Liacos, C.J.
In 1984, at the age of fourteen, the defendant, Jack Matthew Rosenberg, was placed in the custody of the
Department of Youth Services (department), after he pleaded guilty to delinquency by reason of murder and kidnapping.
As the defendant approached his eighteenth birthday, the department issued an order extending his commitment. Following a bench trial and a jury-waived trial de novo, the order was confirmed. In 1988, the department issued another order extending commitment. See G. L. c. 120, § 19 (1988 ed.) (requiring new order within two years). The order was confirmed after a bench trial in the West Roxbury Division of the District Court Department, the original committing court. The defendant sought a jury trial de novo in the appellate session of the Juvenile Court Department. See G. L. c. 120, § 20 (1988 ed.). A jury found beyond a reasonable doubt that the defendant was dangerous by reason of mental illness, and confirmed the department’s order extending commitment of the defendant. A judge of the Juvenile Court confirmed the order in accordance with the jury verdict. This appeal by the defendant challenges his extended commitment to the custody of the department pursuant to G. L. c. 120, §§ 16-20 (1988 ed.).
We allowed the defendant’s application for direct appellate review. We affirm.
The facts adduced at the jury trial are as follows. The Commonwealth presented the testimony of four witnesses duly qualified as experts. Dr. Carol N. Maurer, a supervising psychiatrist at a Pennsylvania treatment center where the department placed the defendant, diagnosed the defendant as being mentally ill by reason of a bipolar affective disorder in remission.
Dr. Maurer also stated in her diagnosis that the defendant suffered from a sexual identity disorder, pedo
philia, and a narcissistic personality disorder. In her opinion, neither pedophilia nor a narcissistic personality disorder was a mental illness, but, she testified, the latter could intensify a bipolar affective disorder. Based on her diagnosis, Dr. Maurer stated her opinion that the defendant was dangerous by reason of mental illness and was “capable, when frustrated, of carrying out the same act [the murder of a child] again.”* *
The Commonwealth’s second witness, Dr. Albert J. Scott, who holds a doctoral degree in education, evaluated the defendant on one occasion at the treatment center. He diagnosed the defendant as suffering from a bipolar affective disorder and a borderline personality disorder with antisocial tendencies. Dr. Scott classified both disorders as mental illnesses and considered the defendant dangerous to himself and others as a result of these disorders.
Dr. Craig Latham, a forensic psychologist with the Department of Mental Health, had examined the defendant on one occasion. He testified that the defendant was a danger to the public, that he “had not received any treatment” for his “sex offender” behavior, and, according to staff reports, had been “committing repeated acts of sexual aggression and manipulative behavior toward younger residents in the program.” In Dr. Latham’s opinion, the defendant exhibited “a pattern of behavior that, if left unchecked or untreated, would continue,” and that the “circumstances [for which the defendant was originally committed] could occur again.” Dr. Latham opined, however, that there was no evidence of bipolar disorder and that pedophilia was not a mental illness. He
concluded that the defendant was not mentally ill and was not subject to commitment.
The Commonwealth presented, as its final witness, Dr. George L. Hardman, a psychiatrist who had examined the defendant once three years before the trial. The doctor’s diagnosis of the defendant included a bipolar affective disorder in remission and a narcissistic personality disorder, both of which Dr. Hardman classified as mental illnesses. Based on his review of the defendant’s records prior to the trial and on the reports of Drs. Maurer, Scott, Latham, and Seghorn, Dr. Hardman stated his opinion that the defendant still suffered from these disorders and still presented a danger.
The defendant presented the testimony of one witness, Dr. Theoharis Seghorn, whose diagnosis and opinion were based on a personal evaluation of the defendant and interview with the staff at the treatment center conducted in conjunction with Dr. Latham, and on evaluation reports in the defendant’s file, as well as on the results of a penile plethysmograph examination (discussed
infra).
This witness concluded that the defendant was not mentally ill and was not dangerous by reason of mental illness.
The defendant alleges the following errors occurred at the jury trial: (1) The judge abused his discretion in failing to declare a mistrial when members of the jury panel were left in a court hallway, prior to the taking of evidence, with members of a group calling themselves “Parents of Murdered Children”; (2) the judge erred in admitting the testimony of Dr. Scott in violation of the psychotherapist-patient privilege; (3) the judge erred in admitting testimony of Dr. Maurer to the extent that her testimony relied on confidential conversations between the defendant and his therapist, Timothy Blackson, allegedly an agent of Dr. Maurer; (4) the
judge erred in admitting the testimony of Dr. Hardman, as it was based on stale information; (5) the judge improperly instructed the jury regarding Dr. Seghorn’s reliance on the results of a penile plethysmograph examination; (6) there was insufficient evidence to prove that the defendant was dangerous by reason of mental illness; and (7) the judge should have excluded testimony regarding personality disorders because such disorders are not mental illnesses under 104 Code Mass. Regs. § 3:01 (a) (1986). The defendant also contends that the two-tier de novo review of a department order extending commitment violated his right not to be put in jeopardy twice and his right to due process; that G. L. c. 120 does not authorize multiple extensions, and that multiple extensions violated his right to due process; that the commitment procedure under G. L. c. 120 denied the defendant his right to equal protection of the laws; and finally, that commitment without proof of a recent overt act demonstrating dangerousness deprived the defendant of his due process rights.
1.
Motion for a mistrial.
The defendant claims that the judge should have declared a mistrial when it was discovered that members of the jury had been seated in a hallway among members of a group known as “Parents of Murdered Children,” all of whom wore buttons bearing the name of the group. The defendant fails to cite any authority to support this claim.
The judge conducted a voir dire examination of the jurors in accordance with the procedure prescribed in
Commonwealth
v.
Jackson,
376 Mass. 790, 800-801 (1978), to determine whether any possible prejudicial information had reached any juror. Having found no prejudice, see
Commonwealth
v.
Palmariello,
392 Mass. 126, 142 (1984) (judge has right to rely on juror statements), the judge took the added precaution of dismissing one juror who sat particularly close to the group. The judge also ordered all the buttons confiscated. The denial of the motion for a mistrial was not an abuse of the judge’s discretion.
Evidentiary Issues.
a.
Psychotherapist-patient privilege.
The defendant argues that it was error to admit the testimony of one expert witness, Dr. Scott, a psychologist who evaluated the defendant, because Dr. Scott failed to give the defendant the appropriate warning prescribed in
Commonwealth
v.
Lamb,
365 Mass. 265 (1974), prior to evaluating him. The judge ruled that the defendant’s conversations with Dr. Scott did not fall within the statutory bounds of the psychotherapist-patient privilege, G. L. c. 233, § 20B (1988 ed.), because Dr. Scott has a doctoral degree in education, not a doctoral degree in the field of psychology.
“The patient-psychotherapist privilege has never been recognized at common law.
Commonwealth
v.
Gordon,
307 Mass. 155, 158 (1940). We are thus not inclined here to extend the patient-psychotherapist privilege beyond the bounds established by the Legislature. See
Usen
v.
Usen,
359 Mass. 453, 457 (1971).”
Commonwealth
v.
Mandeville,
386 Mass. 393, 409 (1982). See
Adoption of Diane,
400 Mass. 196, 201 (1987). There was no error.
The defendant contends that the judge erred in allowing Dr. Maurer, a psychiatrist who qualifies as a psychotherapist under G. L. c. 233, § 20B,
and who allegedly supervised the defendant’s therapist, to testify to her opinion of the defend
ant’s mental state which was based in part on communications between the defendant and his therapist.
The defendant argues that the therapist was an agent of Dr. Maurer, and, therefore, communications between the defendant and his therapist should be privileged to the extent communications between the defendant and Dr. Maurer would be privileged.
The therapist, Timothy Blackson, does not qualify independently as a psychotherapist under G. L. c. 233, § 20B, because he holds neither a medical degree in psychiatry nor a doctoral degree in the field of psychology. We have recognized the possibility of extending this statutory privilege to an agent of a psychotherapist where there exists a confidential relationship between the patient and the psychotherapist, see
Commonwealth
v.
Mandeville, supra
at 409; see also
Commonwealth
v.
Clemons,
12 Mass. App. Ct. 580, 586-587 (1981), and the therapist-psychotherapist relationship also meets the requirements of an agency relationship. Cf.
Kirk-Patrick
v.
Boston Mut. Life Ins. Co.,
393 Mass. 640, 645 (1985) (agency relationship arises when one consents to act on behalf of another and subject to her control); Restatement (Second) of Agency § 1 (1958). There was no such agency relationship shown between Blackson and Dr. Maurer. Additionally, the evidence indicates that the defendant was told on numerous occasions, by his therapist and by his case worker from the department, that conversations with the therapist were not confidential. The defendant testified on voir dire that he had been so informed. We are therefore not inclined to extend the privilege to the conversations between the defendant and his therapist, Blackson.9
b.
Stale testimony.
The defendant contends that the judge should not have admitted the testimony of Dr. Hardman, who examined the defendant on one occasion over three years before the trial. Dr. Hardman based his opinion on that evaluation and a review of the defendant’s records since that time. A judge has broad discretion in determining the admissibility of testimony concerning an evaluation which occurred at some time in the past. See
DeJesus
v.
Yogel,
404 Mass. 44, 47 (1989). The period of time between an evaluation, here Dr. Hardman’s clinical examination of the defendant in 1986, and the testimony of a witness is a factor which the trier of fact may weigh in assessing the credibility of the testimony. The judge did not abuse his discretion by admitting testimony based on an evaluation conducted three years earlier, as well as a review of the defendant’s records since that time. Additionally, the judge could have considered trial testimony by Dr. Latham that “nothing about [the defendant’s] particular pattern of thinking has changed.” There was no error.
c.
The judge’s limiting instructions regarding basis of expert’s opinion.
The defendant claims that the judge improperly instructed the jury that Dr. Seghorn’s opinion of the defendant’s mental state was based “[t]o a great degree” on the results of a penile plethysmograph examination,
which, the judge told the jury, was not commonly used, but which has a “fair degree of reliability.” On the basis of this language, the defendant asserts that the judge essentially told the jury they could discount Dr. Seghorn’s testimony. The defendant exaggerates the effect of these words.
“A trial judge may properly bring to the jury’s attention issues of fact and conflicts of testimony. He may point out factors to be considered in weighing particular testimony. Nothing . . . precludes, or could properly, preclude, such guidance where the judge clearly places the function of ultimate appraisal of the testimony upon the jury.”
Barrette
v.
Hight,
353 Mass. 268, 271 (1967).
Review of the trial transcript reveals that, while the results of the plethysmograph test may not have had great significance in Dr. Seghorn’s over-all evaluation of the defendant, the results were a basis of his opinion as to the defendant’s sexual orientation. The transcript reveals that the judge clearly and extensively instructed the jury that they, not the judge, were the ultimate fact finders who determined what weight and effect to give testimony. There was no error.
d.
Sufficiency of the evidence.
The defendant claims there was insufficient evidence for a jury to find beyond a reasonable doubt that he was dangerous by reason of mental illness. We review the record in the light most favorable to the Commonwealth to determine whether a jury reasonably could find beyond a reasonable doubt that the defendant was dangerous by reason of mental illness.
Commonwealth
v.
Barboza,
387 Mass. 105, 110, cert, denied, 459 U.S. 1020 (1982). See
Commonwealth
v.
Burbank,
388 Mass. 789, 797 (1983).
The standard for extending an individual’s commitment to the control of the department is whether that individual “would be physically dangerous to the public because of the person’s mental or physical deficiency, disorder or abnormality.” G. L. c. 120, § 17. In
Department of Youth Servs.
v.
A Juvenile,
398 Mass. 516, 523 (1986), we held that, because G. L. c. 120 and c. 123 “are concerned with the same general subject matter,” c. 123’s definitions of similar words would apply to c. 120. A “mental or physical deficiency, disorder or abnormality” in c. 120 is defined by the term “mental illness” in c. 123.
Id.
at 524.
Chapter 120’s term
“physically dangerous to the public” is defined by “likelihood of serious harm” in c. 123.
Id.
There was evidence before the jury that the defendant had sexually assaulted and murdered a five year old boy, that nothing about the defendant’s pattern of thinking had changed, and that the defendant was “capable, when frustrated,” of committing the same act of violence again. There was also evidence that the defendant had attempted suicide and had demonstrated sexual aggression and manipulative behavior. Each of the Commonwealth’s witnesses testified that the defendant was a danger to himself and others. Three of those four witnesses testified that the defendant was dangerous by reason of mental illness. There was expert testimony indicating that the defendant suffered from bipolar affective disorder in remission, which is a mood disorder impairing his judgment, a narcissistic personality disorder, which is also a mood disorder, and pedophilia. The expert witnesses disagreed about whether a narcissistic personality disorder is a mental illness. That is a question of fact for the jury.
“Judicial experience with psychiatric testimony makes it abundantly clear that it would be unrealistic to treat an opinion .. . by an expert on either side of . . . [an] issue as conclusive. . . . The law should not, and does not, give the
opinions of experts on either side of . . . [an] issue the benefit of conclusiveness, even if there are no contrary opinions introduced at the trial.”
Commonwealth
v.
DeMinico,
408 Mass. 230, 235 (1990), quoting
Commonwealth
v.
Lamb,
372 Mass. 17, 24 (1977). There was sufficient evidence for a jury reasonably to find beyond a reasonable doubt that the defendant was dangerous by reason of mental illness.
3.
Constitutional Issues.
a.
Constitutionality of the two-tier review system for extending a Department of Youth Services commitment order.
According to the defendant, the two-tier procedure for extending commitment of individuals to the department placed the defendant in double jeopardy. The defendant acknowledges that the two-tier system currently in place in the District Court Department of the Trial Coúrt of this Commonwealth has been found constitutional even where there was insufficient evidence presented at the bench trial to warrant a conviction. See
Justices of the Boston Municipal Court
v.
Lydon,
466 U.S. 294 (1984);
Lydon
v.
Commonwealth,
381 Mass. 356, cert, denied, 449 U.S. 1065 (1980). The defendant argues that the present appeal is distinguishable, because in
Lydon
the defendant had the option of a jury trial in the first instance or a bench trial with a right to seek a jury trial on a determination of guilt at the bench trial, whereas in this case the defendant was given no option to choose a jury trial in the first instance.
The issue of a defendant’s option was not determinative for the United States Supreme Court in
Justices of the Boston Municipal Court
v.
Lydon, supra,
nor was it determinative in this court’s holding in
Gibson
v.
Commonwealth,
381 Mass. 372 (1980), cert, denied, 449 U.S. 1089 (1981), and
A Juvenile
v.
Commonwealth (No. 2),
381 Mass. 379, cert, denied, 449 U.S. 1062 (1980). Contrast
Gibson
v.
Commonwealth, supra
at 378 (Liacos, J., dissenting, with whom Abrams, J., joined). The Court held that, the process does not expose a defendant to double jeopardy. There is no merit to the defendant’s double jeopardy claim under the decided case law.
b.
Multiple extensions of commitment under G. L. c. 120, §19, and due process.
The defendant contends that the statute permitting the extended commitment of an individual to the department is impermissibly vague on the issue of multiple extensions. General Laws c. 120, § 19, set out in the margin,
applies when the department has already secured the confirmation of one extension order. When a person committed as a juvenile has that commitment extended, the department must, according to G. L. c. 120, § 19, make a new order and new application for review if the department deems the individual “physically dangerous to the public because of the person’s mental or physical deficiency, disorder or abnormality,” the requirement for extending commitment under § 17. “Such orders and applications may be repeated at intervals as often as in the opinion of the department may be necessary for the protection of the public, except that the department shall have power, in order to protect other children and adolescents, to transfer the custody of any person eighteen years of age or older to the department of correction
for placement in the appropriate institution.” G. L. c. 120, § 19.
The statute clearly permits at least two extensions.
The defendant next argues that he was denied due process of law because the department is not capable of providing the defendant with the proper treatment for his mental illness. The cases the defendant cites do not support his proposition that a mentally ill person has a constitutional right to be committed to one State agency rather than another. Instead, the cases hold that a person committed for treatment of mental illness has a due process right to receive treatment, see
O’Connor
v.
Donaldson,
422 U.S. 563 (1975);
Rouse
v.
Cameron,
373 F.2d 451 (D.C. Cir. 1966);
Wyatt
v.
Stickney,
325 F. Supp. 781 (M.D. Ala. 1971), and the right to receive the least restrictive or least burdensome control necessary to pursue rehabilitation. See
Thompson, petitioner,
394 Mass. 502, 505-507 (1985);
Commonwealth
v.
Nassar,
380 Mass. 908, 917-918 (1980). There was
no
evidence that the defendant received inadequate treatment or that commitment to the department was unnecessarily restrictive. See
Youngberg
v.
Romeo,
457 U.S. 307 (1982);
Commonwealth
v.
Davis,
407 Mass. 47, 49-51 (1990);
Newton, petitioner,
357 Mass. 346, 352-353 (1970);
Doe
v.
Gaughan,
808 F.2d 871, 877 (1st Cir. 1986). Cf, G. L. c. 120, § 6 (1988 ed.). The defendant was not denied his right to due process of law.
c.
Equal protection.
The defendant’s third constitutional challenge is that the procedural differences between commitments under c. 120, §§ 16-20, and commitments under c. 123, which have as the same purpose the treatment of
mentally ill persons, see
Department of Youth Servs.
v.
A Juvenile,
398 Mass. 516, 523 (1986), denied the defendant equal protection of the laws. Specifically, the defendant indicates that the c. 123 commitment process is initiated by the superintendent of a Department of Mental Health facility, whereas the c. 120 commitment process is initiated by a “bureaucratic order.” Compare G. L. c. 123, § 7
(a)
(1988 ed.), with G. L. c. 120, § 17. The defendant contends that a superintendent of a Department of Mental Health facility is more qualified to determine the committability of "an individual for mental health reasons than is the Department of Youth Services. The defendant also challenges the venue of the committing court, claiming that a “c. 123 court” is more appropriate than a “c. 120 court.” Compare G. L. c. 123, § 7
(a)
(District Court in jurisdiction of superintendent), with G. L. c. 120, § 17 (original committing court). These differences are insignificant and do not rise to the level of an equal protection concern.
Last, the defendant notes that commitment under c. 123 is initially for a period of six months, or in some circumstances, one year, with later extensions for no greater than one year. G. L. c. 123, § 8
(d)
(1988 ed.). The defendant, however, asserts incorrectly that an individual may be committed under c. 120 for two years without review. General Laws c. 120, § 5
(b)
(1988 ed.), requires “periodic reexamination of all persons within [the department’s] control.” The reexaminations “may be made as frequently as the department considers desirable, and shall be made with respect to every person at intervals not exceeding one year.”
Id.
Failure to reexamine an individual within one year entitles that individual to petition the committing court for an order of discharge. G. L. c. 120, § 5
(d)
(1988 ed.). As stated in
Department of Youth Servs.
v.
A Juvenile, supra
at 523, c. 120 and c. 123 “are concerned with the same general subject matter” and “have as their purpose the protection of others from physical harm and the treatment of the mentally ill person.” Given that the statutes have the same purpose, yet one chapter initially commits an individual for a shorter time period,
G. L. c. 123, § 8
(d),
there must be a rational basis for distinguishing between those civilly committed as dangerous by reason of mental illness under c. 123, and those committed as dangerous by reason of mental illness after commitment to the Department of Youth Services. See
Baxstrom
v.
Herold,
383 U.S. 107, 111 (1966);
Doe
v.
Gaughan, supra
at 880-881. A short interval for review of a recently committed person’s condition while in the control of the Department of Mental Health assures familiarity with that individual and the state of his condition. An individual whose commitment is extended by action of the Department of Youth Services under c. 120, has been in the control of the department for some time. The department’s familiarity with that individual’s condition is assured. There is a rational basis for this differentiation. The defendant was not denied equal protection of the laws.
d.
Evidence of dangerousness and due process.
The defendant argues that the judge deprived him of due process by failing to instruct the jury that dangerousness by reason of mental illness must be predicated on evidence of a recent overt act. To support his contention, the defendant cites
Lynch
v.
Baxley,
386 F. Supp. 378, 391 (M.D. Ala. 1974);
Lessard
v.
Schmidt,
349 F. Supp. 1078, 1093 (E.D. Wis. 1972), vacated on other grounds, 414 U.S. 473 (1974); and
Commonwealth
v.
Nassar,
380 Mass. 908, 916-918 (1980). The holdings in
Lessard
and
Lynch
are no longer persuasive, however, in light of
United States
v.
Sahhar,
917 F.2d 1197 (9th Cir. 1990), cert, denied, 111 S. Ct. 1591 (1991) (considering involuntary civil commitment standard). The
Sah-har
court held that “a finding of ‘substantial risk’ [of causing bodily injury] . . . may be based on
any
activity that evinces a genuine possibility of future harm to persons .... Whether that activity occurred recently is but one factor for the [finder of fact] to consider in weighing the evidence.” (Em
phasis in original. Citation omitted.)
Id.
at 1207. “Nor do we believe that due process requires a finding that defendant recently committed dangerous acts. In establishing the clear and convincing evidence standard for civil commitments, [the Supreme Court, in
Addington
v.
Texas,
441 U.S. 418, 427 (1979),] held that commitment must be based on ‘something more serious than is demonstrated by idiosyncratic behavior.’ . . . Otherwise,
Addington
placed no due process limits on the range of evidence a [trier of fact] may consider in reaching its decision.” (Citation omitted.)
Id.
at 1207-1208. Further, contrary to the defendant’s assertion, our decision in
Commonwealth
v.
Nassar, supra,
did not recognize a requirement of a recent overt act as a basis for finding a “likelihood of serious harm.” Rather, referring to
Lessard
v.
Schmidt, supra,
we said that a showing of imminent
future
harm was linked to the enhanced standard of proof beyond a reasonable doubt, and that to “the degree that the anticipated physical harm is serious — approaches death — some lessening of a requirement of ‘imminence’ seems justified.”
Id.
at 917. There is no requirement that a “likelihood of serious harm” be established by evidence of a
recent
overt act. Nor does the statutory definition of “likelihood of serious harm,” G. L. c. 123, § 1, require a recent overt act. The judge made no error in excluding such a requirement from his instructions to the jury.
Judgment affirmed.