MOST young boys who persistently act like girls grow up to be homosexuals or bisexuals, a 15-year study of ''sissy boys'' has shown. According to the findings, neither therapy designed to discourage the extremely feminine behavior nor ideal child rearing could guarantee that the boys would develop as heterosexuals, although parental discouragement of the boys' girlish behavior tended to result in a more heterosexual orientation.
Three-fourths of 44 extremely feminine boys followed from early childhood to adolescence or young adulthood matured as homosexuals or bisexuals, as against only one bisexual among a comparison group of more typically masculine boys.
In many cases parents either overtly or subtly encouraged the feminine behavior. But when parents actively discouraged it and took other steps to enhance a male self-concept, homosexual tendencies of the feminine boys were lessened, although not necessarily reversed. Neither did professional counseling divert a tendency toward homosexuality, although it resulted in more conventional masculine behavior and enhanced the boys' social and pyschological adjustment and comfort with being male.
The study was conducted by Dr. Richard Green, a noted sex researcher who is professor of psychiatry at the University of California, Los Angeles and director of its Program in Psychiatry, Law and Human Sexuality. Details of the findings and implications are described in Dr. Green's new book, ''The 'Sissy Boy Syndrome' and the Development of Homosexuality,'' to be published in February by Yale University Press.
Although the study examined extreme cases of boyhood effeminacy, Dr. Green believes the findings may have relevance to lesser degrees of feminine behavior in boys. Such boys, who may, for example, be athletically inept or prefer music to cars and trucks, often have difficulty making friends with other boys and identifying with typically male activities. Dr. Green suggested that to help the boys think of themselves as male, parents might assist them in finding boy friends who are similarly unaggressive and that the fathers might share in activities the boys enjoy, such as going to the zoo or a concert, rather than insist on taking the boys to athletic events. Counseling to guide such parents and enhance the child's masculine self-image may also be helpful, Dr. Green said.
The study did not examine the development of homosexuality in boys whose childhoods are typically masculine. About one-third of homosexual men recall such masculine boyhoods. Nor does the study suggest that all boys with the sissy-boy syndrome are destined for homosexuality. Indeed, one-fourth of the extremely feminine boys followed to maturity developed as heterosexuals.
According to Dr. Green and other experts familiar with his study, the findings indicate that some children may have an inborn ''receptivity'' to environmental factors that encourage a homosexual orientation. Whether such a predisposition is genetic or the result of prenatal factors, or both, is not known. Recent research in animals suggests that prenatal hormonal influences can interfere with programming the brain of the male fetus and result in the birth of males that act like females.
The study supports a recent Kinsey Institute survey of 1,500 adults that singled out ''gender nonconformity'' in childhood as the most important predictor of homosexuality. Dr. Alan Bell of Indiana University, a director of the Kinsey study, said he was pleased and not surprised that the findings of Dr. Green's prospective study corresponded with the retrospective Kinsey data.
''The pendulum is swinging back to biology,'' Dr. Bell remarked. ''Apparently there is a very important physiological component that plays a big role in determining one's sexual identity.''
As have other recent investigations, including the Kinsey study, the new research challenges long-held psychoanalytic beliefs that dominant, overprotecting mothers and ineffectual fathers are primary ''causes'' of a son's homosexuality.
Rather, the study suggests that some boys are born with an indifference to rough-and-tumble play and other typical boyhood interests and that this indifference alienates and isolates them from their male peers and often from their fathers as well. Dr. Green believes that such boys may grow up ''starved'' for male affection, which prompts them to seek love from men in adolescence and adulthood. To Dr. Bell, however, a sense of difference and social distance from males during childhood is what leads to the romantic and erotic attraction to other males. Tomboys Now Being Studied
Dr. Richard Isay, a New York psychoanalyst whose practice is largely homosexual men, said: ''I would agree with Dr. Green. I too see no support for the notion that binding mothers produce homosexual sons, nor do I see any consistent pattern for absent fathers that I don't also see among heterosexual men in analysis.'' Dr. Isay, who is affiliated with Columbia Psychoanalytic Institute and New York-Cornell Medical School, suggested that the common depiction among homosexual men of an absent, distant father is in fact a defense against an underlying erotic attachment to their fat