(CNN) Most sex education classes begin in high school. But a new study suggests that, no matter where children live, real talk about relationships, identity and sexuality should start even earlier to minimize the negative impacts of gender roles.

You've probably heard it before : More than biology, family, friends and society influence impressions of what it means to be a boy or a girl, placing rigid gender expectations on children from a young age. In recent years, a growing body of research has focused on health inequities that result from enforced gender norms in children.

The study, published Wednesday in the Journal of Adolescent Health, contributes a global perspective to this issue. The key finding: Whether a child is in Baltimore, Beijing or New Delhi, the onset of adolescence triggers a common set of rigidly enforced gender expectations associated with increased lifelong risks of mental and physical health problems.

Researchers with the Johns Hopkins Bloomberg School of Public Health and the World Health Organization collaborated on the Global Early Adolescent Study to identify universal themes in gender identity development across countries and income levels.

"Adolescent health risks are shaped by behaviors rooted in gender roles that can be well-established in kids by the time they are 10 or 11 years old," said Kristin Mmari, associate professor at Johns Hopkins Bloomberg School of Public Health and lead researcher for qualitative research on the Global Early Adolescent Study. "Yet we see billions of dollars around the world invested in adolescent health programs that don't kick in until they are 15, and by then, it's probably too late to make a big difference."

For girls, those risks can include child marriage, pregnancy, leaving school early, sexually transmitted infections and exposure to violence. Boys suffer, too, from increased risk of substance abuse, suicide and shorter life expectancy than women -- especially if they try to challenge masculine norms.

The findings appear in the journal as a series of articles based on interviews in 15 countries with 450 early adolescents and their parents or guardians, totaling nearly 900 people. The experiences described were different, but themes emerged across continents.

Girls are vulnerable, and boys are strong

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The study calls it the hegemonic myth: the perception that men are the dominant sex, strong and independent, while women need to be protected.

This idea starts in early childhood, reinforced by schools, parents and media. Interviews with children and their guardians revealed that the onset of puberty triggers increased reinforcement of pressure to conform to hegemonic sex-typed identities and roles.

While boys, men described having the freedom to come and go as they pleased to pursue education and other opportunities, girls found their mobility and access to education restricted, the study notes. As they enter adolescence, silence and modesty are instilled as desirable values, as girls are pressed to behave in a "modest fashion."

This phenomenon leads to numerous cascading cultural perceptions.

Once puberty hits, it's all about (preventing) sex

Puberty deepens the divide, especially when it comes to sexuality, turning boys into predators and girls into potential targets, the study found.

Messages such as "don't sit like that," "don't wear that" and "boys will ruin your future" reinforce the gender division of power and promote sex segregation with the aim of preserving a girl's sexuality, the study says. "In some places, girls come to internalize these norms to even a greater extent than boys."

In Delhi, once a girl reaches puberty, "her family is concerned with protecting her chastity, preventing elopement and fears stigma from losing family honor," the researchers said. Girls there are not supposed to look at or talk to boys, "as this might raise suspicion that they were initiating romantic relationships."

The researchers also found that, despite efforts to normalize menstruation and breast development, negative societal attitudes further contribute to girls' sense of self-objectification, body shame and lack of agency in sexual decision-making.

"The onset of menstruation appeared to constitute a major concern for parents as they linked puberty with the potential for early romantic and sexual engagements and subsequent risk of adolescent pregnancy," the study said.

'Cover up, and don't go out'

As a consequence of adult perceptions of girls' sexual vulnerability, respondents in nearly every location reported that girls' mobility is far more restricted than it is for boys.

In Ghent, Belgium, respondents spoke of how girls had to take care in selecting their clothing or risk being seen as "too easy" or "prostitutes," making them potential victims of violence.

An 11-year-old in Delhi expressed a similar sentiment: "Parents also tell (girls) not to go out alone. She will not be allowed to do a job. I really don't understand why girls are not allowed to go out of the house."

A young girl in Nairobi, Kenya, described not being able to go out at night because she had grown breasts.

Parental fear for girls' safety and family status is a common rationale for restricting girls' movements, especially in the Middle East and among some immigrant groups in Western Europe. But such concerns seem minor compared with the potential consequences.

"This constricted spatial mobility of girls is linked to restricted knowledge and power and can have profound health implications for girls in the form of more susceptibility to sexual violence and poor access to reproductive health services, in addition to loss of prestige especially among poor families," the study said.

Boys are trouble

Despite the freedom and benefits conferred upon them, boys are still perceived as a danger to girls because of their vulnerability -- a bias with negative impacts for both genders, researchers said.

In Delhi and Shanghai, girls said they were discouraged from interacting with boys, becoming friends them or forming romantic relationships.

"I don't make friends with boys, as my parents asked me not to," a 12-year-old in Delhi said.

"Maybe I befriend a girl and someone would see me and would go tell her dad, and problems happen," a boy in Assiut, Egypt, said.

Gender-nonconforming has consequences

A recurring theme in conversations with adolescents and adults was those who defied gender expectations, resulting in "significant sanctions and pressures to conform," the study said.

The study focuses on three common manifestations of defiance: boys wearing nail polish, girls playing football or soccer, and gender-"inappropriate" clothing.

For young children, it may start as acceptable to cross gender boundaries. But once it becomes clear that a behavior is socially defined as typical for the other sex, "it is shunned out of fear of being ostracized," the study said.

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Though boys too resist typical masculine norms, researchers discovered a greater willingness among girls to challenge gender stereotypes, Moreover, "tomboy" behavior in girls enjoyed more peer and parental acceptance than their male "sissy" counterparts, who faced significantly more stigma and rejection, the study said.

"A potential explanation for this may be that girls who 'act like boys' display masculine characteristics associated with power and dominance," researchers surmised.

Conversely, "boys who 'act like girls' are generally not granted the same social acceptance because of the lower power or prestige associated with femininity traits and behaviors."