OPINION
MACY, Justice.Appellant Arlen Joe Price appeals from his conviction for first-degree murder.
We affirm.
Appellant presents the following issues for our review:
I. Because the jury instructions and the form of the verdict gave the jury the choice between two theories of the first degree murder — premeditated or felony murder — and it is impossible to determine upon which theory the jury based its verdict, and because the court determined at the end of the defense’s case that there was insufficient evidence of aggravated robbery as a matter of law, Appellant is entitled to a new trial.
II. Because as a matter of law there was insufficient evidence of intent to commit larceny, the Appellant is entitled to a reversal and a remand for the purposes of retrial on the premeditated theory, without the possibility of felony murder.
III. The trial court erred when it conditioned the admiss[i]bility of the testimony of defendant’s expert upon the laying of a proper foundation i.e. when the court required the testimony of the defendant before permitting testimony of the expert.
IV. Expert testimony from Dr. Brian Miracle should have been allowed as to (a) whether he thought the Appellant did the actual stabbing of the victim and (b) why the defendant would come into court and say that he had committed the murder he had been accused of, when he had not.
During the evening of February 25,1989, Appellant and a man referred to as “Chief” went to the home of Appellant’s uncle, Lewis “Sonny” Price, Jr., near Jackson, Wyoming. Appellant was carrying a handgun, and Chief was carrying a knife. After Sonny invited Appellant and Chief into his trailer, Appellant brandished the gun and told Sonny that he was going to “make him pay.” Sonny and Appellant began to wrestle, and the gun discharged a bullet which did not hit anyone. Chief went to Appellant’s aid, jumped on Sonny, and stabbed him in the side. Appellant got up off the floor, checked to see if anyone in the neighborhood was aware of what was taking place, grabbed the knife, and stabbed Sonny to death.
The two men left the trailer and went to Appellant’s mother’s house to change out of their bloody clothes. Appellant told his mother that he killed Sonny, and then he and Chief departed for Salt Lake City, Utah. After they spent time in Salt Lake City trying to borrow money, Appellant and Chief went to Nevada. They eventually arrived in Las Vegas and began to gamble. On March 4, 1989, Chief took Appellant’s car without Appellant’s knowledge and drove to Lake Mead National Recreation Area where park rangers shot and killed him when he threatened them with a *911gun. The police apprehended and arrested Appellant in a casino after they received a telephone call from an individual whom Appellant told about the killing.
After he was taken into custody, Appellant told police officers that he went to Sonny’s place to kill him, but he stated that he did not attempt to rob Sonny. He said that he killed Sonny to pay him back for the pain he had caused Appellant’s family, especially his younger brother, Timmy. Appellant and Timmy claimed that Sonny had raped them, and Timmy claimed that Sonny forced him to perform bestialities.
Appellant was charged with first-degree murder in violation of Wyo.Stat. § 6-2-101 (1988), amended by 1989 Wyo.Sess.Laws ch. 171, § 1 (effective March 6, 1989),1 and aggravated robbery in violation of Wyo. Stat. § 6-2-401 (1988).2 On May 9, 1989, a jury found Appellant guilty of first-degree murder, and the trial court subsequently sentenced him to serve a life term in the Wyoming State Penitentiary.
Although Appellant raises four issues for our consideration, we consolidate them into the following two questions which encompass Appellant’s arguments: (1) Is Appellant’s conviction for first-degree murder supported by sufficient evidence of felony murder; and (2) did the trial court err by limiting the scope of testimony given by a psychologist who testified on behalf of Appellant?
Sufficiency of the Evidence
Appellant’s first two issues challenge the sufficiency of the evidence which supports Appellant’s first-degree murder conviction. At the trial, the State sought Appellant’s conviction for first-degree murder on the basis of premeditation or, alternatively, for felony murder with robbery as the underlying felony and for aggravated robbery. Appellant moved for a judgment of acquittal on both charges pursuant to W.R.Cr.P. 30. The trial court refused to send the issue of aggravated robbery to the jury but determined that the jury should decide whether Appellant committed or attempted to commit robbery. The jury found Appellant guilty of first-degree murder, but the verdict form did not state whether the basis for the conviction was premeditation or felony murder with rob-' bery as the underlying felony.3 Appellant now asserts that the record does not contain sufficient evidence to support a conviction for robbery or attempted robbery and that:
When a count is brought on two separate theories, and there is insufficient evidence on one of the theories, if it is impossible to determine upon which theory the conviction rests, the verdict is to be set aside, and a new trial granted. Yates v. U.S., 354 U.S. 298, 77, S.Ct. 1064, 1 L.Ed.2d 1356 (U.S.Cal. June 17, 1957); Stromberg v. California, 283 U.S. 359, 51 S.Ct. 532, 75 L.Ed. 1117 (U.S.Cal., May 18, 1931); Cloman v. State, 574 P.2d 410 (Wyo., 1978).
This Court previously dealt with a similar fact situation in Cloman v. State, 574 P.2d 410 (Wyo.1978). In that case, the appellant was convicted of first-degree murder, but the ambiguous verdict form and jury instructions produced the following question: “Did the jury find evidence of premeditated murder or felony-murder in the commission of a robbery, or both?” *912Id. at 412. We upheld the conviction for first-degree murder because the evidence “justified the holding that the jury found both premeditated murder and felony-murder in the commission of a robbery.” Id. See United States v. Natelli, 527 F.2d 311 (2d Cir.1975), cert. denied 425 U.S. 934, 96 S.Ct. 1663, 48 L.Ed.2d 175 (1976).4 If each alternative ground for a defendant’s first-degree murder conviction is supported by substantial evidence, we will not set aside the conviction solely because we are unable to determine which ground served as the basis for the jury’s decision.
Appellant does not dispute the sufficiency of the evidence demonstrating that he committed a premeditated murder, so we must determine if the record reveals sufficient evidence which indicates that he killed Sonny in the perpetration of a robbery or an attempted robbery. Section 6-2-401(a) provides:
(a) A person is guilty of robbery if in the course of committing a crime defined by W.S. 6-3-402 he:
(i) Inflicts bodily injury upon another; or
(ii) Threatens another with or intentionally puts him in fear of immediate bodily injury.
Wyo.Stat. § 6-3-402(a) (1988) is a subsection of our larceny statute, and it states: “A person who steals, takes and carries, leads or drives away property,of another with intent to deprive the owner or lawful possessor is guilty of larceny.” Wyoming’s attempt statute provides in pertinent part:
(a) A person is guilty of an attempt to commit a crime if:
(i) With the intent to commit the crime, he does any act which is a substantial step towards commission of the crime. A “substantial step” is conduct which is strongly corroborative of the firmness of the person’s intention to complete the commission of the crime * * *[.]
Wyo.Stat. § 6-l-301(a)(i) (1988). To ascertain if sufficient evidence of robbery or attempted robbery exists,
[w]e examine all the evidence in the light most favorable to the State * * *.
“[I]t is not whether the evidence establishes guilt beyond a reasonable doubt for us, but rather whether it is sufficient to form the basis for a reasonable inference of guilt beyond a reasonable doubt to be drawn by the jury when the evidence is viewed in the light most favorable to the State.
* * * * * *
“It is not our function to weigh the evidence for a determination as to whether or not it is sufficient to establish guilt beyond a reasonable doubt. We have consistently held that even though it is possible to draw other inferences from the evidence presented, it is the responsibility of the jury to resolve conflicts in the evidence.”
Mendicoa v. State, 771 P.2d 1240, 1243 (Wyo.1989) (quoting Broom v. State, 695 P.2d 640, 642 (Wyo.1985) (citations omitted)).5
At the trial, Appellant’s mother testified about the conversation she had with Appellant on the night of the killing. According to her, Appellant told her that he pulled out his gun and that
he asked for Sonny to open the safe to give him some money, and Sonny told him he couldn’t open the safe for him. And then he got down on his knees and grabbed ahold of Arlen and told him, begged him not to do this to him. Told him to just go away; that he would for*913get about it. And then they got to wrestling and the gun went off, but he said he didn’t know if he hit Sonny or not.
That testimony, in addition to Appellant’s statements that he stabbed Sonny at least four times and the fact that Sonny died from knife wounds, viewed in the light most favorable to the State, is sufficient evidence to uphold Appellant’s conviction for killing a human being during the attempted perpetration of a robbery (felony murder) under § 6-2-101.
The Admissibility of Psychological Testimony
Appellant’s final two issues challenge the district court’s evidentiary rulings which restricted the testimony of a psychologist who testified on Appellant’s behalf. At the trial, Appellant claimed that he lacked the mental capacity to form the requisite specific intent necessary to maintain a first-degree murder conviction. To prove his contention, Appellant informed the district court that he intended to call a psychologist who would testify about Appellant’s mental condition. The court stated that Appellant would have to lay a foundation for the psychologist’s testimony by testifying himself because Appellant consulted the psychologist only for purposes of litigation, because Appellant had not pleaded not guilty by reason of insanity, and because the psychologist was not a witness to the crime. After the court informed Appellant of his right not to testify, Appellant took the stand and admitted killing Sonny.
Appellant contends that the district court forced him to give up his right not to testify by exclusively requiring his testimony as a foundation for the psychologist’s testimony. We have stated many times that making the decision to allow an expert to testify is within the sound discretion of the trial court. Braley v. State, 741 P.2d 1061 (Wyo.1987); Buhrle v. State, 627 P.2d 1374 (Wyo.1981). The district court erred, however, when, before the psychologist took the stand, it determined that Appellant’s testimony was necessary to establish an adequate foundation for the psychologist’s testimony. Regardless of whether the district court based its directive on the relevancy requirement of W.R.E. 402,6 the standards for expert testimony specified in W.R.E. 702 and 703,7 or the hearsay rule, W.R.E. 802,8 a trial judge may not rule prospectively on a party’s ability to establish a proper foundation for expert testimony-
We hold that the district court’s error was harmless, however, because it did not prejudice Appellant’s substantial rights. An error warrants reversal if it is prejudicial and if it affects an appellant’s substantial rights. Otherwise, the error is harmless. Loomer v. State, 768 P.2d 1042 (Wyo.1989). The district court’s establishment of a foundation prerequisite was a harmless error for two reasons. First, because Appellant’s theory of defense was that his mental condition prevented him from forming the requisite specific intent for premeditated murder and not that he did not kill Sonny, his in-court admission that he killed Sonny did not incriminate him more than he had already been incriminated.
*914Second, the district court did not force Appellant to testify about anything other than the information the district court required as a foundation for the psychologist’s testimony. Appellant could have limited the scope of the State’s cross-examination. Once Appellant began testifying about Sonny’s death, he opened the door for the State’s inquiry into that matter. W.R.E. 611(b) provides:
(b) Scope of cross-examination.— Cross-examination should be limited to the subject matter of the direct examination and matters affecting the credibility of the witness. The court may, in the exercise of discretion, permit inquiry into additional matters as if on direct examination.[9]
Several federal courts have determined that, when a defendant takes the witness stand, the prosecution’s cross-examination is limited to matters reasonably related to the subject matter of direct examination. Aldridge v. Marshall, 765 F.2d 63 (6th Cir.1985), cert. denied 474 U.S. 1062, 106 S.Ct. 810, 88 L.Ed.2d 785 (1986); United States v. Hernandez, 646 F.2d 970 (5th Cir.), cert. denied 454 U.S. 1082, 102 S.Ct. 638, 70 L.Ed.2d 617 (1981); United States v. Panza, 612 F.2d 432 (9th Cir.), cert. denied 447 U.S. 925, 100 S.Ct. 3019, 65 L.Ed.2d 1118 and 447 U.S. 926, 100 S.Ct. 3020, 65 L.Ed.2d 1118 (1980). As a result, a defendant may testify in an effort to lay the foundation for the testimony of another witness without opening the door for the State to ask questions on cross-examination about the crime which the defendant is accused of committing. “The witness himself, certainly if he is a party, determines the area of disclosure and therefore of inquiry.” Brown v. United States, 356 U.S. 148, 155, 78 S.Ct. 622, 627, 2 L.Ed.2d 589, 72 A.L.R.2d 818 (1958). See also McGautha v. California, 402 U.S. 183, 91 S.Ct. 1454, 28 L.Ed.2d 711 (1971); and Harrison v. United States, 392 U.S. 219, 88 S.Ct. 2008, 20 L.Ed.2d 1047 (1968).
Appellant also contends that the district court erred when it prevented the psychologist from testifying about whether Appellant actually killed Sonny and about Appellant’s motivation for claiming that he killed Sonny. At the trial, the following colloquy occurred between Appellant’s attorney and the psychologist:
Q. Do you have .an opinion as to whether or not Arlen Price was able to control his behavior in so far as it related to Sonny Price?
A. Control his behavior. I think that he was so obsessed and that he was compelled to do what he did, and I think that had he not done it some two or three—
MR. ROGERS: Objection. He’s going beyond the scope of the question.
THE COURT: True. Sustained.
Q. Did you talk to him yesterday about whether suppose, for instance, Sonny Price was in this courtroom today and with the police all around what would he have to do? Did you talk to him about that?
A. Yes.
Q. And what did he tell you?
A. That he would kill him right here.
Q. And is that — what does that have to do with your diagnosis here?
A. It’s a continuation of this obsession. Total obsession takes up a lot of his daily time, and state of mind of what has been inflicted upon him now. He wants to handle it.
Q. Now, you’ve spent a lot of time in this ease examining the evidence, haven’t you?
A. Yes.
Q. And I guess you’ve already testified that you’ve spent a great deal of time talking to the witnesses including Arlen Price; isn’t that right?
A. Yes.
Q. Is there any doubt in your mind as to whether or not Arlen killed Sonny Price?
A. None whatsoever, as far as his being there. I have talked with him at great length. I have some real reservations whether or not he actually did the crime.
*915Q. Why is that?
A. Because—
MR. ROGERS: Objection, Your Honor. Invades the province of the jury.
THE COURT: Sustained.
In the discussion which followed, the district court reiterated the rule that a diminished capacity defense does not exist in Wyoming. The court stated that the psychologist could not “testify about the mental state or condition constituting an element of the crime or a defense; that the ultimate issue in that instance is left for the trier of fact.” The court also expressed concern about allowing the psychologist to testify that Appellant did not kill Sonny and that Appellant had a motive for lying about the matter because such testimony would be contrary to Appellant’s testimony.
We first note that the district court was correct when it stated that Wyoming has not recognized a diminished capacity defense.10 In Dean v. State, 668 P.2d 639, 645 (Wyo.1983), this Court affirmed the district court’s décision to reject diminished capacity and irresistible impulse instructions because
the legislature has set forth the standard [in Wyo.Stat. §§ 7-11-301 to -304 (1987) [11] relative to the mental condition which will constitute a defense to a criminal charge. Such standard should not be increased or decreased. ,
That rule applies with equal force to Appellant’s claim that his mental condition (obsessive/compulsive) prevented him from forming the requisite specific intent necessary to sustain a first-degree murder conviction.
Appellant contends that the district court erred by preventing the psychologist from testifying as to whether he thought Appellant actually killed Sonny because that testimony embraced the ultimate issue. We have rejected the rule that an expert cannot testify on the ultimate issue in a case. Reed v. Hunter, 663 P.2d 513 (Wyo.1983). W.R.E. 704 states: “Testimony in the form of an opinion or inference otherwise admissible is not objectionable because it embraces an ultimate issue to be decided by the trier of fact.” W.R.E. 702 mandates that the testimony “assist the trier of fact.” 12
We hold, however, that the district court did not abuse its discretion when it prevení-*916ed the psychologist from testifying about whether Appellant killed Sonny. Not only would the testimony have failed to assist the jury with its decision, but that testimony would have challenged the veracity of Appellant’s testimony.
The psychologist attempted to say that the evidence he reviewed led him to doubt that Appellant actually killed Sonny, despite Appellant’s testimony to the contrary. The combination of his understanding of the facts of the crime with his medical opinion that Appellant would be compelled to take credit for the killing led the psychologist to doubt the veracity of Appellant’s testimony. In Smith v. State, 564 P.2d 1194, 1200 (Wyo.1977), we held that an expert may not testify about the veracity of a defendant’s version because “it assumes the function of the jury.” See also Lessard v. State, 719 P.2d 227 (Wyo.1986). In addition, the psychologist’s testimony indicating he doubted that Appellant killed Sonny did not assist the jury. While the psychologist may have been an expert for the purposes of determining an individual’s mental condition, he was in no better position than the jury to decide whether Appellant actually stabbed Sonny to death.
Affirmed.
CARDINE, J., files a specially concurring opinion. URBIGKIT, C.J., files a dissenting opinion.. At the time of the murder, § 6-2-101 provided:
(a) Whoever purposely and with premeditated malice, or in the perpetration of, or attempt to perpetrate, any sexual assault, arson, robbery, burglary, escape, resisting arrest or kidnapping, or by administering poison or causing the same to be done, kills any human being is guilty of murder in the first degree.
(b) A person convicted of murder in the first degree shall be punished by death or life imprisonment according to law.
. Section 6-2-401(c) provides:
(c)Aggravated robbery is a felony punishable by imprisonment for not less than five (5) years nor more than twenty-five (25) years if in the course of committing the crime of robbery the person:
(i) Intentionally inflicts or attempts to inflict serious bodily injury; or
(ii) Uses or exhibits a deadly weapon or a simulated deadly weapon.
. The verdict form stated in part: "We, the jury, find the defendant, X Guilty_ Not Guilty of murder in the first degree.”
. We quoted the rule from Yates v. United States, 354 U.S. 298, 312, 77 S.Ct. 1064, 1073, 1 L.Ed.2d 1356 (1957), which states that the " 'proper rule to be applied is that which requires a verdict to be set aside in cases where the verdict is supportable on one ground, but not on another, and it is impossible to tell which ground the jury selected.’ ” Cloman, 574 P.2d at 412.
. Appellant’s second issue challenges the district court's decision to deny his motion for acquittal under W.R.Cr.P. 30. When reviewing a district court’s decision to deny a motion for acquittal, we apply the standard quoted in Mendicoa and determine whether sufficient evidence exists to sustain the charges. Chavez v. State, 601 P.2d 166 (Wyo.1979); Cloman, 574 P.2d 410.
. W.R.E. 402 states:
All relevant evidence is admissible, except as otherwise provided by statute, by these rules, or by other rules prescribed by the Supreme Court. Evidence which is not relevant is not admissible.
. W.R.E. 702 provides:
If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.
W.R.E. 703 states:
The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to him at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence.
.W.R.E. 802 provides:
Hearsay is not admissible except as provided by these rules or by other rules adopted by the Supreme Court of Wyoming or by statute.
. W.R.E. 611(b) is identical to Fed.R.Evid. 611(b).
. We have recognized a limited exception for the defense of automatism. Fulcher v. State, 633 P.2d 142 (Wyo.1981). See also Polston v. State, 685 P.2d 1 (Wyo.1984).
. Section 7 — 11—301(a)(iii) provides:
(a)As used in this act:
******
(iii) “Mental deficiency” means a defect attributable to mental retardation, brain damage and learning disabilities!.]
Section 7-11-302 provides:
(a) No person shall be tried, sentenced or punished for the commission of an offense while, as a result of mental illness or deficiency, he lacks the capacity, to:
(i) Comprehend his position;
(ii) Understand the nature and object of the proceedings against him;
(iii) Conduct his defense in a rational manner; and
(iv) Cooperate with his counsel to the end that any available defense may be interposed.
Section 7-11-304 provides in pertinent part:
(a) A person is not responsible for criminal conduct if at the time of the criminal conduct, as a result of mental illness or deficiency, he lacked substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of law. As used in this section, the terms mental illness or deficiency mean only those severely abnormal mental conditions that grossly and demonstrably impair a person's perception or understanding of reality and that are not attributable primarily to self-induced intoxication as defined by W.S. 6-l-202(b).
(b) As used in this section, the terms "mental illness or deficiency” do not include an abnormality manifested only by repeated criminal or otherwise antisocial conduct.
(c) Evidence that a person is not responsible for criminal conduct by reason of mental illness or deficiency is not admissible at the trial of the defendant unless a plea of “not guilty by reason of mental illness or deficiency" is made. A plea of “not guilty by reason of mental illness or deficiency" may be pleaded orally or in writing by the defendant or his counsel at the time of his arraignment. The court, for good cause shown, may also allow that plea to be entered at a later time. Such a plea does not deprive the defendant of other defenses.
. See supra note 7.