In November, 1988, a Barnstable County grand jury indicted George Conefrey for indecent assault and battery on his eight year old daughter, whom we shall refer to as Lisa, “on divers times and dates during 1986.”1 See G. L. c. 265, § 13B. The defendant was tried and convicted of violating G. L. c. 265, § 13B, in 1989, but the conviction was reversed on appeal because the defendant did not have a meaningful opportunity to exercise his right to self-representation. Commonwealth v. Conefrey, 410 Mass. 1 (1991). After retrial in August, 1991, the defendant was convicted for a second time.
In this appeal, the defendant alleges error in the trial judge’s refusal to give two jury instructions: (1) that the jury must unanimously find one specific incident of indecent assault and battery in order to convict the defendant; and (2) that there is an element of sexual purpose or intent in G. L. c. 265, § 13B. In addition, the defendant argues for the first time on appeal that G. L. c. 265, § 13B, is unconstitutionally vague and overbroad. Although we agree with the defendant that there was error in the refusal to instruct on “specific unanimity,” we affirm because the error did not prejudice the defendant.
The evidence at trial was that approximately seven incidents of indecent assault and battery occurred: three at the defendant’s apartment in Falmouth and four at another apartment in West Falmouth. Lisa gave no specific dates for any of the incidents. Six of the incidents were of the same general description (hereafter the “bed incidents”): Lisa and the defendant were sitting on his bed watching television, and the defendant dropped his pants, exposed his penis and asked her to touch it. Although Lisa said that she did not want to touch his penis, the defendant said “it was okay” *292and took her hand and put it there. The defendant also told Lisa that her mother “didn’t need to know about it.”
One of the seven incidents occurred in the shower at the defendant’s apartment in West Falmouth (hereafter the “shower incident”). Lisa and the defendant were showering together, and the defendant rubbed his penis against Lisa’s stomach.
In fresh complaint testimony, a Department of Social Services social worker corroborated Lisa’s description of the “bed” and “shower” incidents of indecent assault and battery. A second fresh complaint witness, a State police investigator, corroborated Lisa’s testimony regarding the six “bed” incidents. The Commonwealth did not present any other witnesses, and the defendant did not testify on his own behalf.
1. Alleged errors in jury instructions. Where there is a timely objection at trial and compliance with Mass.R.Crim.P. 24(b), 378 Mass. 895 (1979),2 the issue of whether the judge erred in charging the jury is properly preserved. The standard of review is prejudicial error rather than the standard of whether the omission of the instructions created a substantial risk of a miscarriage of justice. Cf. Commonwealth v. Preziosi, 399 Mass. 748, 752 (1987).
“The legal adequacy of a particular instruction to the jury can only be judged in the context of the whole charge, and not on the basis of limited or isolated portions of it.” Commonwealth v. Carrion, 407 Mass. 263, 270 (1990). The judge began with general instructions on reasonable doubt, declaring that the jury was “bound to act conscientiously upon the evidence” to determine if the proof of the defendant’s guilt was “proof beyond surmise or conjecture or speculation or some doctrine of probabilities or chances.” After giving further general instructions on the credibility of witnesses and on the drawing of inferences, the judge instructed on the elements of indecent assault and battery. “[I]n order *293to prove that a touching is indecent,” the judge explained, “it must be established that there was an act or a series of acts; and here in this allegation the Government has to establish that there was a series of acts, at least two, that were, indeed, indecent.”3 Near the conclusion of the charge, while addressing the foreperson and explaining the necessity for polling the jury, the judge declared that “there is no verdict until every juror subscribes to that verdict.” Finally, the judge defined the verdict as “the truth as best twelve honorable jurors can determine that truth to be.”
a. Jury instruction on specific unanimity. Near the conclusion of the jury charge, the trial judge gave a general unanimity instruction: “there is no verdict until every juror subscribes to that verdict. In short, whatever the verdict is, it has to be unanimous.” The defendant had requested, instead, that the judge give a specific unanimity instruction: “[i]n order to convict the defendant you must unanimously agree beyond a reasonable doubt upon at least one incident.” When defense counsel objected at the conclusion of the jury charge, the judge refused to give the instruction he requested.4 The defendant contends that the judge’s failure to instruct the *294jury on “specific unanimity” violated his Federal constitutional rights.
Courts .have credited juries with “using robust intuition and good common sense [to] understand and act on the understanding that a general instruction about unanimity implicitly calls for more specific unanimity.” Commonwealth v. Lemar, 22 Mass. App. Ct. 170, 172 (1986). Where a general unanimity instruction is given, we presume that each juror followed it unless there is evidence to the contrary. Commonwealth v. Sylvester, 400 Mass. 334, 340 (1987). “[A] specific unanimity instruction indicates to the jury that they must be unanimous as to which specific act constitutes the offense charged”; i.e., as here, which particular incident of indecent conduct led to the conviction for indecent assault and battery. Commonwealth v. Keevan, 400 Mass. 557, 566-567 (1987).
Our appellate courts have twice addressed the need for a specific unanimity instruction. In Commonwealth v. Lemar, 22 Mass. App. Ct. at 172, this court “assume[d] . . . that a judge would be obliged to give a specific [unanimity] instruction if the defendant requested it in accordance with the normal procedure of Mass.R.Crim.P. 24(b), 378 Mass. 895 (1979).” In Commonwealth v. Comtois, 399 Mass. 668, 676-677 n.11 (1987), the court commented, without deciding, that a judge should honor a defendant’s request for a specific unanimity instruction. In both the Comtois and Lemar cases, the question was whether a judge’s failure to give a specific unanimity instruction sua sponte (where none was requested at trial) created a substantial risk of a miscarriage of justice. This is the first case involving allegations of separate incidents of criminal conduct in which we address the refusal to give a specific unanimity instruction after a proper request and a timely objection. Cf. Commonwealth v. Ramos, 31 Mass. App. Ct. 362, 367 (1991) (no error where judge rejected defendant’s request for specific unanimity instruction, where prosecution alleged one event and alternative theories of guilt).
*295We do not constrain jurors to agree unanimously on one theory of culpability for a single charged offense, but we may require that a conviction be based on unanimous agreement as to which specific act constitutes the offense charged. See Commonwealth v. Ramos, 31 Mass. App. Ct. at 367. Where a statute disjunctively proscribes several methods for committing an offense, the Commonwealth can prove a violation of that statute by proof of any one method; unanimity as to a certain method (e.g., type of weapon, mental state) is not required. See Commonwealth v. Murphy, 415 Mass. 161, 164 (1993). See also Schad v. Arizona, 501 U.S. 624 (1991) (held specific unanimity not required as to mental state for first degree murder, where evidence supported conviction for premeditated or felony murder). Where there are “alternative sets of facts or episodes which support a finding of guilty” (e.g., “divers” dates or times), however, the jury must reach a unanimous verdict as to at least one of those specific sets of facts or episodes. See Commonwealth v. Comtois, 399 Mass. at 676; Commonwealth v. Lemar, 22 Mass. App. Ct. at 173.5
As we noted in Lemar, 22 Mass. App. Ct. at 172, the issue of specific unanimity “has had some vogue in Federal courts.”6 See United States v. Holley, 942 F.2d 916, 926 (5th Cir. 1991), cert. denied, 114 S. Ct. 77 (1993)(general unanimity instruction usually sufficient unless there is “genuine possibility” that jury were confused or that basis of con*296viction was different jurors concluding that defendant committed different acts). Accord United States v. Schiff, 801 F.2d 108, 114-115 (3d Cir. 1986), cert. denied, 480 U.S. 945 (1987); United States v. Sanderson, 966 F.2d 184, 187 (6th Cir. 1992); United States v. Sayan, 968 F.2d 55, 65 (D.C. Cir. 1992); United States v. Hager, 969 F.2d 883, 891 (10th Cir.), cert. denied, 506 U.S. 964 (1992); United States v. Gruenberg, 989 F.2d 971, 975 (8th Cir.), cert. denied, 114 S. Ct. 204 (1993).
“[Jjuror disagreement as to the critical facts of the offense might reflect a ‘reasonable doubt’ that the defendant actually engaged in criminal activity.” United States v. Correa-Ventura, 6 F.3d 1070, 1078 (5th Cir. 1993) (general unanimity instruction sufficient where jury not likely to confuse evidence with respect to use of various firearms in determining whether defendant knowingly carried a single firearm during drug trafficking activities). “[F]actual concurrence must be viewed on a case-by-case basis to . . . ensure that the purposes of unanimity are satisfied.” Id. at 1082. See and compare United States v. Payseno, 782 F.2d 832, 835-837 (9th Cir. 1986)(specific unanimity instruction required where complex evidence of separate acts of extortion at different locations and times by various individuals created risk of jury confusion). See generally Howe, Jury Fact-Finding in Criminal Cases: Constitutional Limits on Factual Disagreements Among Convicting Jurors, 58 Mo. L. Rev. 1, 21-47 (1993). It is incumbent upon the court to determine “which ‘fact[s] [are] necessary to constitute the crime,’ . . . and to require consensus on these ‘facts.’ ” United States v. Correa-Ventura, supra at 1078, quoting from In re Winship, 397 U.S. 358, 364 (1970).7
*297In light of Massachusetts and prevalent Federal precedent, we hold that it is error to decline to give an instruction on specific unanimity that the defendant has properly requested. “[I]f evidence of three different episodes of sexual abuse is offered to the jury, any one bundle of evidence being sufficient to support a guilty finding, the jury is to be instructed that they must be unanimous in their findings as to any specific bundle of facts; they may not mix nonunanimous findings about the several episodes to come up with a general verdict of guilty.” Commonwealth v. Ramos, 31 Mass. App. Ct. at 366 (interpreting Commonwealth v. Lemar, 22 Mass. App. Ct. at 172). Such was the danger here in the absence of a specific unanimity instruction. If a defendant makes the tactical decision that specific unanimity is consistent with the theory of defense and takes the required steps to request an instruction, that defendant is entitled to receive it. Compare Commonwealth v. Corriveau, 396 Mass. 319, 340 (1985)(trial judge not required to give instruction concerning alibi unless defendant requests it), with Commonwealth v. Gilchrist, 413 Mass. 216, 218-219 (1992)(trial judge required to give instruction concerning defendant’s right not to testify upon defendant’s request).
The Commonwealth has, nevertheless, made a sufficient showing that this error was harmless beyond a reasonable doubt. See Commonwealth v. Hanger, 377 Mass. 503, 511 (1979); Commonwealth v. Freeman, 29 Mass. App. Ct. 635, 642-643 & n.9 (1990) (discussing Chapman v. California, 386 U.S. 18, 20-24 [1967], and harmless error analysis). See also Commonwealth v. MacDonald (No. 1), 368 Mass. 395, 399 (1975), and cases cited. Contrast Commonwealth v. Rios, 412 Mass. 208, 214 (1992) (violation of right to confrontation held prejudicial where effect of having defendants *298in courtroom was impossible to demonstrate). While the defendant’s right to unanimity is certainly a “substantial” one, it appears that the “subjective state of certitude” guaranteed by In re Winship, 397 U.S. at 364, was provided in the circumstances of this case.
The evidence here was not complex, contrast United States v. Payseno, 782 F.2d at 835-837, and Lisa’s descriptions of the “bed incidents” and the “shower incident” were not likely to confuse the jury. Compare United States v. Correa-Ventura, 6 F.3d at 1086. Assuming as we must that the jury followed the judge’s instructions in convicting the defendant, the jury unanimously determined that at least two indecent acts occurred in the events recounted by Lisa. (See the text accompanying note 3, supra.) Constituting the two indecent acts were either two “bed incidents” or one “bed incident” coupled with the “shower incident.” In either scenario, a “bed incident” led to a unanimous verdict to convict the defendant. Because Lisa’s testimony regarding all six “bed incidents” was uniform, no rational juror could have concluded that one of the bed incidents constituted indecent assault and battery but the other five did not. Moreover, the quantity and quality of the evidence was sufficient for a conviction based on Lisa’s general description of all of the “bed incidents.”8 Thus, following the instructions, in order to find at least two indecent acts, the jury must have found unanimously that at least one of the “bed incidents” occurred, *299which is all that was required to convict the defendant. See Commonwealth v. Comtois, 399 Mass. at 676. In effect, then, the defendant received the same safeguard of unanimous factfinding that he would have received through an instruction on specific unanimity. Under these circumstances, there was no prejudice in denying the request for a specific unanimity instruction.
b. Jury instruction on elements of indecent assault and battery on a child. The defendant requested that the judge instruct the jury that the Commonwealth was required to prove, beyond a reasonable doubt, “that the touching was for the purpose of sexual arousal, gratification, or offense.” The judge did not make any reference to sexual purpose or arousal in his instructions.9 At the conclusion of the jury charge, when defense counsel objected, the judge refused to give the instruction.
The defendant constructs the following argument. By failing to include his requested instruction on sexual arousal, G. L. c. 265, § 13B, invites confusion over what is indecent. Unless the statute is restricted to a touching made for sexual gratification, a “parent could be convicted for patting his minor child on the rear or bathing his infant or disabled child.”
The defendant’s proposed jury instruction is an- erroneous statement of law because it suggests that indecent assault and battery on a child is a specific intent crime. The judge correctly instructed the jury that the “touching” that is alleged to be indecent should be “intentional and deliberate.” See Commonwealth v. Knap, 412 Mass. 712, 714-715 *300(1992).10 Placed “in the same category as statutory rape,” indecent assault and battery on a child is a strict liability crime. Ibid.
The definition of indecent assault and battery on a child turns not on the state of mind of the defendant but on the nature of the conduct itself, viewed objectively in light of “contemporary moral values.” To be “indecent,” an act need not be for the purpose of sexual gratification or arousal. The measure of indecency is “common understanding and practices.” Commonwealth v. De La Cruz, 15 Mass. App. Ct. 52, 59 (1982)(interpreting G. L. c. 265, § 13H, prohibiting indecent assault and battery on “person who has attained age fourteen”), quoting from Commonwealth v. Jarrett, 359 Mass. 491, 497 (1971). See Commonwealth v. Gallant, 373 Mass. 577, 589-590 (1977). Like other statutes enforcing “community values [that] still strongly condemn the sexual victimization of minors,” the purpose of G. L. c. 265, § 13B, is to protect minors from sexual exploitation. Commonwealth v. Benoit, 26 Mass. App. Ct. 641, 645 (1988) (interpreting G. L. c. 272, § 3 5A, prohibiting “unnatural and lascivious acts”).
Instructing on an element of intent for sexual arousal or gratification alters the accepted meaning of the term “indecent.” The nature of the defendant’s particular indecent acts suggests implicit limits on the concept of “indecency.” Commonwealth v. Perretti, 20 Mass. App. Ct. 36, 44 (1985) (interpreting G. L. c. 265, § 13H). The acts classified as “indecent” in this case were well within the boundaries established in Commonwealth v. De La Cruz, 15 Mass. App. Ct. at 59 (fondling of breasts and touching of buttocks and pubic area constituted indecent assault and battery), and in Commonwealth v. Mosby, 30 Mass. App. Ct. 181, 185 (1991) (placing of hand on buttocks in suggestive manner constituted in*301decent assault and battery). Thus, the trial judge appropriately rejected the defendant’s proposed jury instruction on intent.
2. The constitutionality of G. L. c. 265, § 13B. The defendant did not raise this issue in a pretrial motion under Mass.R.Crim.P. 13(c), 378 Mass. 872 (1979), or in his argument for a required finding of not guilty. See Commonwealth v. Jasmin, 396 Mass. 653, 655 (1986)(challenge to vagueness statute as applied “need not be raised until the Commonwealth has presented its evidence”); Commonwealth v. Oakes, 407 Mass. 92, 94 (1990). Nevertheless, he argues that his motion for a required finding of not guilty (without explicitly raising an overbreadth or vagueness argument) and his requests for jury instructions sufficiently preserved his appellate rights. Because neither the motion nor his requests placed the issue before the trial judge, appellate review of the constitutionality of the statute as applied should be limited to whether there was a “serious and obvious error creating a substantial risk of a miscarriage of justice.” Commonwealth v. Oakes, supra at 94-95, quoting from Commonwealth v. Pares-Ramirez, 400 Mass. 604, 609 (1987).
The defendant hinges his argument on the premise that G. L. c. 265, § 13B, infringes on “family life” activities that are constitutionally protected, “sweeping within its reach behavior which nearly everyone would consider decent and appropriate.”11 A statute “is not vague simply because ‘it re*302quires a person to conform his conduct to an imprecise but comprehensible normative standard. . . .’ Commonwealth v. Williams, 355 Mass. [302,] 304 [1985], quoting from Commonwealth v. Orlando, 371 Mass. 732, 734 (1977).” Commonwealth v. Benoit, 26 Mass. App. Ct. at 646 (unsuccessful vagueness challenge to prohibition of “unnatural and lascivious act”). See Commonwealth v. Gallant, 373 Mass. at 589 (unsuccessful constitutional challenge to statute prohibiting “unnatural sexual intercourse” with a child under sixteen). The term indecent provides a “reasonable opportunity for a person of ordinary intelligence to know what is prohibited.” Commonwealth v. Jasmin, 396 Mass. at 655 (vagueness challenge to “anti-drug paraphernalia act”).
The conduct in which the defendant engaged falls well within the boundaries of previous decisions upholding convictions for indecent assault and battery on a child. See Commonwealth v. De La Cruz, 15 Mass. App. Ct. at 59 (which predated all 1986 incidents in this case); Commonwealth v. Bibby, 35 Mass. App. Ct. 938, 942 (1993) (denying vagueness challenge to G. L. c. 272, § 5 3A, prohibiting sexual conduct for a fee, this court declared “there is no mystery that fondling a woman’s breasts is sexual conduct”). In his charge, the judge properly used specific examples of indecent conduct and called upon the jurors to determine whether the defendant’s acts were “intentional acts of lewd exposure offensive to one or more persons.” See Commonwealth v. Benoit, 26 Mass. App. Ct. at 649. Thus, in light of the defendant’s unavailing arguments that G. L. c. 265, § 13B, is unconstitutional, there is no substantial likelihood that justice was miscarried in the circumstances of this case.
Judgment affirmed.