An innate sissiness is ''not the answer to all homosexuality,'' Dr. Marmor said, ''but it is a factor that plays a role in a substantial number of male homosexuals.'' He added that homosexuality could also develop from a seriously distorted family environment but that ''it is much harder to develop that way, without a biological predisposition.''

Boys who participated in Dr. Green's study were first examined in early childhood, when their parents became concerned about the boys' persistent feminine behaviors and dislike of activities boys usually like. Many of the boys also repeatedly said they wanted to be girls. At the outset, Dr. Green thought he was examining the origins of male transsexuals -boys who grow up thinking they are girls trapped in male bodies and who may later seek sex-change surgery. However, only one of the feminine boys became a transsexual.

In an interview, Dr. Green pointed out that the boys he studied were notably different from other children. While many, if not most, young children - boys as well as girls - occasionally dress up in their mothers' clothes, put on makeup or jewelry, play with dolls or assume the role of the opposite sex in fantasy play, the boys in Dr. Green's study did so almost exclusively. They spurned typical boy games, rough-housing and sports and instead would play with Barbie dolls for hours, frequently don female clothing and nearly always assume a female role when playing house. Many followed their mothers around the house, mimicking the mothers' activities.

The boys and their parents were interviewed every few years, and some were seen several times a year in therapeutic counseling aimed at intercepting the boys' feminine tendencies and encouraging more ''gender-appropriate'' activities. Parental Actions Cited

Although Dr. Green found no evidence that the parents ''created'' feminine boys (many, in fact, had other sons who were normally masculine), certain parental attitudes and actions were correlated with a stronger homosexual orientation. One of the earliest influences was the prenatal desire on the part of either parent, and the father in particular, that the child be a girl. After the boy was born, the parents often considered their son to be an especially beautiful infant. Even strangers who admired the baby tended to make comments like ''what a pretty little girl.''

One of the most important factors related to a more homosexual orientation in adolescence and adulthood was how parents responded to the boys when they dressed up as girls and pretended to be girls. Many of the parents, Dr. Green said, thought it was cute and directly or indirectly encouraged the cross-gender behavior. For example, photographs of the boys dressed as girls were found in many family albums of feminine boys but in none of the albums of the comparison group of masculine boys.

No relationship was found between later homosexuality and the amount of time a boy spent with his mother. In fact, many of the feminine boys spent less time with their mothers than did the masculine boys. Nor was there any link to a mother-dominated household.