The defendant was found guilty under G. L. c. 273, § 11, which provides that “Whoever, not being the husband of a woman, gets her with child shall be guilty of a misdemeanor.” Prior to the trial he moved to dismiss the complaint on the ground that G. L. c. 273, § 11, “on its face” denies the defendant equal protection of the laws guaranteed by arts. 1 and 10 of the Declaration of Rights of the Massachusetts Constitution and by the Fourteenth Amendment to the Constitution of the United States. The motion to dismiss asserted that § 11 imposes criminal sanctions on a man found guilty of fathering a child out of wedlock, but not on the woman who is the mother of the same child. The motion to dismiss was denied. The defendant was sentenced to three months in a house of correction, suspended for six years, with probation conditioned on his payment of weekly amounts to support the child and to satisfy the confinement expenses. The defendant argues here only his exception to the denial of his motion to dismiss. We ordered the case transferred here from the Appeals Court.

Relying on traditional equal protection standards, the defendant contends that the distinction made by § 11 denies equal protection of the laws because it is not substantially and rationally related to a permissible legislative purpose. Furthermore, he argues that, even if there were a valid legislative purpose for the distinction, any sex-based classification would be inherently suspect and could be upheld only if necessary to achieve a compelling State interest. From this, the defendant asserts that there is no compelling State interest in distinguishing between males and females in assigning criminal responsibility for conceiving a child out of wedlock.

Since 1913 the statutes of the Commonwealth have expressed in a criminal context a father’s responsibility for fathering an illegitimate child. St. 1913, c. 563. As to *615the father, the statute “makes the begetting of an illegitimate child a misdemeanor, and the non-support of such child a distinct and continuing offence. It utilizes the probation collection system, incidental to criminal proceedings; and adopts the practice of the uniform desertion act . . . [see now G. L. c. 273, §§ 1-10], so far as it is applicable.” Commonwealth v. Dornes, 239 Mass. 592, 594 (1921). See G. L. c. 273, § 16.

The Commonwealth argues that in practice § 11 is used only (a) to determine the paternity of the child, (b) to impose on the father the obligation to contribute toward the expenses of the pregnancy and of the confinement (see G. L. c. 273, § 13), and (c) to establish or enforce the father’s obligation to share in the support of the child. See Commonwealth v. Baxter, 267 Mass. 591, 595 (1929); Lombard, Family Law, § 453, p. 437, and § 474, p. 469 (1967). The Commonwealth recognizes that the father may receive a suspended sentence which is subject to revocation for failure to comply with the court’s orders, but claims that, because § 11 is merely a paternity and support statute, the distinction between fathers and mothers is rational and permissible.

The judicial scrutiny required in cases involving distinctions based solely on sex is stricter than that required under traditional equal protection standards. The application of this stricter standard of review has led the Supreme Court to reject many sex-based classifications.1 A sex-based classification “must rest upon some ground of difference having a fair and substantial relation to the *616object of the legislation, so that all persons similarly circumstanced shall be treated alike.” Reed v. Reed, 404 U. S. 71, 76 (1971), quoting from F. S. Royster Guano Co. v. Virginia, 253 U. S. 412, 415 (1920).2

Judging § 11 by this standard, we discern no permissible legislative goal which rationally is achieved by making a father, but not a mother, guilty of conceiving a child out of wedlock. The statutory purposes advanced by the Commonwealth support a paternity and child support statute, but not a criminal sanction directed solely against the father and addressed expressly to the act of begetting. We are aware of no other possible justification for the distinction in § 11 between men and women in a criminal context which would satisfy the appropriate equal protection test. The imposition of a criminal sentence on the father alone for begetting the child violates the equal protection of the laws.3

*617Although this criminal aspect of § 11 cannot survive constitutional challenge, § 11 is not similarly defective to the extent that it is used to establish paternity and oblige the father to contribute toward pregnancy and childbirth expenses. Because of significant circumstantial differences between unwed fathers and unwed mothers, it is permissible for the Legislature to focus statutory attention exclusively on the fathers of illegitimate children. Our paternity statutes were enacted to deal with the problem of defaulting fathers. Commonwealth v. Dornes, 239 Mass. 592, 594 (1921). See G. L. c. 273, § 15. There was no showing that unwed mothers presented a similar social problem. Furthermore, statutes obligating a father to support his illegitimate child eliminated a sex-based discrimination. At common law, the father of an illegitimate child had no duty to contribute to the support of his child (Commonwealth v. Dornes, supra, at 593-594), whereas the mother did have such a duty (Commonwealth v. Hall, 322 Mass. 523, 528 [1948]). In addition, because the woman carries and bears the child, the pressures of society make it nearly impossible for her successfully to deny parenthood or to avoid responsibility for the child. The father, on the other hand, because not visibly linked to the child, often is unaware of the tie or denies it, and thus the fact of his parenthood is more difficult to prove. For these reasons, defaulting fathers have posed a significant social problem, while defaulting mothers have not.

We are satisfied, then, that the paternity and child support provisions of § 11 rest on more than a sex-based *618classification. They reflect significant circumstantial differences between the fathers and the mothers of illegitimate children. Adapting the language of the Supreme Court in Schlesinger v. Ballard, supra, at 508, to these circumstances, we believe that “the different treatment of men and women . . . [under § 11] reflects, not archaic and overbroad generalizations, but, instead, the demonstrable fact that male and female . . . [parents of illegitimate children] are not similarly situated . . ..” Thus there is no violation of equal protection in this noncriminal aspect of § 11.4

We conclude that the defendant’s motion to dismiss the complaint was properly denied. A complaint under § 11 may be used to initiate a proceeding to adjudicate paternity. See G. L. c. 273, § 12. Such an adjudication provides a basis for an order directing the father to contribute toward pregnancy expenses (see G. L. c. 273, § 13) and the support of the child (see G. L. c. 273, §§ 15 and 16). In such proceedings, the relative obligations of the father and the mother must be determined by applying an identical standard. Here an order directing the defendant to make certain payments would have been proper. Thus the motion to dismiss the complaint was denied correctly, and the defendant’s exception must be overruled.

*619As we have indicated, however, a criminal conviction and sentence may not be imposed constitutionally under §11. The sentence imposed on the defendant is invalid and should be vacated. An adjudication of paternity and an order for the payment of pregnancy expenses and support of the child may be entered in this proceeding.5

So ordered.

(concurring in the result). I agree with the court that there are “significant circumstantial differences between unwed fathers and unwed mothers,” and that the paternity and child support provisions of G. L. c. 273, § 11, rest on more than a sex-based classification. Cf. Commonwealth v. Chapman, 2 Mass. App. Ct. 878, 879 (1974). Hence those provisions do not deny the defendant equal protection of the laws. The same differences seem to me to justify differentiation between the unwed father and the unwed mother in the imposition of criminal sentences. It seems clear, however, that it is no longer permissible to punish unnatural sexual conduct of consenting adults in private. See Commonwealth v. Balthazar, 366 Mass. 298, 301, n. 2 (1974). Punishment of natural sexual conduct and of its natural consequences must be subject to the same principle. Even though the defendant has not argued this point, therefore, I concur in the court’s decision to vacate his criminal sentence.