The opinion of the court was delivered by: Justice Miller:
Following a jury trial in the circuit court of Cook County, the defendant, Norma Jefferson, was found guilty of attempted first degree murder and aggravated battery and was sentenced to 35 years' imprisonment for those offenses. The appellate court affirmed the defendant's convictions and sentence in an unpublished order. No. 1-96-1163 (unpublished order under Supreme Court Rule 23 (166 Ill. 2d R. 23)). We allowed the defendant's petition for leave to appeal (166 Ill. 2d R. 315(a)), and we now affirm the judgment of the appellate court. The facts in the case are not in dispute, and they may be stated briefly. The events giving rise to the present case occurred on March 25, 1992. That morning, the defendant took her daughter, Danisha, to Cook County Hospital. Dr. Demetra Soter examined Danisha the next day, March 26. Danisha had been born on February 8 of that year and was less than seven weeks old at the time. At trial, Dr. Soter testified that an external examination of the child showed swelling and bruising on her forehead, and she had two black eyes. Danisha was placed in intensive care and was intubated and put on a respirator. According to Dr. Soter, a CT scan performed when the child was admitted to the hospital revealed subdural hematomas on both the left and right sides of the head. Other tests performed at that time disclosed the existence of retinal hemorrhaging. Dr. Soter explained that both types of injuries are classic symptoms of shaken baby syndrome. Dr. Soter said that a second CT scan, which was taken on March 26, and an EEG also done that day showed severe brain damage from a lack of oxygen to the brain. Dr. Soter explained that this was a distinct injury, unrelated to shaken baby syndrome, and would have been the result of strangulation or suffocation.
Another witness at trial, Dr. Padma Sundaram, testified regarding Danisha's development; Dr. Sundaram is a physician at Schwab Rehabilitation Center and has treated Danisha. Dr. Sundaram stated that the child has severe cerebral palsy as a result of shaken baby syndrome. Dr. Sundaram last examined Danisha in April 1995, a month before trial, when Danisha was more than three years old. According to Dr. Sundaram, Danisha's motor skills were less than those of a four-month-old, and her mental capacity was equivalent to that of a child five or six months old. Dr. Sundaram stated that Danisha will never be able to walk, sit up, turn over, or fully talk, and that she will always require constant care.
The defendant gave law enforcement authorities several different accounts of the events leading up to the child's injuries. The defendant initially told investigators that she was cleaning in another room in the apartment when she heard Danisha crying. The defendant's nephew, Jacob, who was then eight years old, told her that another child was bothering Danisha. The defendant warned the children to leave Danisha alone, and she then resumed cleaning. Danisha began crying two more times, and on the last occasion the defendant noticed that Danisha was having difficulty breathing and that white matter was coming from her mouth. The defendant said that she then shook Danisha to help her breathe.
The defendant later told investigators that on the morning of March 25 she left the apartment to go to a nearby store, leaving Danisha with Jacob and three other young children. The defendant said that upon her return about 15 or 20 minutes later, Jacob told her to check on Danisha. The defendant found that Danisha was having trouble breathing and that there was white matter around the child's mouth. The defendant said that she shook Danisha to help her breathe. The defendant then wrapped Danisha in a blanket and ran downstairs to the security desk, where she asked the guard to call an ambulance.
The defendant eventually made an inculpatory statement to investigators. In the statement, the defendant said that on the morning of March 25 she left the apartment to go the store, leaving Danisha with Jacob and several other children. When the defendant returned, she noticed a scratch below Danisha's left eye and redness around her right eye. The defendant said that she then began watching the "Perry Mason" show on television. Danisha began crying, and the defendant said that she became angry with that, because the crying was interfering with the program. The defendant explained that she also felt angry because her sister owed her money. According to the defendant, she grabbed Danisha's throat to make her stop crying. The defendant then gave Danisha a bottle, but she resumed crying a little later. The defendant said that she then struck Danisha in the face with the palm of her hand. About 20 or 30 minutes later, the defendant noticed white foaming material like thick saliva coming from Danisha's mouth. The defendant picked up Danisha and shook her; the child's head rolled back and forth and her body was limp. The defendant then wrapped the baby in a blanket and ran downstairs, where she asked a security guard to summon an ambulance.
The defendant testified in her own behalf at trial. She stated that she left the apartment briefly on the morning of March 25 and, upon returning, found that Danisha had saliva coming out of her nose. Danisha was limp when the defendant picked her up, and she would not awaken. The defendant said that she then wrapped the baby in a blanket and took her downstairs to the security desk, where she asked the guard to call an ambulance. The defendant testified that she signed an inculpatory statement because a police officer told her that Danisha had only several hours to live, and that if she signed the statement she could see her child, talk to her parents, and go home.
During cross-examination of the defendant, the prosecution requested a Discussion with the trial Judge outside the jury's presence. The State sought to present evidence about the defendant's agreement to take a polygraph examination and her later decision, before the test could be conducted, to give an inculpatory statement. Over the defendant's objection, the trial Judge allowed the introduction of this evidence. In response to further questioning, the defendant testified that she agreed to take a polygraph test and that one was scheduled for her. The defendant denied, however, that she later knocked on the door of the room where she was being held and said that she wanted to give a statement.
In rebuttal, a detective testified that the defendant agreed to take a polygraph test. The detective scheduled one with an examiner and told the defendant that an appointment had been made for that evening. According to the detective, about 5 or 10 minutes later, the defendant knocked on the closed door of the interview room where she was being held. The detective opened the door, and the defendant explained that she wanted to talk to him. The detective and an assistant State's Attorney entered the room, and the defendant said that she wanted to tell the truth. The defendant then gave the statement in which she admitted choking and shaking her daughter. The assistant State's Attorney who took the defendant's statement also testified in rebuttal, and she denied that the defendant was promised anything in exchange for her statement or was told that her child had only several hours to live. At the Conclusion of the trial, the jury found the defendant guilty of attempted first degree murder and aggravated battery. Following a sentencing hearing, the trial Judge sentenced the defendant to 35 years' imprisonment. The appellate court affirmed. The court rejected the defendant's argument that reversible error occurred when the trial Judge allowed the prosecution to present evidence about the defendant's agreement to undergo a polygraph exam. The appellate court believed that the evidence was admissible for the limited purpose of explaining the circumstances in which the defendant's confession was made. In the alternative, the appellate court believed that any error in the introduction of the polygraph testimony was harmless, given the overwhelming proof of the defendant's guilt. We allowed the defendant's petition for leave to appeal (166 Ill. 2d R. 315(a)).
The only issue presented by the defendant in this appeal is whether the trial Judge erred in allowing the State to introduce evidence about the defendant's agreement to undergo a polygraph examination. The defendant argues that the evidence was prejudicial and inadmissible because it violated the longstanding rule in Illinois barring the introduction of evidence regarding polygraph examinations. In response, the State maintains that the defendant, through her own testimony, opened the door to the presentation of this evidence. The State notes further that the defendant did not take the scheduled examination and that no polygraph results were introduced into evidence in this case.
The defendant correctly observes that the general rule in Illinois is to preclude introduction of evidence regarding polygraph examinations and the results of those tests. People v. Triplett, 37 Ill. 2d 234 (1967). In People v. Baynes, 88 Ill. 2d 225 (1981), this court found error in the introduction at trial of the results of a polygraph examination taken by the defendant; the error was held to be reversible even though the defendant, before the examination, had agreed with the prosecution that the test results could be admitted in evidence. In People v. Gard, 158 Ill. 2d 191 (1994), this court found plain error in the introduction, without objection by the defense, of testimony regarding the polygraph testing of a prosecution witness. The problems with polygraph evidence are twofold. First, polygraphy is not sufficiently reliably to establish guilt or innocence. Second, the quasi-scientific nature of the test may lead a trier of fact to give the evidence undue weight, notwithstanding its lack of reliability. People v. Taylor, 101 Ill. 2d 377, 391-92 (1984); see generally United States v. Scheffer, 523 U.S. __, __, 140 L. Ed. 2d 413, 419-22, 118 S. Ct. 1261, 1265-67 (1998). Our cases have also held that polygraph evidence is inadmissible in capital sentencing hearings. People v. Pecoraro, 175 Ill. 2d 294, 315 (1997); People v. Sanchez, 169 Ill. 2d 472, 493 (1996); People v. Szabo, 94 Ill. 2d 327, 362 (1983).
The rule of exclusion is not without exception, however. More than 30 years ago, in People v. Triplett, 37 Ill. 2d 234 (1967), this court, in an opinion by Justice Schaefer, suggested that polygraph evidence might be admissible to explain why a defendant made a confession. After reviewing Illinois statutes and case law barring the introduction of polygraph evidence, the court stated:
"Despite the strong aversion thus manifested, by statute and decisions, it can be argued that a different result should follow when the issue is the voluntariness of a confession. It can be said that the fact that the confession followed a polygraph examination is a relevant circumstance and that it is the fact of the examination, rather than its result, that is significant." Triplett, 37 Ill. 2d at 239.
The court concluded that a new trial was necessary in that case, however, because no limiting instruction had been given to the jury ...