then yelled for Albertson to leave.
 The defendant went back into his house. The defendant testified that at that time, he did not know what to do because Albertson appeared "just out of it." The defendant got a pistol, went back outside, told Albertson that he had a gun, and told him to leave. Albertson yelled something unidentifiable back. Albertson stood on the seat of the four-wheeler and yelled something about the walking stick that he was still holding. The defendant testified that he then felt a sharp pain in his hip and started to fall backwards and heard a blasting sound above his head.
 The defendant testified that at that time, he did not realize how the gun had fired. The defendant had not cocked the gun. The defendant saw Albertson fall down. The defendant went to check on Albertson and did not get a response. The defendant called an operator for assistance and tried CPR, to no avail. The defendant testified that the gun had not been cocked and he had not aimed it at Albertson. The defendant stated that he had not come out of the house with the intent to shoot Albertson and had not wanted to shoot him.
 The defendant testified that he had suffered numerous injuries from 20 years' working in the construction industry. The defendant stated he has a pinched nerve in his hip and suffers from carpal tunnel syndrome.
 In addition to other witnesses, the defendant presented testimony from his personal physician of the previous 20 years, Dr. Timothy Garrett. Dr. Garrett testified that he had treated the defendant for back pain, a sciatic nerve in the hip, and carpal tunnel syndrome. The defendant had reported numbness and tingling in his fingers due to the carpal tunnel syndrome. He also had spasms in his hip that could cause him to buckle and lose balance while walking.
 Detective Richard Simer, an expert in the functioning of a firearm, from the City of Centralia police department, testified that he had examined the .22-caliber Colt Peacemaker revolver that had been involved in the incident. Simer also described the fanning and cocking measures of firing a shot. According to Simer, his examination revealed that the trigger pull was very slight and hardly required any pressure at all to pull the trigger back if the hammer was not cocked. He testified that this model of handgun was developed in the late nineteenth century and is famous as the gun used by cowboys in the Old West. Simer testified that it was possible on some occasions to strike the hammer and for the gun to go off by accident. According to Simer, when the firearm was first developed, the recommended procedure for carrying the firearm was to always carry it on an empty cylinder, even when it was placed in a holster. The reasoning behind this recommendation was that if someone fell off his horse or otherwise fell down, the firearm would not fire because there would be no cartridge under the hammer.
 The State also presented several witnesses. County Coroner Mark Curtis testified that he participated in the autopsy on Albertson. Curtis testified that one shell had been fired and several shells remained in the handgun from which Albertson was shot. He stated that it appeared that the bullet which had struck Albertson was going upward at impact. According to a toxicology report, Albertson had methamphetamine, amphetamine, ephedrine, and pseudoephedrine in his blood.
 Michael Cooper, a forensic scientist for the Illinois State Police, testified that he had examined the firearm that had been used in the incident. Cooper stated that it was a single-action revolver, which, in contrast to double-action revolvers, requires a person to bring the hammer back or cock the gun prior to shooting. Cooper described two methods for firing a shot: cocking and fanning. He stated that in cocking, the shooter sets the cock by bringing the hammer back and then fires by pulling the trigger and that in fanning, the shooter holds the trigger down and pulls the hammer back and then releases it, as shown in movies about the Old West.
 The 9-1-1 call from the defendant after the incident was played. In the recording, the defendant states, "[Albertson] jerked the gun out of my hand."
 Rusty Heindselman testified that when the defendant returned home from Grayville, the defendant immediately got into an argument with Albertson. According to Heindselman, both the defendant and Albertson were in bad moods. He stated that Albertson was outside and the defendant went outside to check what was wrong with Albertson. The defendant returned in a few minutes, apparently after getting into an argument, and stated, "He is not going to talk to me that way," and picked up a walking stick before returning outside to talk to Albertson. When Heindselman went outside to see what was going on, he observed the defendant and Albertson on the ground wrestling near the barn door. Albertson apparently won the match and took the walking stick away from the defendant. Albertson walked away and the defendant went back inside. The defendant then came back outside carrying a handgun. Heindselman asked the defendant what he was doing and the defendant replied, "It's cocked and it's ready." Heindselman heard a gunshot and saw Albertson fall. Heindselman testified that the defendant told him several times the shooting was an accident.
 The court found the defendant guilty of one count of first-degree murder. At the conclusion of ...