DUFFLY, J.

A family dispute over ownership of what had been the family home in Woburn prompted Frances M. Sparrow2 to file a complaint in the Superior Court against her sister, Susan A. Demonico, and Susan's husband, David D. Demonico.3 Prior to trial, the parties resolved their differences by a settlement agreement reached during voluntary mediation. When Sparrow sought an order enforcing the agreement, a Superior Court judge denied her motion, concluding in essence that, due to mental impairment, Susan lacked the capacity to contract at the time of agreement. Sparrow appealed and, after initially vacating the order denying enforcement and remanding the case for findings of fact, Sparrow v. Demonico, 73 Mass.App.Ct. 1121 (2009), the Appeals Court reversed the judge's order and remanded the case for entry of an order enforcing the settlement agreement. Sparrow v. Demonico, 77 Mass.App.Ct. 1120 (2010).

We granted further appellate review to consider whether a party can establish that she lacked the capacity to contract, thus making the contract voidable by her, in the absence of evidence that she suffered from a medically diagnosed, long-standing mental illness or defect. We conclude that our evolving standard of contractual incapacity does not in all cases require proof that a party's claimed mental illness or defect was of some significant duration or that it is permanent, progressive, or degenerative; but, without medical evidence or expert testimony that the mental condition interfered with the party's understanding of the transaction, or her ability to act reasonably in relation to it, the evidence will not be sufficient to support a conclusion of incapacity. Because the evidence was insufficient to support a determination of incapacity in this case, we vacate the motion judge's order and remand for entry of an order enforcing the settlement agreement.

Background. Sparrow's complaint, filed initially in July, 2003, and later amended, alleged that Sparrow was entitled to a one-half interest in the Woburn property, consistent with the wishes of her (and Susan's) now-deceased mother, under theories of constructive and resulting trusts. Susan, who resided in the Woburn property at the time of the mediation, and David, who had been separated from Susan for several years and was no longer residing with her, asserted that they were the sole owners of the property, as reflected in a deed, and denied that Sparrow had any interest in it.4 Shortly before what was scheduled to be a final pretrial conference, the parties sought to achieve a settlement through voluntary mediation and the matter was removed from the trial list.

The parties and the attorneys who were representing them in the Superior Court proceeding participated in mediation on October 19, 2006. Sparrow contends that the case was settled during this mediation by an agreement that the Demonicos would sell the property and pay Sparrow $100,000 from the sale proceeds. When Sparrow sought an order enforcing the agreement, alleging that the Demonicos "reneged on their obligations under it," the Demonicos claimed that the agreement was unenforceable because Susan had, in their view, experienced a mental breakdown during the mediation and thus lacked the capacity to authorize settlement. At an evidentiary hearing on the motion, David and Susan were the only witnesses and no exhibits were admitted, although they were marked for identification.5 The motion judge denied Sparrow's motion on the basis that "the purported agreement may have been the product of an emotionally overwrought state of mind on the part of Susan Demonico."6 The case proceeded to trial by jury before a different judge, who, at the close of evidence, allowed the Demonicos's motion for a directed verdict on all counts. Sparrow appealed from the judgment and the denial of her motion to enforce the mediated settlement agreement.

The Appeals Court concluded, in an unpublished memorandum and order issued pursuant to its rule 1:28, that the motion judge's determination that Susan may have been emotionally overwrought was not grounds to avoid the contract, vacated the order denying the motion to enforce it, and remanded for "express findings of fact on the extent of Susan's impairment at the time that she authorized the settlement agreement." Sparrow v. Demonico, 73 Mass.App.Ct. 1121 (2009). On remand, the motion judge issued written findings and an order denying Sparrow's motion to enforce the settlement agreement based on his determination that Susan "was mentally incapacitated on the day of the mediation," and thus that "she was not able to understand in a reasonable manner the nature and consequences of what was happening and did not have an ability to comprehend the transaction or its significance and consequences."

Sparrow again appealed and a different panel of the Appeals Court, in another unpublished memorandum and order, reversed, concluding that although "the evidence supported a finding that Susan was extremely upset and mentally distressed during — and by — the mediation, . . . it does not support a finding that she was mentally incapacitated to the extent required by our cases." The court noted that decisions that have concluded contracts were void due to incapacity have done so only where medical evidence demonstrated a permanent, progressive, degenerative or long-term illness "that has been diagnosed by a mental health professional." Sparrow v. Demonico, 77 Mass.App.Ct. 1120 (2010).

Findings of fact. We summarize the motion judge's subsidiary findings of fact, which we accept as not clearly erroneous, see Kendall v. Selvaggio, 413 Mass. 619, 620 (1992) and Mass. R. Civ. P. 52(a), as amended, 423 Mass. 1402 (1996), and include additional details from evidence that the judge implicitly credited. See Commonwealth v. Isaiah I., 448 Mass. 334, 337 (2007), and cases cited. "[T]o ensure that the ultimate findings and conclusions are consistent with the law, we scrutinize without deference the legal standard which the judge applied to the facts." Kendall v. Selvaggio, supra at 621.

On the date of the scheduled mediation, Susan drove from her home to David's residence. From there, David drove them to the location of the mediation session because, in David's view, Susan was not capable of driving to the mediation. The mediation began at approximately 9 A.M. and ended at 3 P.M. The judge, crediting David's testimony, found:

The judge noted Susan's testimony that she had been taking a medication, Zoloft, prior to the mediation, but that she had stopped taking the medication at some point before the mediation, and that she cried much of the day; he specifically credited Susan's testimony that she "was out of control emotionally during the mediation" and found also that "she was not thinking rationally" on that day.7

As noted, both sides were represented by counsel throughout the mediation. At some point before they departed from the mediation session, the Demonicos authorized their attorney to execute a settlement agreement on their behalf.8 According to the terms of a written agreement titled, "Memorandum of Settlement," which was signed by Sparrow, her attorney, and the Demonicos's attorney, and witnessed by the mediator, the Demonicos agreed to pay Sparrow "the settlement amount of $100,000.00" from the proceeds of the sale of the property, which would occur "as soon as practicable," and in any event within a specified timeframe. The agreement also set forth other affirmative requirements regarding the marketing and sale of the property.

Discussion. A settlement agreement is a contract and its enforceability is determined by applying general contract law. Warner Ins. Co. v. Commissioner of Ins., 406 Mass. 354, 360 n. 7 (1990). It has been long established that a contract is voidable by a person who, due to mental illness or defect, lacked the capacity to contract at the time of entering into the agreement.9 See, e.g., Bucklin v. National Shawmut Bank, 355 Mass. 338, 341 (1969); Gibson v. Soper, 6 Gray 279 (1856). The burden is on the party seeking to void the contract to establish that the person was incapacitated at the time of the transaction. Meserve v. Jordan Marsh Co., 340 Mass. 660, 662 (1960). See Wright v. Wright, 139 Mass. 177, 182 (1885).

a. Standard for determining contractual incapacity. As Justice Holmes observed, it is a question of fact whether a person was competent to enter into a transaction — that is, whether the person suffered from "insanity" or "was of unsound mind, and incapable of understanding and deciding upon the terms of the contract." Id. at 182-183. In Reed v. Mattapan Deposit & Trust Co., 198 Mass. 306, 314 (1908), we described this inquiry as the "true test" of mental incapacity:

We applied this test, also known as the "cognitive test," see Ortelere v. Teachers' Retirement Bd. of the City of N.Y., 25 N.Y.2d 196, 202 (1969) (Ortelere), without significant modification for fifty years thereafter. See Meserve v. Jordan Marsh Co., supra (to avoid contract based on mental incapacity requires showing that person was "in such a state of insanity" at time of contracting as to render person "incapable of transacting the business"); Adams v. Whitmore, 245 Mass. 65, 68 (1923) (incapacity requires proof that person "was too weak in mind to execute the deed with understanding of its meaning, effect and consequences"); Sutcliffe v. Heatley, 232 Mass. 231, 232 (1919) (test is whether person "could not understand the nature and quality of the transaction or grasp its significance").

Over time, however, the traditional test for contractual incapacity, both in Massachusetts, see Krasner v. Berk, 366 Mass. 464, 467-468 (1974), and in other jurisdictions, see, e.g., Ortelere, supra, evolved to incorporate an increased understanding of the nature of mental illness in its various forms. See Matter of the Marriage of Davis, 193 Or.App. 279, 285-287 (2004) (Deits, C.J., concurring) (discussing evolution of incapacity standards); 5 R.A. Lord, Williston on Contracts § 10:8 at 341-343 (4th ed. 2009) (historically, little distinction made between different kinds of mental incompetency or illness; "[t]he law now recognizes a wide variety of types and degrees of mental incompetency and distinctions among the various types of mental illness"). Based on this understanding, we adopted a second, alternative test for incapacity.

In Krasner v. Berk, supra, we recognized that there may be circumstances when, although a party claiming incapacity has some, or sufficient, understanding of the nature and consequences of the transaction, the contract would still be voidable where, "by reason of mental illness or defect, [the person] is unable to act in a reasonable manner in relation to the transaction and the other party has reason to know of his condition."10 Id. at 468, citing Ortelere, supra at 204-205.11 This modern test — also described as an "affective" or "volitional" test — recognizes that competence can be lost, not only through cognitive disorders, but through affective disorders that encompass motivation or exercise of will. See Ortelere, supra at 204-205, quoting Note, Mental Illness and the Law of Contracts, 57 Mich. L.R. 1020, 1036 (1959) (recommending "that a complete test for contractual incapacity should provide protection to those persons whose contracts are merely uncontrolled reactions to their mental illness" as well to those who could not understand nature and consequences of their actions). See also Gore v. Gadd, 268 Or. 527, 528-529 (1974) (under affective test, person such as one who is manic-depressive psychotic, although aware of nature and consequences of conduct, may still be considered incompetent because mental illness "impel[s person] to act irrationally" and such person is "incapable of making a rational judgment in the execution of the transaction").

Under this modern, affective test, "[w]here a person has some understanding of a particular transaction which is affected by mental illness or defect, the controlling consideration is whether the transaction in its result is one which a reasonably competent person might have made." Krasner v. Berk, supra at 469, quoting Restatement (Second) of Contracts § 18C comment b (Tent. Drafts Nos. 1-7, 1973). See Green, Proof of Mental Incompetency and the Unexpressed Major Premise, 53 Yale L.J. 271, 307 (1944). Also relevant to the inquiry in these circumstances is whether the party claiming mental incapacity was represented by independent, competent counsel. See Willett v. Webster, 337 Mass. 98, 103 (1958). Contrast Farnum v. Silvano, 27 Mass.App.Ct. 536, 537, 540 (1989) (plaintiff's mental competence had begun to fail several years before she delivered deed to defendant; she was represented in transfer of real estate by lawyer selected and paid for by defendant, whose mission "was to effect the transaction," rather than to protect plaintiff's interests).12

b. Evidence of contractual incapacity. We begin by observing that the evidence required to support a finding of incapacity to contract, whether considered under the traditional or modern standard, need not in all cases demonstrate that a party suffers from a mental illness or defect that is permanent, degenerative, progressive, or of significant duration. Although such incapacity has historically been established by evidence of a long-standing mental illness, nothing in our jurisprudence requires such evidence. The inquiry as to the capacity to contract focuses on a party's understanding or conduct only at the time of the disputed transaction. See Krasner v. Berk, supra at 468; Meserve v. Jordan Marsh Co., 340 Mass. 660, 662 (1960). Based on the evolving understanding of mental illness, we do not preclude the possibility that a party could establish an incapacity to contract without proof of a mental condition that is permanent, degenerative, progressive, or long standing.

The Demonicos contend that this is such a case; that the evidence established Susan's incapacity without showing a permanent, degenerative, progressive, or long-standing mental illness. They point to evidence that Susan's asserted mental impairment arose and was limited to the period of the mediation session, and argue that this evidence was sufficient to support a conclusion of incapacity, despite the lack of medical evidence or expert testimony as to the nature of Susan's mental impairment and its effect on her decision-making ability.

We have not previously addressed whether medical evidence is required to establish an incapacity to contract, and the Demonicos have not directed our attention to case law in other jurisdictions that would support their contention. In our prior decisions concerning the issue of incapacity to contract, however, evidence of mental illness or defect has been presented consistently through medical evidence, including the testimony of physicians and mental health providers or experts, in addition to lay testimony.13 Moreover, in other contexts, we have held that a lay witness is not competent to give an opinion as to mental condition. See Commonwealth v. McDermott, 393 Mass. 451, 454 (1984). Cf. Commonwealth v. Schulze, 389 Mass. 735, 738-740 (1983). Expanding on this analysis, we conclude that medical evidence is necessary to establish that a person lacked the capacity to contract due to the existence of a mental condition.

In other contexts, the reliance on medical and expert evidence is routine when addressing issues of mental illness, capacity, and competence. See, e.g., Commonwealth v. DiPadova, 460 Mass. 424, 426-428 (2011) (criminal responsibility); O'Rourke v. Hunter, 446 Mass. 814, 822 (2006) (testamentary capacity). See also Vitek v. Jones, 445 U.S. 480, 495 (1980), quoting Addington v. Texas, 441 U.S. 418, 429 (1979) (question of mental illness in civil commitment context is "essentially medical," which "turns on the meaning of facts which must be interpreted by expert psychiatrists and psychologists").

Where the issue is the capacity to contract, we have looked to medical providers or experts to explain whether, and to what extent, a person's mental condition has affected the ability to understand the nature of the transaction and its consequences, or to explain why, despite the intellectual and cognitive ability to understand, the person is unable to act reasonably in making the decision. See, e.g., Krasner v. Berk, supra at 466; Meserve v. Jordan Marsh Co., supra at 663-664. See also T. Grisso & P.S. Appelbaum, Assessing Competence to Consent to Treatment 10, 20-23 (1998) (legal standards for competence, including competence to contract, "focus on certain functional abilities on which the law relies to structure its thinking about competence, based on information that clinicians and others can provide about patients' functional deficits or strengths in decision making").

The inquiry in this area is in contrast to other areas of the law, such as tort claims of emotional distress or mental anguish, where the inquiry is whether the evidence supports a plaintiff's claim to have experienced emotional distress or mental anguish, rather than on the impact of that state on the ability to comprehend. See Sullivan v. Boston Gas Co., 414 Mass. 129, 138 (1993). Yet even in the context of an action sounding in tort, we have often considered medical corroboration to be a highly probative, if not essential, component of a plaintiff's case. See id. at 137-138; Payton v. Abbott Labs, 386 Mass. 540, 556 (1982). See also Bresnahan v. McAuliffe, 47 Mass.App.Ct. 278, 285 (1999) (more difficult, but not impossible, for plaintiff to meet burden in absence of medical evidence). Such corroboration not only guards against feigned or fraudulent claims of mental distress, but also alleviates the concern "that even honest plaintiffs erroneously might convince themselves that they suffer from emotional distress . . . thereby compounding the problem of fraudulent lawsuits." Sullivan v. Boston Gas Co., supra at 133, citing Payton v. Abbott Labs, supra at 547. These concerns apply with at least equal force where the question is the capacity to contract due to mental impairment. See Ortelere, supra at 206 ("nothing less serious than medically classified psychosis should suffice or else few contracts would be invulnerable to some kind of psychological attack").

Here, there was lay evidence, credited by the judge, that Susan's speech was "slurr[ed]," that she was in a state of uncontrollable crying, and that she had experienced an inability to focus or "think rationally" throughout the day of the mediation. Susan testified also that she had recently discontinued taking the prescribed medication Zoloft. However, she presented no medical evidence regarding a diagnosis that would have required her to take the medication, or the effect, if any, that ceasing to take the medication would have had on her medical or mental condition. There was also no evidence that Susan's condition at the mediation was related to or caused by her discontinuing the medication.14

"A non-expert is competent to testify to the physical appearance and condition and acts of a person both for their probative value for the jury and for the purpose of furnishing facts as the basis of hypothetical questions for experts." Cox v. United States, 103 F.2d 133, 135 (7th Cir.1939). See Brown v. United States Fid. & Guar. Co., 336 Mass. 609, 614 (1958) (wife's testimony about nervous condition of husband prior to his death, although admissible as matter of common observation by lay person, not sufficient to prove whether disease was proximate or contributing cause of death). However, there was no expert or medical testimony to explain the effect of Susan's experiences or behavior on her ability to understand the agreement, to appreciate what was happening, or to comprehend the reasonableness of the settlement terms or the consequences to her of authorizing the settlement. Without such medical evidence, there was no basis to conclude that Susan lacked the capacity to contract. See Farnum v. Silvano, supra at 539-540 (plaintiff, who "suffered mental disease which had manifested itself in erratic and irrational conduct and was confirmed by diagnostic test," did not possess "requisite contextual understanding" of transaction). Contrast Grindlinger v. Grindlinger, 10 Mass.App.Ct. 823, 823 (1980) ("fatigue and anxiety" not sufficient to invalidate separation agreement).

The evidence did not support a conclusion that, under the traditional test for incapacity, Susan was incapable of understanding the nature and quality of the transaction, or of grasping its significance. Indeed, based on Susan's testimony, she understood at the time that she was participating in a mediation to discuss settlement of the lawsuit; she was aware that the subject of the mediation was to resolve the dispute regarding the family home in Woburn; she participated in the mediation and listened to the arguments of counsel; and she "couldn't believe how things [were] turning out."

It is apparent from Susan's testimony that, even if she suffered from a transient mental defect, or "breakdown" as the judge concluded, she had at least some understanding of the nature of the transaction and was aware of its consequences. Under the modern test to establish Susan's incapacity, the evidence was similarly insufficient. There was no evidence that the settlement agreement was unreasonable, or that a reasonably competent person would not have entered into it.15

Conclusion. Because the evidence does not support a conclusion that Susan lacked the mental capacity to authorize settlement on the day of the mediation, it was error to deny Sparrow's motion to enforce the agreement. The order denying the motion to enforce the mediated settlement agreement is vacated. The case is remanded to the Superior Court for entry of an order that the settlement agreement be enforced.

So ordered.