While the issue is apparently one of first impression in many jurisdictions, a handful of courts nationally have addressed the relevancy and admissibility of evidence of pre- or post-will-execution mental capacity—normally deemed irrelevant to will-execution mental capacity—where it has been shown that the testator suffered from a permanent mental deficiency. Importantly, as observed by the U.S. Supreme Court, where evidence is developed of permanent or continuing mental incapacity, the burden properly shifts to the will proponent to prove a lucid interval, rather than the normal burden upon the contestant to prove incapacity, as continued mental incapacity is legally presumed:
In addition to the proof . . . of his undoubted insanity prior [to] and for some time subsequent to [the will execution], there was slight evidence of insane acts during the month of February, though there was no opinion expressed by anyone that he was incapable of making a valid deed or contract. The whole testimony regarding his insanity was duly submitted to the jury, who were instructed that if they found his insanity to be permanent in its nature and character, the presumptions were that it would continue, and the burden was upon the defendant to satisfy the jury by a preponderance of testimony that he was, at the time of executing the will, of sound mind. There was no error in this instruction.
Keely v. Moore, 196 U.S. 38, 46-47 (1904) (emphasis added). This legal presumption is also expressed in major secondary authority regarding testamentary capacity, citing Keely and a number of similar cases nationally:
Insanity of a confirmed or permanent nature, shown to have afflicted the testator at one time, is presumed to have continued to the subsequent time of the execution of the will. This presumption is limited to insanity of a continuing and permanent, or fixed and habitual, nature, as distinguished from temporary mental weakness, and may be rebutted by evidence that the incapacity did not exist when the will was executed. Evidence of the testator's condition before the execution of the will will not support an inference of mental incapacity at the time of the execution of the will in the absence of evidence of the permanence or continuity of the condition.
79 Am. Jur. 2d Wills § 92 (Westlaw May 2016 Update) (footnotes omitted). Thus, courts have held nationally that evidence of a testator's mental capacity during periods remote from the time of will execution is both admissible and relevant to the issue of testamentary capacity, so long as there is a prior showing that the testator's mental condition was of a permanent or ongoing nature.
In view of the testimony, mainly of Dr. Woods, as to the permanent and progressive character of testator's mental disability, the hospital records are not too remote in point of time from the execution of the will.
Ipsen v. Ruess, 35 N.W.2d 82, 88 (Iowa 1948) (admitting as relevant to testamentary capacity hospital records of the decedent's mental capacity for periods of a year before and four years after will executions because the testator's mental condition was of a permanent nature). Thus, a two-part rule allowing will contestants to overcome the legal presumption of testator capacity is expressed very clearly in a recent Alabama decision:
[I]t is presumed that every person who has attained the age of majority has the capacity to make a valid will, and the person challenging the will bears the burden of proving a lack of testamentary capacity. This may be shown either by  evidence of "a permanent type of insanity prior to the execution of the will" . . . or  evidence indicating that the testatrix "did not have testamentary capacity at the time she executed the will."
Horton v. Raspberry, 852 So. 2d 155, 157 (Ala. Civ. App. 2002) (quoting Fletcher v. DeLoach, 360 So. 2d 316, 318 (Ala. 1978)). Maryland case law has also set forth a clear rendering of this rule regarding the shifting burden of proof regarding testamentary capacity upon a contestant's showing of a permanent or continuing mental impairment:
The presumption of the law being in favor of sanity and testamentary capacity, the evidence to support a caveat on the issue of insanity must ordinarily tend to show either that  the testator was of unsound mind at the time of the execution of the will, or that  he was affected with permanent insanity prior to its execution, in which latter case the burden of proof of capacity is shifted to the caveatee.
Acker v. Acker, 192 A. 327, 331 (Md. 1937).
An Illinois judicial opinion confirms that upon evidence of a permanent or progressive mental incapacity, evidence of the testator's mental condition before or after supposed will execution is wholly relevant and admissible to the question of testamentary capacity:
[P]roof of the mental condition of the testator a reasonable time before or after the making of a will may be received where it tends to show mental condition at the time of the execution of the instrument. The mental condition shown to exist is presumed to continue if it be of a continuing nature.
Cent. Bank-Granite City v. Ziaee, 544 N.E.2d 1121, 1130 (Ill. App. Ct. 1989) (quoting Peters v. Peters, 33 N.E.2d 425, 427 (Ill. 1941)).
Given the universe of circumstances that can impact when and how the testator allegedly executed a will, it is improper to exclude pretrial potential evidence to be developed at trial, as evidentiary relevancy and admissibility should be decided on a case-by-case basis during the trial.
Thus, where evidence exists of continuing or permanent mental incapacity, will contestants should raise this issue, as such evidence of the testator's mental incapacity during periods even years prior to will execution may overcome the legal presumption that the testator possessed the capacity to make a will.