By fifth grade, boys know that there are certain rules on the playground. They know that they're not supposed to cry, especially in front of other boys, or show they are scared.

"If a boy doesn't do something he normally would do -- he's scared to play tackle football -- some people would yell at him, 'sissy' or 'baby' or 'wimp,' " said 11-year-old Thomas (not his real name).

Thomas said he burst into tears recently because other kids were teasing him and he got into an argument with one of his friends.

"I totally burst out crying after lunch, but normally boys try to keep that in or hold it until they're alone," Thomas said.

Even in this new millennium, boys struggle with the unwritten, unspoken rule that it's not acceptable for them to express sadness and other emotions, said psychologist Dr. Dan Kindlon, co-author with Dr. Michael Thompson of the 1999 bestseller "Raising Cain."

"Emotions become this foreign territory for boys," Kindlon said in a recent lecture.

Feminists helped broaden the definition of what it is to be feminine in the 1960s and '70s, Kindlon said. As a result, everyone accepts that girls should be able to play sports and do math and that they can have careers and be wives and mothers, he said. That has given girls the flexibility to know they can be leaders and still be emotional and nurturing.

But the definition of masculinity has not evolved as much, Kindlon said. Boys don't learn that they can be strong and nurturing and they still view displays of emotions as weak. So they are less flexible.

"We've got to expand that if we want our boys to be more mentally healthy," Kindlon said.

Many boys today are in trouble, Kindlon says. They are far more likely than girls to have behavior problems, get involved with drugs and alcohol and commit crimes.

Studies show that parents, especially fathers, tend to steer boys away from discussions about their inner life. Parents spend more time talking about feelings with their daughters.

Studies also show that parents use harsher punishment -- including physical punishment -- with their sons than their daughters. But that tends to increase bad behavior and makes boys more aggressive.

Boys also have a difficult time in school because classrooms, which are mostly taught by female teachers, are not set up for boys. Boys tend to be more active, more impulsive and less mature than girls, so they fall behind and are reprimanded more, Kindlon said.

The playground culture also can be cruel for boys, and it is another place where boys need to be taught empathy, Kindlon said. He quoted a 15-year-old boy who said that everyone in middle school and high school was bullied.

Kindlon believes the reasons for these problems have more to do with our culture than with inherent, biological sex differences. Males in the United States are more likely to be aggressive and do not have the tools to cope with strong, negative emotions, he said.

The key to solving boys' deeper problems is to teach them "emotional literacy," Kindlon said. Parents, particularly fathers, can do this by talking about their emotions and exploring others' feelings.

Since boys don't respond well to direct questions about their feelings, Kindlon said he often asks boys for advice about other kids instead. He might ask a boy whose parents are divorcing, for example, for advice about another boy who's upset because his parents are splitting up.

Fathers are particularly important in boys' lives because they serve as role models, Kindlon says. They are the ones who can teach boys that they can be tough and still talk about their feelings. Fathers also need to be more involved in their children's lives.

In the end, boys need to learn that it is courageous to open themselves up and show their feelings, Kindlon said, including deep fears about everyday situations. He quotes Mark Twain, who said, "Courage is resistance to fear, mastery of fear -- not absence of fear."